How Does LED Brightness Vary with Current?

Objective
The goal of this project is to measure the light output of an LED as a function of current through the LED.
Introduction
Today's electronic devices such as computers, handheld video games, and MP3 players are all based on components made of materials called semiconductors. Semiconductors have properties that are intermediate between conductors and insulators. Diodes, for example, are a semiconductor device that allow current to flow in only one direction. In the forward direction, diodes act like a conductor. In the reverse direction, diodes act like an insulator.
An LED (light-emitting diode) is a special kind of diode that produces light (see Figure 1).

Figure 1. A red LED (top). The longer lead is the anode (+) and the shorter lead is the cathode (&minus). In the schematic symbol for an LED (bottom), the anode is on the left and the cathode is on the right (Hewes, 2006).When current flows through the diode in the forward direction, some of the current is converted into light of a specific color (i.e., wavelength). The color of the light depends on the material from which the semiconductor is made. LEDs are available in many different colors.
As the current through the LED increases, the brightness also increases. Typically, the recommended current for an LED is 20 mA or less. Above this value, the lifetime of the LED will be decreased significantly. Far above this value, the LED will fail catastrophically, like a flashbulb.
To keep the LED current at a reasonable level, LEDs are typically connected in series with a current-limiting resistor, as shown in Figure 2.

Figure 2. Schematic diagram of an LED in series with a 1kΩ resistor (Hewes, 2006).

The voltage drop across an LED is about 2 V (except for blue or white LEDs, where the voltage drop is about 4 V). In the circuit in Figure 2, the voltage drop across the resistor will be 9 − 2 = 7 V. Using Ohm's law, the current, I, through the resistor will be V/R = 7 V/1kΩ = 7 mA.
Figure 3 (below) shows you how to use Ohm's Law to calculate what size resistor you need to limit the current through the LED to the desired value. The voltage drop across the resistor will equal the supply voltage minus the voltage drop across the LED (or, VS − VL). You can then use Ohm's Law to calculate the resistance, R, needed to produce a desired current, I:
R = (VS − VL)/I.
So, if the supply voltage is 9 V, what resistor would you need for a 20 mA current? R = (9 − 2)/0.02 A = 350Ω. For more details, and a set of online calculators, see the LED references in the Bibliography section (Hewes, 2006; Ngineering, 2003).

Figure 3. Schematic diagram showing how to use Ohm's Law to calculate the correct value for the current-limiting resistor (Hewes, 2006).

In this project, you will use a variety of different resistors in series with an LED to make circuits with smaller and larger currents. You'll use a simple light-to-voltage converter circuit to measure the output of the LED. How will LED output change with current?

Terms, Concepts and Questions to Start Background Research
To do this project, you should do research that enables you to understand the following terms and concepts:
1 semiconductor,
2 light emitting diode (LED),
3 voltage (V),
4 current (I),
5 resistance (I),
6 Ohm's law (V = IR, or I = V/R, or R = V/I).

Questions
You have a 4.5 V voltage source connected in series with a 470Ω resistor and a standard red LED. Assuming that the voltage drop across the LED is 1.7 V, how much current would you expect to flow through the circuit?
What resistance would you need in the above circuit in order to produce a 20 mA current?

Bibliography
On this page you can build virtual circuits with batteries and resistors, then test your circuit by throwing a switch to light up a bulb. If there's too much current, the virtual light bulb blows up, too little current, and the bulb won't light. When you get the current right, the bulb glows brightly.Unknown, 1999. "Ohm's Law." Physics Department, University of Oregon. [accessed December 13, 2006] http://zebu.uoregon.edu/nsf/circuit.html#Ohm
These webpages have useful information on LEDs:
Hewes, J., 2006. "Light Emitting Diodes (LEDs)," The Electronics Club, Kelsey Park Sports College [accessed December 15, 2006] http://www.kpsec.freeuk.com/components/led.htm.
Ngineering, 2003. "LED Calculators," Ngineering.com [accessed December 15, 2006] http://www.ngineering.com/LED_Calculators.htm.
The data sheet for the light-to-voltage converter has complete specifications for these devices:TAOS, Inc., 2006. "TSL12S, TSL13S, TSL14S Light-to-Voltage Converters," [accessed December 15, 2006] http://www.taosinc.com/images/product/document/TSL12S-E20.pdf.
This webpage shows you how to read the value of a resistor from the colored stripes:Engstrom, S., 2006. "Resistor Color Codes," SamEngstrom.com [accessed December 15, 2006] http://www.samengstrom.com/nxl/3660/4_band_resistor_color_code_page.en.html.

Materials and Equipment
To do this experiment you will need the following materials and equipment (unless otherwise specified, part numbers are from Mouser Electronics):

• light-to-voltage converter (part number 856-TSL14S-LF),
• for building the light detection circuit you can use:
  1. solderless breadboard (part number 517-922306), or
  2. you can solder the circuit together and install them in a small enclosure (you'll need to drill a hole and position the sensor so that light can reach it),
• 6 fresh AA batteries (or freshly-charged AA batteries, if you use rechargeables),
• 2 battery holders for 3 AA batteries (part number 12BH431-GR),
• alligator clip leads (part number 13AC010),
• 1/4-watt resistors with the following values:
  1. 165 Ω (part number 271-165-RC),
  2. 330 Ω (part number 271-330-RC),
  3. 665 Ω (part number 271-665-RC),
  4. 1330 Ω (part number 271-1.33K-RC),
  5. 2670 Ω (part number 271-2.67K-RC),
  6. 10 kΩ (part number 271-10K-RC);
• 5 red LEDs, (40–60 mcd@20 mA; e.g., part number 638-204IT),
• one digital multimeter (DMM):
  1. If you want one-stop shopping, Mouser has moderately priced (about $40) DMMs from BK Precision (part number 615-2703B) and ExTech (part number 685-MN26T), however,
  2. the TM-162 is about half the price: part number TM-162 from TechBuys.Net.
  

