Build a Simple AC Generator

Introduction

How do you create electrical energy from mechanical energy? Build a simple generator that will light an LED when the device is spun.

Concepts

• Electric generators
• Alternating current
• Electromagnetism
• Faraday’s law

Materials

Safety Precautions

Exercise caution working with neodymium magnets—they are very powerful, and will snap together easily. Wear gloves to protect your fingers during the preparation. The magnets are fragile and may shatter if dropped or if they hit another object too hard. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.

Disposal

All materials may be saved and stored for future use. Do not store magnets near electronics.

Prelab Preparation

1. Measure the length of the cardboard tube and mark the center.
2. Push the tip of the nail through the cardboard tube at the center mark and drop the nail straight down. Poke a hole through the other side as well.
3. Make a short (0.5 cm) diagonal cut at the top of the tube to hold the wire.
4. Unwind a small length from the coil of magnet wire (about 10 cm), and hook it into the cut (see Figure 1).
5. Wind the wire around the cardboard tube at least 300 times. Be cautious to not pull too tightly, as the thin wire may snap. Note: The more coils, the easier it will be to light the LED.
6. Once you have the desired length, spool a bit of excess and cut the wire. Note: Skip this step if you intend to vary the number of turns.

7. Make another diagonal cut on the opposite end of the cardboard-tube generator and secure the wire end in it.
8. Use the sandpaper to remove the enameled coating on the two ends of the wire coiled around the generator.
9. (Optional) Test the coil using the multimeter’s “continuity beeper” function to ensure you have a complete circuit. If the test indicates you do not have a complete circuit, remove more enamel from the ends of the wire. Check the resistance to ensure the total resistance is less than 100 Ω.
10. Widen each hole by inserting the nail through one hole at a time and then wiggle the nail using circular motions until the nail can spin easily.
11. Test the nail’s ability to spin by holding the nail steady, then spinning the cardboard tube. The tube should spin easily, without much friction to slow it down. If not, repeat step 10. Leave the nail in its slots (see Figure 2).
12. Test that the generator works before demonstrating it by following steps 13–16.

Procedure

13. Show students the generator along with the two magnets.
14. Carefully detach the magnets from each other. Slowly bring one inside the generator, gripping it loosely so it will attach to the nail on its own. It should attach by the circular end (one of its poles).
15. Reach in with your fingers and grip the magnet attached to the nail to hold it in place. Grasp the other magnet firmly, and slowly and carefully insert it into the other end of the tube. If opposite poles are facing each other, a pulling force will be felt. If a force of repulsion is felt, flip the magnet around. Gripping both magnets firmly, bring the second magnet as close as possible before releasing it, allowing it to attach to the other side of the nail (see Figure 3).
16. Wind each shaved end of the wire around the LED’s leads, ensuring the leads do not touch each other (see Figure 4).
17. Turn off or dim the lights in the room and spin the nail with both hands. Increase the spinning speed until you can see the LED flicker on and off (see Figure 4).

Student Worksheet PDF

12736_Student1.pdf

Teacher Tips

• A complete circuit is essential for success in this laboratory activity. If a voltage drop over the wire is not produced, the LED will not light. Verify that the ends of the wires are well sanded and not touching each other to ensure good conductivity.
• Although the magnets will introduce a resistance force when spinning the nail, it should still not be difficult to spin. If it is, wiggle the nail through each individual hole to make it easier.
• To facilitate winding the coils, set up the spool on a stand or stabilized rod so it can be held in place but freely rotate.
• A greater number of turns can increase the generator’s efficiency.
• If you cut the wire too short, and the generator does not light the LED due to lack of coils, sand the ends of the coiled wire and the wire spool, and twist them together. Test on the edges of the shaved region to make sure you have a circuit through the twisted wires, and continue spooling.
• Test the amount of AC current and voltage used by measuring with a multimeter. Spin the magnets faster and watch the numbers go up!
• A square coil shape will also work. Find or create a small cardboard box and similarly wind the coil around it.
• The magnets are fragile and may crack or chip if they are dropped, or allowed to snap together with excessive force. Use caution when handling.
• Keep neodymium magnets away from computer disks or other magnetic strips such as credit cards. They will quickly erase the magnetized data.
• LEDs will burn out easily with too much current, and are thus usually connected in series with a resistor. Connect the generator to a multimeter set to measure alternating current and ensure the current does not exceed 30 mA, the maximum current a typical LED can take.

