IGCSE Physics / Electric motors and electromagnetic induction
Electromagnetic induction
Edexcel International GCSE Physics (4PH1), chapter 21: Electric motors and electromagnetic induction, specification point 6.15. Free lesson.
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Electromagnetic induction is the production of a voltage when a conductor moves through a magnetic field or the magnetic field through it changes. It explains how a current can be produced without a battery, and what affects the size and direction of the induced voltage.
When a voltage is induced
A voltage is induced when a conductor cuts across magnetic field lines, or when the magnetic field through a conductor or coil changes. A current flows only if the conductor is connected in a complete circuit. A voltage can be induced even if the conductor is not connected to anything. With no movement and no changing field, no voltage is induced.
What happens when a wire moves through a magnetic field
Connect a wire to a sensitive meter, which can show a very small voltage. When the wire moves across the field lines between the poles of a magnet, the meter flicks. Reversing the movement reverses the direction of the induced voltage. Moving the wire down across the field lines makes the meter flick one way. Moving it back up makes the meter flick the other way. Holding the wire still gives a reading of zero. Moving it along the field lines also gives zero, because it does not cut across them.
What happens when a magnet moves into a coil
A coil connected to a sensitive meter shows induction as the magnetic field through it changes. Pushing the magnet in increases the field through the coil, while pulling it out decreases the field. The change reverses, so the meter flicks in the opposite direction. Push the magnet in: the meter flicks one way as the field through the coil increases. Hold the magnet still: the meter reads zero because the field is not changing. Pull the magnet out: the meter flicks the other way as the field decreases. Moving the coil instead of the magnet works just as well. What matters is their relative…
How to make the induced voltage bigger
A bigger voltage is induced when the magnetic field through the conductor changes more quickly or by a greater amount. In a coil, the voltages induced in each turn add together. Move the magnet or wire faster, so field lines are cut more quickly. Use a stronger magnet, so there are more field lines to cut. Use a coil with more turns, so the voltage induced in each turn adds to the total.
When an induced current flows
Induction produces a voltage, but that does not always mean a current flows. A current needs a complete circuit, so a coil that is not connected in a complete circuit can have an induced voltage without a current. Say that a voltage is induced in the coil. Add that a current flows only if the circuit is complete.