Disclaimer: We occasionally provide information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. We receive no consideration, financial or otherwise, from suppliers for these listings. (The sole exception is the Amazon.com link.)

Experimental Procedure

Building the Light Detection Circuit
1) The circuit is very simple. The light-to-voltage converter is an integrated package that contains a photodiode and an amplifier. The functional block diagram is shown below.

Light-to-voltage converter functional block diagram (TAOS, Inc., 2006).

Light (indicated by arrows) illuminates the photodiode sensor and generates a current. The operational amplifier (or "op amp," symbolized by the large triangle in the diagram) produces an output voltage that is proportional to the intensity of the light illuminating the photodiode.

2) A drawing of the actual component is shown below. The round window contains the light-sensitve region. The component has three pins, as shown.
Pin 1 should be connected to ground (black wire from the battery holder).
Pin 2 should be connected to the positive supply voltage (red wire from the battery holder). The supply voltage should be between 2.5 and 5.5 V DC, so you can use either 2 or 3 AA batteries.
Pin 3 is the output voltage, a signal that is proportional to the amount of light falling on the sensor.

3) Here is a schematic diagram of the complete circuit. In addition to the light-to-voltage converter, there is only one more component: a 10 kΩ resistor (RL). Connect the resistor from pin 3 to ground, as shown.

4) The output signal is the voltage drop across the 10 kΩ resistor. To read the output, use one alligator clip lead to connect the positive lead of the resistor to the red probe of your DMM, and another clip lead to connect the grounded lead of the resistor to the black probe of your DMM. Set your DMM to read up to 5 DC volts (usually the 20 V range).

5) You can easily build the circuit on a solderless breadboard.

The photograph below shows a small breadboard. The breadboard has a series of holes, each containing an electrical contact. Holes in the same column (examples highlighted in yellow and green) are electrically connected. When you insert wires into the holes in the same column, the wires are electrically connected. The gap (highlighted in orange) marks a boundary between the electrical connections. A wire inserted in one of the green holes would not be connected to a wire inserted in one of the yellow holes. Integrated circuits, such as the oscillator used in this project, should be inserted so that they span the gap in the breadboard. That way, the top row of pins is connected to one set of holes, and the bottom row of pins is connected to another set of holes. If the integrated circuit was not spanning a gap in the breadboard, the pins from the two rows would be connected together (shorted), and the integrated circuit wouldn't work. Finally, the two single rows of holes at the top and bottom (highlighted in red and blue) are power buses. All of the red holes are electrically connected and all of the blue holes are electrically connected. These come in handy for more complicated circuits with multiple components that need to be connected to the power supply.

An example of a solderless breadboard. The highlighting shows how the sets of holes are electrically connected. The red and blue rows are power buses. The yellow and green columns are for making connections between components. Integrated circuits are inserted to span the gap (orange) so that the two rows of pins are not connected to each other.

6) Alternatively, if you have experience with a soldering iron, you can make the circuit in a small enclosure. You'll have to drill a hole and position the light-to-voltage converter so that light can reach its sensor.

7) Test the circuit with your DMM. Use clip leads to connect the DMM across the 10 kΩ resistor, and set the multimeter to read DC volts (the maximum signal will be about 5 V). When you shine a flashlight directly on the sensor, your multimeter should read between 1 and 5 V (depending on the brightness of the flashlight, and how close it is to the sensor). When you cover the sensor, the multimeter should read close to 0 V.

Building the LED Circuit
The LED circuit is very simple. As discussed in the Introduction, you should always use a current-limiting resistor in series with the LED.
Use a clip lead to connect the red wire of the battery holder to one lead of the 165Ω resistor.
Use a clip lead to connect the other resistor lead to the longer lead (anode) of the LED.
Gently bend the ends of LED leads apart from one another so that the clip leads won't accidentally short the circuit.
Use a clip lead to connect the shorter lead (cathode) of the LED to the black wire of the battery holder. That's it!

Measuring LED Light Output
Taking care not to disconnect the clip leads, position the LED so that its top is pointing directly at the sensor window of the light-to-voltage converter.
Check the reading on the DMM. If the LED is too close, it will drive the light detection circuit to its maximum response (about 4.5 V, with 3 AA batteries). We say that the response is saturated, because the detector cannot increase its output if it detects more light. You want to avoid this condition, because if the detector is in saturation, you will not get an accurate reading of the intensity of the LED. Move the LED away from the detector until the voltage reading on the DMM starts to drop.
Measure the distance between the LED and the detector, or, better yet, fix the LED in place. You want the LED at the same height as the detector window, with the top of the LED facing directly at the window. The distance between the LED and the detector should be exactly the same for all of your measurements.
Record the voltage reading on the DMM.
Change the resistor in the LED circuit. Swap out the 165Ω resistor and replace it with the 330Ω resistor.
With the LED at exactly the same distance from the sensor, again measure and record the voltage reading on the DMM.
Repeat for each of the resistors (165Ω–2.67kΩ).

