The generator effect
- Electromagnetic induction is the production of a voltage (and hence a current, if there is a complete circuit) in a conductor whenever the conductor experiences a changing magnetic field
- This is sometimes called the generator effect. It is the reverse of the motor effect:
- For the motor effect, a current must already be running in the conductor; the magnetic field then pushes the wire sideways
- For the generator effect, no current is needed to start with; the voltage appears spontaneously when the conductor cuts through magnetic field lines
Two ways to get a changing field
There are two equally good ways to set up the changing field that produces induction. The physics is the same in both:
- Move a conductor through a stationary magnetic field. A wire pushed between the poles of a magnet sweeps through the field lines and a voltage is induced along its length
- Move a magnet relative to a stationary conductor. A bar magnet pushed in and out of a coil of wire makes the magnetic field inside the coil change, inducing a voltage across the ends of the coil
Factors that change the size of the induced p.d.
- The induced voltage gets bigger when:
- the speed of relative motion between magnet and coil is faster (faster sweep through the field lines means more field lines cut per second)
- the number of turns on the coil is larger (every turn picks up its own contribution; they add together)
- the strength of the magnet is greater (a stronger field means more field lines for the same volume, so more get cut per second)
- the cross-sectional area of the coil is larger (more wire is moving through the field)
Factors that change the direction of the induced p.d.
- The induced voltage reverses direction when:
- the direction of motion is reversed (pulling out instead of pushing in)
- the magnet is flipped (N pole leading instead of S)
- Both of these "flip" the way field lines are being cut, and so flip the polarity of the induced p.d. The size of the p.d. is not affected; only its sign
Exam-language reminders
- Use "add more turns to the coil", not "add more coils". A coil is the whole component, and the loops in it are turns
- Use "a stronger magnet", not "a bigger magnet". Magnet size and magnet strength are not the same thing
- Always say the conductor must cut magnetic field lines to induce a p.d. If the wire is moved parallel to the field, no lines are cut and no voltage is induced