The Idea
Michael Faraday discovered that a moving magnetic field causes an electrical current to flow in a wire. Most of the electricity generated throughout the world today is based upon this historic discovery. Power plants routinely convert mechanical energy into electrical energy. This experiment explores the physical principle that makes this possible.
What You Need
- bar magnet
- coil of insulated wire—the more coils, the more pronounced the effect. Thin "magnet" wire insulated with clear or colored enamel can work fine.
- galvanometer (a very sensitive ammeter)
Method
- Wrap the insulated wire in a coil around a cylindrical cardboard form. Use the smallest diameter that will enable the bar magnet to pass through. Prewound coils are available.
- Connect the two ends of the wire to the positive and negative terminals of the galvanometer.
- Predict what you think will happen if the magnetic is placed inside the coil. Try it and observe the response on the galvanometer.
- Move the magnet back and forth in the coil. Observe the deflection on the galvanometer.
- Move the magnet back and forth outside the coil and observe the effect. What happens if the coil moves while the magnet is stationary?
- If it is possible to increase or decrease the number of coils, you can evaluate its effect on the amount of current that can be generated.
- Based on your observations, describe how a magnet can produce an electric current.
The apparatus is shown in Figure 108-1.
Expected Results
A magnet produces a current in a wire only when the magnet is moving. A stationary magnet will not generate a current.
The faster the relative motion between the magnet and the coil, the greater the current. The larger the number of coils—with all else equal—the greater the current. A more powerful magnet produces a greater current.
Why It Works
A magnetic field itself does not produce an electric current. A changing magnetic field is required to produce an electrical current. This is addressed in mathematical detail by Maxwell's laws for those who want to pursue it further.
Other Things to Try
A nice way to display these results is to attach the coil to a voltage sensor and use this to generate a graph of voltage versus time. Moving the magnet back and forth in the coil results in an alternating current (AC). If you hang the magnet on a spring and have it oscillate up and down in the coil, you will have a simple model of an AC generator.
A good follow-up is to investigate a model electrical generator, which can also generate a similar AC current.
The Point
The significance of this project is to show how mechanical energy is converted into electrical energy. The key parts of an electrical generator are a magnet and a coil of wire. Electricity flows when the coil and magnet move relative to each other.
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