Measuring LED Current
You also need to measure the current in the LED circuit with each of the different resistors (165Ω–2.67kΩ). If you have two DMMs, you can use one to measure the voltage of the light detector circuit, and the other to measure the current in the LED circuit. If you have a single DMM, then you have to make the current measurements separately.
To measure current, connect the DMM in series with resistor and LED.
Use a clip lead to connect the red wire of the battery holder to one lead of the 165Ω resistor.
Use a clip lead to connect the other resistor lead to the longer lead (anode) of the LED.
Gently bend the ends of LED leads apart from one another so that the clip leads won't accidentally short the circuit.
Use a clip lead to connect the shorter lead (cathode) of the LED to the red probe of the DMM. Note that some DMMs have separate sockets for the red probe for reading current and voltage. Make sure that the red probe is in the correct socket for reading current.
Use a clip lead to connect the black probe of the DMM to the black wire of the battery holder.
Set the DMM to read DC current in the 200 mA range. (For resistors > 165Ω, you will probably want to switch to the 20 mA range.)
Record the current reading for each circuit.

Analyzing Your Results
Make a graph of the LED intensity, expressed as voltage output from the light detection circuit (y-axis), vs. the LED current, in milliamps (x-axis).
What is the relationship between LED current and light intensity?

Variations
An LED can easily be powered by 2 AA batteries instead of 3. With two batteries, the supply voltage will be 3.0 V instead of 4.5 V. If you were to use a 3 V supply for the LED circuit, can you figure out the value of the resistor you would need in order to limit the LED current to 20 mA? Which additional resistors would you need in order to replicate this experiment using a 3 V supply for the LED circuit? Try it out!
What happens if you increase the LED current beyond 20 mA? Calculate the resistor value you would need to limit the LED current to 40 mA. Design an experiment to find out if the LED intensity at 40 mA is twice the intensity at 20 mA.
For an experiment that investigates LED current in circuits powered by solar cells, see the Science Buddies project: How Does Solar Cell Output Vary with Incident Light Intensity?

Electronics & Electrical Engineering Projects

Stop for a minute and try to imagine your world without electrical power and electronic gadgets. No convenient appliances in the kitchen, no electric lights. No computers, MP3 players, television or video games. Your life would be completely different, wouldn't it? Electricity and electronics are so central to modern life that, paradoxically, they're easy to overlook. If you've ever wondered how electricity is generated, or how motors or electromagnets work, if you'd like to build your own radio, or some other electronic device, then this could be the place to find your next science fair project idea. In the brackets are the difficulty levels which would guide you from where to start.

• Crank up the music (1)
• Pencil resistors(2)
• Induction:How many coils do i need?(3)
• Rock on ! Recording Digital Data with Magnets(4)
• Building an electric motor(5)
• How Does LED Brightness Vary with Current?(6)
• Testing a Parabolic Reflector with Light from an LED(7)
• Make Your Own Low-Power AM Radio Transmitter(8)
• Recording on a Wire(9)
  

Building an Electric Motor!!

Objective
The objective of this project is to build a simple electric motor from scratch.

Introduction
Electric motors are everywhere; even your computer has electric motors to power its cooling fans and hard disks. Building a simple DC electric motor is a great way to learn how they work, and it's really fun to watch your creation spin.

Terms, Concepts and Questions to Start Background Research
To do an experiment in this area, you should do research that enables you to understand the following terms and concepts:
armature or rotor,
commutator,
brushes,
field magnet,
electromagnet, and
the operating principles of a DC motor.

More advanced students will also want to study:
right-hand rule,
induction, and
back EMF.

Bibliography
Here are some resources to get you started:
How Electric Motors Work: http://electronics.howstuffworks.com/motor.htm
Simple Electric Motors: http://www.simplemotor.com/

Find instructions for the simple motor described below at any of these sites:
http://www.scitoys.com/scitoys/scitoys/electro/electro.html#motor
http://www.exploratorium.edu/snacks/stripped_down_motor.html
http://fly.hiwaay.net/~palmer/motor.html

Experimental Procedure

The motor is simply a battery, a magnet, and a small coil of wire you make yourself. There is a secret to making it (which I will of course share with you) which is at the same time clever and delightfully simple.
What you will need:
A battery holder, such as Radio Shack #270-402 (holds a "C" cell) or #270-403 (holds a "D" cell).
A battery to fit the holder.
A magnet such as Radio Shack #64-1877, #64-1895, #64-1883, #64-1879, or #64-1888.
Some magnet wire such as Radio Shack #278-1345. We want enamel coated 22 gauge (or thicker) wire. We will only need about a yard of wire, so the Radio Shack package will make a dozen motors or more.
Some heavier wire such as Radio Shack #278-1217 or #278-1216. We want bare wire of 18 or 20 gauge, so we will be removing the plastic insulation from the wires listed above. We will need less than a foot of this wire per motor.
We start by winding the armature, the part of the motor that moves. To make the armature nice and round, we wind it on a cylindrical coil form, such as a ball point pen or a small AAA battery. The diameter is not critical, but should be related to the wire size. Thin wire requires a small form, thick wire requires a larger form.

