Interaction between electric & magnetic fields
The phenomenon of electromagnetic induction is responsible for some of the most important devices that technology has brought us. They include electromagnets, motors, generators and transformers.
Induction is a product of the interaction of a magnetic field with an electric field. When a permanent magnet is moved near a conductor (causing the conductor to experience a changing magnetic field) a current flows in the conductor. And when charges flow inside a wire, they always generate a small magnetic field – one that can be amplified using the right conductor geometry.
Because currents generate magnetic fields, those fields can be used to generate currents on other conductors, so a current in one conductor can be used to generate a current in another conductor through free space. It's almost magical.
Play the animation once or twice to get an idea of how induction works. The changing magnetic field, caused by the physical movement of a permanent magnet, causes an electromotive force on the electrons in the wire, resulting in a current.
Faraday's law
Faraday's law describes all induction phenomena, but its mathematics is beyond the scope of this page, so we'll settle for a qualitative description of the law. It says:
The induced potential (force that can move electrons) in a closed electrical circuit is equal to the negative of the rate of change of the magnetic field that lies inside of the circuit.
Notice that the magnetic field must be changing in order for a potential to form.
The animation above is a good example. If the magnet were inside of the wire coil, but stationary, then the magnetic field would be static (unchanging), and therefore could not produce an electromotive force (or EMF or "voltage" or potential) on the electrons in the wire.
When the magnetic field changes, a potential is produced. Its strength is proportional both to the amount of movement and to the volume of the magnetic field enclosed by the wires.
Electromagnetic induction
- A changing magnetic field can produce a potential, and thus a current in an electric circuit that encloses it.
- An electric current (the movement of charges in a conductor) can produce a magnetic field in a nearby paramagnetic object. A paramagnetic object is something that can be magnetized.
Examples: devices that run on induction
This will just be a brief tour of some of the important devices we commonly use that rely on the principle of electromagnetic induction. Each has (or will have) a page of its own. Follow the links for more detailed coverage.
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Electromagnets
When we circulate a current through a coil of wire, we generate a magnetic field that will attract or repel a permanent magnet (depending on orientation), and that will attract paramagnetic materials. Electric current always generates a magnetic field, though that field can be quite weak. When we coil wires, each adds to the magnetism of the electromagnet. A typical electromagnet will have hundreds or thousands of wire coils.
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Generators
A generator typically consists of a coil of wire that spins inside of a fixed magnetic field. There's no difference between moving the coil and moving the magnets in terms of Faraday's law; the coil, which is connected to an external circuit, still experiences a changing magnetic field (B-field). A generator coil can be turned by a jet of steam, as in a nuclear power plant, by a turbine placed in the path of falling water at the bottom of a dam, by a windmill or by other means.
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Motors
You can think of a motor as a generator in reverse. A coil of wires on a movable rotor is energized with current. The magnetic field generated by the coil opposes the permanent B-field of the magnets that surround the coil. The resulting magnetic repulsion, if cleverly exploited (and it is), keeps the rotor turning inside the magnet. The rotor can be connected to a machine such as the axle of a car to do useful work.
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Transformers
You can think of a motor as a generator in reverse. A coil of wires on a movable rotor is energized with current. The magnetic field generated by the coil opposes the permanent B-field of the magnets that surround the coil. The resulting magnetic repulsion, if cleverly exploited (and it is), keeps the rotor turning inside the magnet. The rotor can be connected to a machine such as the axle of a car to do useful work.
qualitative / quantitative
A qualitative description of a phenomenon uses words and ideas, like "The rock was heavy." A quantitative description involves numbers, usually of a precision appropriate to the thing being described: "The mass of the rock was 22.4 Kg."
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