Sample Data

Record your observations about what the generator is made of and how it works.

Student answers will vary.

Answers to Questions

1. What are the components needed to make a generator?
   

   To make a generator you need conducting loops of wire, a strong magnetic field, and some method of changing the field going through the loops.

2. Will spinning the nail slowly light the bulb? Quickly? Explain.
   

   Spinning the nail slowly will not light the LED. Spinning quickly increases the change in magnetic flux over a shorter period of time, which increases the voltage. If you put more mechanical energy into the system, you get more electrical energy out.

3. Does the voltage produce direct current (DC) or alternating current (AC)? Describe the observations supporting your answer, and explain why this type of current is produced.
   

   The voltage induces alternating current. The magnetic field is switching directions, therefore the current switches direction. Since the LED will only light when the current travels in the correct direction, the LED flickers on and off as the current switches direction.

Discussion

A generator is a device used to convert mechanical energy into electrical energy. The source of the mechanical energy can be many things—a windmill, a water wheel and even a human being. A generator operates the opposite of an electric motor, which converts electrical energy into mechanical energy. In fact, the two devices can be used interchangeably. Providing mechanical energy to the motor by spinning its shaft will cause it to produce electrical energy, and providing electrical power to a generator will make the shaft spin. However, the designs of a generator and motor are typically such that it’s more efficient to convert energy in one direction only.

The key to both generators and motors is the interactions between electricity and magnetism, or electromagnetism. Although scientists had earlier noticed similarities between electricity and magnetism, it wasn’t until 1819 that Hans Christian Oersted (1777–1851) observed a physical connection between the two. While lecturing on electrical circuits, he connected a circuit near a compass. To his surprise, the compass needle was deflected when current was flowing through the circuit. Oersted had discovered that a current-carrying wire produces a magnetic field (see Figure 5).

Oersted’s discovery helped scientists understand how electricity could create magnetism, but it took a few years more to understand how magnetism would generate electricity. In 1831, Michael Faraday (1791–1867) discovered the relationship through careful experimentation that “any change in the magnetic environment of a coil of wire will cause a voltage to be “induced” in the coil.” This is known as Faraday’s Law—a changing magnetic field produces a voltage called electromotive force, or emf. This is the essence behind this demonstration of how a generator works. A changing magnetic environment—the spinning magnet—induces a voltage in the coils. When voltage is induced, electrons flow through the wire. The amount of voltage or emf induced depends on the strength of the magnetic field, the number of wire coils, and the speed at which the wire or magnets are moving.
Generators today usually contain a large, stationary permanent magnet, called the stator, and a moving coil of wire, called the armature. The mechanism to rotate the coil of wire in the magnetic field is called the rotor. In the demonstration device, the magnet itself is rotating, changing the direction of the magnetic field, and thus changing the magnetic flux. Because the magnetic field is rotating, the direction of the induced current is also changing, producing alternating current, or AC. Reaching a critical voltage, called the knee voltage, will allow the LED to light. However, LEDs are unipolar, meaning they will only work when current is flowing in one direction. This accounts for the flicker when spinning the magnet. Since the current is alternating, the LED will only light when the current is flowing in the correct direction (see Figure 6).

A mathematical description of Faraday’s Law is shown in Equation 1.
where

Emf is the electromotive force or voltage (V)
N is the number of turns in a coil of wire
ΔΦ is the change in magnetic flux (webers)
Δt is the change in time

Magnetic flux (Φ) is defined as the average magnetic field multiplied by the perpendicular area it penetrates. The equation is:
where

B is the magnetic field
A is the perpendicular area

Faraday’s law shows that the greater the change in flux occurring, the larger the voltage. Using this, Faraday was able to create the first simple generator, a design similar to the one employed in this activity. This meant that instead of creating voltaic cells—batteries—using expensive chemicals, current could be produced continuously, provided one had a constant source of spinning motion. The stronger the magnet, the more turns in the coil, and the faster you can spin the magnets are all directly related to how much voltage a generator will produce. The magnets used in this demonstration are made with neodymium, a rare earth element and are some of the strongest permanent magnets available conventionally.