Leaving a couple of inches of wire free at one end, wind 25 or 30 turns arounf the coil form. Don't try to be neat, a little randomness will help the bundle keep its shape better. The coil will end up looking like the photo below:

Now carefully pull the coil off of the form, holding the wire so it doesn't spring out of shape

To make the coil hold its shape permanently, we will wrap each free end of the wire around the coil a couple of times, making sure that the new binding turns are exactly opposite each other, so the coil can turn easily on the axis formed by the two free ends of wire, like a wheel.

It is not necessary, but I usually wrap a couple turns around these binding turns as well, threading the wire into the space between the large coil and the small coils that hold it together. This makes for a neat, tight package.

If this method of holding the coil together is too difficult, feel free to use scotch tape or electrical tape to do the job. The important thing is to keep the coil together, and to have the two ends of the wire anchored well, and aligned in a straight line, so they form a good axle.

Now is where the secret trick comes in, the thing that makes the motor work. It is a secret trick because it is a small and subtle thing, and is very hard to see when the motor is running. Even people who know a lot about motors may be puzzled until they examine it closely and find the secret.

Hold the coil at the edge of a table, so the coil is staight up and down (not flat on the table), and one of the free wire ends is lying flat on the table. With a sharp knife, remove the top half of the insulation from the free wire end. Be careful to leave the bottom half of the wire with the enamel insulation intact. The top half of the wire will be shiny bare copper, and the bottom half will be the color of the insulation.

Do the same thing to the other free wire end, making sure that the shiny bare copper side is facing up on both wire ends.

The idea behind the trick is that the armature is going to rest on two supports made of bare wire. These supports will be attached to each end of the battery, so electricity can flow from one support into the armature and back through the other support to the battery. But this will only happen when the bare half of the wire is facing down, touching the supports. When the bare copper half is facing up, the insulated half is touching the supports, and no current can flow.

The next step is to make the axle supports. These are simple loops of wire that hold up the armature and allow it to spin. They are made of bare wire, since they will also act to get electricity to the armature.

Take a stiff piece of bare wire (copper or brass will work, as will a straightened paper clip) and bend it around a small nail to make a loop in the middle, as shown in the photo below. Do the same to another wire, so you have two supports.

The base for this first motor will be the battery holder. It makes a nice base because it is heavy when the battery is installed (so the motor won't wobble) and because it has convenient holes in the plastic where we can attach the bare wire armature supports.

Attach the support wires securely to the battery holder by winding the free ends several times through the small holes in the plastic at each end. Bend the support wires so the rings are just far enough apart for the armature to spin freely. Bend them apart a little and insert the armature into both rings, then bend them back so they are close to the coil, but not touching it.

Insert the battery into the holder. Place the magnet on top of the battery holder just underneath the coil. Make sure the coil can still spin freely, and that it just misses the magnet.

The finished motor looks like this:

Note that there is a strip of paper stuck in between the battery and the electrical contact in the holder. This is the on/off switch. Remove the paper to allow electricity to flow into the motor, and replace the paper when you want to stop the motor and save the battery.

Spin the armature gently to get the motor started. If it doesn't start, try spinning it in the other direction. The motor will only spin in one direction.

If the motor still doesn't start, carefully check all the electrical connections. Is the battery connected so one support touches the positive end of the battery, and the other touches the negative end? Is the bare copper half of the armature wire touching the bare support wires at the bottom, and only at the bottom? Is the armature freely spinning?

If all these things are correct, your little motor should be spinning around at a pretty fast rate. Try holding it upside down. The motor should spin in the opposite direction if the magnet is on top instead of on the bottom. Try turning the magnet upside down and see which direction the motor spins. If you want a motor that has the magnet on the side instead of the top or bottom, you can simply make a new armature, but this time lay the coil flat on the table when you scrape the insulation off of the top half of the free wire ends.

If u thought this was a childs game then u can go on to make larger and faster motors.

Rock On! Recording Digital Data with Magnets

Objective
The goal of this project is to determine the maximum "recording density" for storing digitized information using a grid of bar magnets. You'll learn about how information is digitized, and how the digitized information is stored magnetically. The project helps you to understand the limitations on information density recorded on magnetic materials.

Introduction
Today, magnetic disk drives are used to store and retrieve information for many different applications. Digital video recorders (DVRs), MP3 players, and gaming systems are all examples of popular products that use a disk drive to store and retrieve information. Some other applications that you may not be familiar with are global positioning systems (GPS), banking systems, and automobiles.
How are all these different kinds of information stored on magnetic disk drives? How much data can fit in a given amount of space on a disk? How is data erased from a disk? This project will help you answer these questions as you learn how magnetic materials are used to store information.
Information like words, music, or movies is translated into a format that can be saved onto various permanent storage devices, like a magnetic disk drive. This translation is called digitization, which means that the information is converted into a stream of numbers. The smallest unit for digital information is called a bit. A bit can be either 0 or 1, that's it. By stringing together a series of bits, larger numbers can be represented. For example, a byte is a sequence of 8 bits. A byte can encode 28 (= 256) unique values. If you want to learn more about encoding information in bits and bytes, see the Science Buddies project Bits, Bytes, and Bases: Write a JavaScript Binary/Decimal/Hexadecimal Converter.
Letters, numbers, and other symbols for printed text are digitized using a standard code, called ASCII (ASCII is an acronym for American Standard Code for Information Interchange). Each character is assigned a unique code. Click here for a Table of 7-bit ASCII Character Codes. Using the ASCII table, what sequence of bits would correspond to the word "digitize?"
In this project, you will digitize a short piece of text (e.g., the name of your favorite band) using the ASCII representation of the text. Next, you will use bar magnets to represent the individual bits of the digitized text. The orientation of the magnet will determine whether it represents a 0 or a 1. You'll see how close together your magnets can be packed while still preserving your stored information. Finally, you'll see how easily you can erase your stored information with a permanent magnet.

Terms, Concepts and Questions to Start Background Research
To do this project, you should do research that enables you to understand the following terms and concepts:
bit,
byte,
ASCII,
magnet,
bar magnet.

Questions
Before MP3 players and DVRs, what was used to store and retrieve music and video?
What are other examples of storage and retrieval methods or systems?
Why do you think disk drives devices like iPODs and DVRs are now replacing tape-based devices like Walkmans and VCRs?
Materials and Equipment
To do this experiment you will need the following materials and equipment:
a print-out of the ASCII code and binary code for 26 letters,
plastic/glass tray with square/rectangular compartments,
a few dozen 1 inch bar magnets,
paper and pencil,
ruler,
one big horseshoe magnet.

Experimental Procedure
In this experiment, you will digitize a short piece of text (e.g., the name of your favorite band) using the ASCII representation of the text. Next, you will use bar magnets to represent the individual bits of the digitized text. The orientation of the magnet will determine whether it represents a 0 or a 1. You'll see how close together your magnets can be packed while still preserving your stored information. Finally, you'll see how easily you can erase your stored information with a permanent magnet.
You'll need a word or phrase to digitize. Pick the name of your favorite musician or band, your favorite song, or some other word or phrase.
Make a table to translate your chosen word or phrase into the binary ASCII code.

Now use individual bar magnets to represent each bit of the coded word or phrase. We'll say that a magnet with its N pole facing right is a 1, and a magnet with its N pole facing left is a 0. (To make your code easier to see, you may want to color the N half of each magnet.) Place one magnet in each compartment of your plastic tray, arranging them according to the binary code for your word or phrase.
What is the information density of your recording? How many bits per sqaure inch? Just for comparison, a 1990 hard disk could store 1 billion (1,000,000,000) bits per sqaure inch, and a 2006 hard disk can store 100 billion (100,000,000,000) bits per square inch.
Gently tap your tray of magnets. What happens to the arrangement? Are some of the magnets attracted (or repelled) by their neighbors?
If the magnetic material on a recording surface could move around like your bar magnets, how do you think this would affect the durability of the recording?
For the next part, you'll be working with the binary code for a single letter. Choose one letter from your word or phrase to use for this.
On a piece of paper, draw 8 squares, about the same size as the compartments in your tray. Arrange the magnets on the paper squares to represent the code for your chosen letter.
Do the magnets interact with each other now that there is no longer a wall separating them?
1. If the magnets do not interact, try again with smaller squares. How small can your squares be without the magnets interacting with their neighbors?
2. If the magnets do interact, try again with larger squares. How large do the squares need to be so that the magnets are not disturbed by their neighbors?

What is the highest recording density you can achieve when you place the magnets on a piece of paper (in number of bits per square inch)?
Again arrange your bar magnets on the paper to encode your chosen letter. Take the horseshoe magnet and, holding it a foot above the paper (measure with the ruler), pass it over the bar magnets. Did any of the bar magnets move?
1. If the bar magnets did not move, lower the horseshoe magnet slightly and try again. At what height does the horseshoe magnet move the bar magnets?
2. If the bar magnets did move, raise the horseshoe magnet slightly and try again. At what height does the horseshoe magnet first stop moving the bar magnets?
3. What does this tell you about erasing data stored on magnetic recording media?

How Many Coil Turns Do I Need?

Objective
In this experiment you will experiment with induction and test if the number of turns of wire will affect the amount of electricity in a circuit.
Introduction
Did you know that the electricity induced from a magnet (electromagnetic electricity), a battery (voltaic electricity) and lightning (static electricity) are all the same? This was shown in 1832 by a famous scientist named Michael Faraday. But his most famous experiment was in 1831, when he made an "induction ring" and discovered something called electromagnetic induction: the "induction" or generation of electricity in a wire by means of the electromagnetic effect of a current in another wire (EIA, date unknown).
Faraday's experiments form the basis of most modern technology, and he is remembered as one of the world's greatest experimental physicists. He invented the first electric generator and is also known as the father of the electric transformer, the electric motor, and electrolysis. He wrote the "Law of Induction" and is known for the "Faraday Effect." Because of his important discoveries, two units in physics were named in his honor: the farad (for capacitance) and the faraday (as a unit of charge).
In this experiment you will build a simple induction circuit to test the properties of electromagnetic induction. You will change the number of turns of wire in the circuit to investigate the relationship between the number of turns and the amount of electricity that is induced. Will more turns increase or decrease the amount of electricity in the circuit?
Terms, Concepts and Questions to Start Background Research
To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!
electricity
circuit
electrons
current
conductor
wire gauge

Questions
How do electrons flow through wire?
How is current measured?
Does the gauge of wire effect the flow of electrons in a circuit?

Bibliography
Kuphaldt, T.R., 2002. "Lessons In Electric Circuits: Volume VI, Chapter 4," Internet FAQ Archives: Online Education. [accessed May 1, 2006] http://www.faqs.org/docs/electric/Exper/EXP_4.html
Energy Quest, 2002. "How Does a Transformer Work?" California Energy Commission. [accessed May 1, 2006] http://www.energyquest.ca.gov/how_it_works/transformer.html
EIA, Date unknown. "Famous People in Energy and Science," Place: Energy Information Administration (EIA), U.S.Dept. of Energy (DOE). [accessed May 1, 2006] http://www.eia.doe.gov/kids/history/people/pioneers.html#Ohm

Materials and Equipment
5 toilet paper tubes
electrical tape
26 gauge enameled magnet wire (Radio Shack sells a package with 3 spools, 1 roll each of 22, 26, and 30 gauge.)
scotch tape
very large bar or horseshoe magnet, no more than 1" in diameter (so that it will fit into the paper tube)
DC Microammeter:
You'll need an analog meter that can read in the range of 25–100 microamps of current.
One possibility for this project is a DC microampere panel meter by Simpson, which are available from Mouser Electronics (www.mouser.com/simpson). The nice thing about these meters is that it is bidirectional: you'll be able to see both positive and negative currents without reversing the polarity of your connections. The not-so-nice thing about these meters is that they are expensive! Any one of the following should work for this project (the 25-0-25 is the most sensitive):
Another possibility is to find an analog multimeter that can read DC current in the microampere range. The disadvantage of these meters is that the needle on the meter only moves one way, so they only measure positive current. To see current flowing in the opposite direction, you'll need to reverse the polarity of your connections to the meter.
The B&K Precision model 114B analog multimeter will work for this project. It is also available from Mouser Electronics (www.mouser.com/bk) , part number 615-114B, approximate price $40 (Nov., 2006).
You can often find inexpensive vintage analog multimeters on eBay. If you decide to go this route, make sure that any meter you bid on meets the required specifications (able to read 25–100 microamps of current).
Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies receives no consideration, financial or otherwise, from suppliers for these listings. (The sole exception is the Amazon.com link.)
Experimental Procedure
Wrap each tube with a different number of turns of wire: 100, 200, 300, 400, or 500. If you run out of space it is okay to overlap the turns of wire, as long as you keep the wire neat and tight. You can overlap layers of wire by winding an equal number of turns in one direction, securing it with scotch tape, and then winding an equal number of turns in the other direction. For example, when making a coil of 400 turns go 200 turns in one direction, cover the coil with a layer of scotch tape, and then go 200 more turns back in the other direction.
Leave about 3 inches of loose wire on either end of the coil for making connections with the ammeter to measure your current. Use scotch tape to wrap around and secure the wire tightly, so that the coil does not unravel.
Get your parent's help to scrape off 1/2 inch of material from each end of the loose wire with a sharp knife. Attach alligator clips to the naked ends of the wire.
Attach the alligator clips to the terminals of the DC Microammeter.
Now move one pole of the large magnet into the center of one of the toilet paper tubes, starting with the coil that has the largest number of turns. What happens?
Try moving the magnet back and forth in the tube. Now what happens? Is there a difference when you move it fast vs. slow? Does the direction of movement make any difference?
Try the other pole of the magnet. Now what happens?
Determine the best way to generate a current in your experiment, which magnet you will use, which end, and how you will move it around. Keep these the same for each of your experiments as a control.
Test each coil by placing it around the steel bar and measuring the number of microamps produced. Use the DC microammeter according to the manufacturers directions, and write down how much current is produced by each coil, measured in microamps (μA).

Make a graph and analyze your results.
Which tube produced the most electricity? How did the number of turns of the coil relate to the amount of electricity that was produced?

Variations
Try using different materials to induce a current in your circuit. Try comparing steel, iron, copper, nickel, aluminum, or any other metal rod you can find at your local hardware or hobby store. Which rod materials work the best? Are there any materials which do not work? What do you think this means?
You can use the same principle of magnetic induction to build a very simple generator. Try the Science Buddies project Shaking Up Some Energy to see how it works.
You can also use a similar circuit to make an electromagnet. Try the Science Buddies experiment Magnets and Charge or Measure Your Magnetism to see how the number of turns can affect the strength of the magnetic field.

Pencil Registors

Objective
In this experiment you will test if the length of a pencil resistor effects the output of a circuit.
Introduction
The existence of electricity has been known since the ancient Greeks used to rub pieces of amber with fur to make static electricity. Benjamin Franklin is credited with the first demonstration that the electricity in lightening and static electricity are the same in his famous, but very dangerous experiment. It took hundreds of years for thinkers, inventors and scientists to learn how to control and harness the power of electricity.
The first great achievement was the discovery of the concept of a circuit in 1800 by an Italian named Alessandro Volta. He showed that electricity flows through a circuit, and that a circuit needs to be complete, or closed, in order to work. He also invented the first battery, and we use the word Volt to identify the units of electricity.
In 1820, André-Marie Ampère published his explanation of Hans Christian Orsted's discovery that magnetic needles could be deflected by an electric current. Ampère's work, later refined by James Clerk Maxwell, firmly established the connection between electricity and magnetism. The movement of electricity through a circuit is called "current", and we measure the current flowing through a circuit in Amperes (often abbreviated "amps").
The next great discovery was by a German school teacher named Georg Simon Ohm in 1826, who had been a student of Volta. He discovered that some materials slowed down, or resisted, the movement of electricity. He found out that there was a relationship between the amount of electricity in a circuit, the movement of electricity through the circuit and the resistance of the circuit. The unit for resistance, Ohms, is named in his honor.
Even though Volta, Ampère and Ohm had paved the way for the first circuits, a real use for electricity still had not been shown and it was mainly a novelty. The first useful invention using electricity was the electric telegraph in 1832, which was used to send messages by code over long distances. But the first practical invention using electricity was the incandescent light bulb by Thomas Edison in 1877.
Electricity is a very important part of our modern world and none of the modern technology we use today could exist without it. All of our modern day gadgets, appliances and electronics use the power of electricity to work. It is the careful balance of parts of a circuit, batteries, wires and resistors; and the completeness of a circuit, which allow electricity to be useful, and not harmful.
In this experiment you will put these pieces together to build your own simple circuit and use it to investigate resistors. What do resistors do, and why are they useful? How will changing the size of the resistor effect the circuit? By varying the size of the resistor, and looking at the effect on a light bulb, we will determine how resistors work in a circuit.



Terms, Concepts and Questions to Start Background Research


To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

• electricity


• circuit


• resistor


• current


• conductor


• insulator

Bibliography
Here are some great internet resources available:
Surf this website for kids by the First Energy Corporation. Find out about electricity, history, efficiency and safety while having fun too! They also provide an excellent glossary: 2005. "Electric Avenue." First Energy Corp. Akron, OH. [12/13/05] http://www.firstenergycorp.com/kids/
Thelwell, Andy, 2005. "The Blobz Guide to Electrical Circuits." Staffordshire University, UK. [12/13/05] http://www.andythelwell.com/blobz/
The best place to buy parts for exploring and playing with electricity will probably always be Radio Shack. Find all of your supplies on the online catalog: 2005. "Radio Shack: Cables, Parts & Connectors". Radio Shack Corp. Fort Worth, TX. [12/13/05] http://www.radioshack.com/category/index.jsp?categoryId=2032058

Also try these great books:
Glover, David, 1993. "Batteries, Bulbs, and Wire." Kingfisher, New York, NY.
Berger, Melvin, 1989. "Switch On, Switch Off." Harper Trophy, New York, NY.
Cole, Joanna and Degen, Bruce, 1997. "The Magic School Bus and the Electric Field Trip." Scholastic Books, New York, NY.

Materials and Equipment
#2 pencils
insulated alligator clip set
9 V battery
9 V battery connector (optional)
small light bulb rated at 9 V
small light bulb holder
ruler
automatic pencil sharpener
popsicle stick
a coping saw (you will need your parents help with this)

Experimental Procedure
Set up your circuit board that you will use to test your resistors. You will need three pieces of wire with an alligator clip at each end. You can make your own, or you can buy an insulated alligator clip lead set from a store like Radio Shack.
Take one wire and attach one end to one terminal of the battery by clipping the alligator clip securely to one of the terminals.
Attach the other end of that wire to one terminal of the light bulb holder contact screw using the alligator clip.
Using a new wire, attach one end to the other contact screw of the light bulb holder with the alligator clip.
Screw the light bulb securely into the light bulb holder.
Before you start your experiment, you need to make sure your circuit works. Touch the two ends of the empty alligator clips to each other, making sure to hold onto the insulated sleeve so you won't get a shock. Does your light turn on? If it does, move on to the next step. If not, go back to step number 1 and check over your circuit to see if everything is connected correctly.
Next you will make your pencil resistors to test in your circuit. You will be making several different resistors of different sizes by cutting pencils to different lengths and sharpening both ends of the pencil. You will need your parent's help for this part.
With your parent's help and using a small coping saw, cut the pencils to different lengths. The pencil lengths for this experiment should offer a nice variety of small to large sizes, and be at regular intervals, such as 2 inches, 4 inches, 6 inches, etc...
After you cut each pencil, use the pencil sharpener to sharpen both ends of the pencil fragment. Don't worry about changing the lengths of your pencils, because you will be measuring them in the next step.
Use a ruler to measure each piece of pencil from tip to tip of the sharpened pencil lead. Remember to write down and keep a record of your results!

Next, place each pencil resistor one at a time into the circuit between the alligator clips by clipping onto the pencil lead portion at the tip of each end of the pencil. It is important to make sure the clips are attached to the graphite and not to the wood, because wood is an insulator and is not a conductive material.

Look at the light each time you connect one of your pencil resistors to the circuit. Make a record of your observation, and try to use a number scale to describe what you see. For example, you might use a scale of 1 to 5, where 1 is dark and 5 is bright.
Remember that piece of wire and that wooden popsicle stick? These are your "control" groups. Put them into your circuit and rate them using the same method and scale you used to test your pencils. The extra piece of wire is the "positive control." The popsicle stick is called a "negative control."

Variations
This experiment can be just the beginning to having fun building your own circuits. Here are many ways to make your experiment unique:
Try using the same circuit set-up to test different materials around your house to see if they are insulators or conductors. You might be surprised that some common household materials can be tricky to predict!
In our experiment, we are using a battery as a source of energy. How do you think different kinds of batteries would work in this circuit? Can you make a hypothesis of how the strength of the battery would relate to the length of pencil you could use?
Can you think of a way to rearrange this circuit to make a battery tester? Try testing batteries around your house with your Battery Tester.
Can you think of other energy sources to use for this experiment? Try using a solar cell, or a wind vane...
Advanced. A light is just one way to test the amount of resistance in the circuit. Another more careful way is to use Ohm's law. First, use a digital multimeter to measure the voltage drop across the pencil. Set the multimeter to read DC volts. With your circuit connected, and the light bulb on, touch the positive probe (red) of the multimeter to the clip on the side of the pencil connected to the positive terminal of the battery. Touch the ground probe (black) of the multimeter to the clip on the side of the pencil connected to the negative terminal of the battery. Write down the voltage reading. Next, measure the current, or flow of electricity in the circuit. The multimeter should be connected in series with pencil resistor and the light bulb, and the multimeter should be set to read DC current. Write down the current reading. Now you can calculate the resistance of the pencil, in Ohms, by dividing the voltage, in volts, by the current, in amperes. This method will give you more accurate data of the effect of your pencil resistors on the voltage supplied to the light bulb.

Crank Up the Music!

Objective
In this experiment you will investigate how crank powered appliances work by testing how the number of cranks is related to the amount of power produced.
Introduction
Have you used a hand crank powered radio or flashlight? These appliances can come in handy when you are in the outdoors without electricity. Hand cranked radios are becoming important for children in third world countries where electricity in the home is very rare (Cahill, 2004). In developing countries, families use hand cranked radios for news, education and entertainment.

Above a young boy listens to a Lifeline radio in Mugumbazi, Rwanda. (Cahill, 2004; Image from the Freeplay Foundation)

A hand cranked light or radio uses a generator to make electricity to power the device. A generator is usually built using a combination of an electrical coil and a magnet, which will make electricity when they are moved with respect to one another. You provide the movement necessary by cranking, which moves the coil in the generator. Here is a description of how a generator works from the Creative Science Centre:
A generator works by a magnetic field inducing a voltage into a coil of wire. Important points to note are that the voltage increases as the number of turns of wire on the coil, the size of the coil and the strength of the magnetic field increases. The magnetic field (or the coil) needs to be in constant motion to produce/induce the electricity into the coil. This can be done by moving the magnet or by moving the coil—the effect is the same. The coil (or the magnet) needs to move in such a way that the coil continually passes through the magnetic field.
The Iron nail is also important in our simple generator as it tends to concentrate the magnetic field. As the coil is wound around the nail it tends to draw in more magnetic flux into the area of the coil which boosts the overall efficiency of the device and increases the voltage that is produced.
The type of wire in the coil is also important. For example, thick wire means there will be less power loss, but the down side is that the coil will get very large when a great number of turns is needed. In a practical generator some trade off has therefore to be found between the size of magnet, coil and the wire. (Hare, 2006)
In this experiment you will test the connection between the turning of the coil of a generator and the power produced by using a hand cranked radio. How will the number of turns affect the length of time the radio will play?

Terms, Concepts and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!

• alternative energy
• power
• generator
• electricity
• magnet
• coil
• charge

Questions

How do hand crank radios work?
How do the number of cranks relate to playing time?
How do the number of cranks relate to the amount of power produced?

Bibliography
This site describes how an electrical generator is made and gives some background on how generators work: Hare, J., 2006. "Making an Electrical Generator," The Creative Science Centre (CSC) based at the University of Sussex at Brighton. [accessed: 3/18/06] http://www.creative-science.org.uk/gen1.html
This blogger shows us how to turn a hand crank flashlight into an iPod charger: Hoekstra, M., 2005. "How-to Hand Crank Power Your iPod," GeekTechnique.org [accessed: 3/18/06] http://geektechnique.org/index.php?id=236
This site has a java applet you can use to make printable, color graphs of your data: NCES, 2006. "Create a Graph," National Center for Education Statistics (NCES) U.S. Dept. of Education. [accessed: 3/3/06] http://nces.ed.gov/nceskids/createagraph/
Read about how hand crank radios are changing the lives of children in rural Africa: Cahill, P., 2004. "Bringing radio to rural Africa: Spreading information through crank-ups," MSNBC. [accessed: 3/20/06] http://www.msnbc.msn.com/id/4953281/

Materials and Equipment

• hand crank radio (or flashlight)


• stopwatch


• notebook and pencil


• graph paper

Experimental Procedure
1. For this experiment you will need a hand cranked radio, which can be found with outdoor/camping or emergency supplies. You can also use a hand cranked flashlight, but this makes the experiment a bit more challenging because you have to watch the light until it goes out. With a radio, you can listen for the radio to stop playing while doing another quiet activity.
2. You will need to make a data table in your notebook before you begin:

3. Crank the radio five times, let go and start your stop watch. When the radio stops playing, stop the stop watch and record the time in your data table. (If you are using a flashlight, watch the light and record the time the light turns off.)

4. Crank the radio ten times, let go and start your stop watch. When the radio stops playing, stop the stop watch and record the time in your data table.

5. Continue the procedure, each time adding five more cranks and timing the play time with your stop watch. Write each result in your data table.

6. Make a graph of your data. You can either make a bar graph or a line graph. You can make your graph by hand, or use a site like Create A Graph to make your graph on the computer.

7. What do your results mean? Does the data increase, or decrease? Does cranking the radio more times give it more power?

Variations
For a more advanced experiment, you can do several trials of the experiment and graph all of your results. By making a dot-plot of your results, you can use a ruler to draw a line of best fit through your data. By measuring the slope of this line, you can make an equation for the relationship between cranks and power in this type of appliance.
If your parents agree, try taking apart the radio and investigating the crank mechanism. Can you remove it from the device and try to use it to power some other device? Can you repeat the experiment using a voltmeter to quantify the data in another way? Read about this inventor's story of building a Hand Cranked iPod Charger. What other hand cranked appliances can you invent?