Connect a coil to nothing but a galvanometer, push a magnet into it, and the needle moves. When the magnetic field through a coil changes, a current appears in the coil: this is electromagnetic induction.
When the magnet stops, the needle returns to zero, and pulling the magnet out tips it the other way. Faster motion and more turns of wire both give a larger current.
In the diagram above, a + speed brings the magnet closer and a − speed takes it away, and each time the pole that forms at the end of the coil switches so that it opposes the motion.
The generator below uses the same idea, spinning a coil to make alternating current that keeps changing direction.
A Moving Magnet Makes a Current
🧲 What Faraday Saw
①While a magnet is pushed into a coil, the galvanometer needle swings one way
②If the magnet stops inside the coil, the needle returns to zero
③When the magnet is pulled out, the needle swings the other way
④Moving the coil instead of the magnet produces a current just the same
Coil and Magnet Experiment
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Galvanometer needle and induced pole of the coil as a magnet is pushed in and pulled out
30%
In 5
💡 What Makes the Induced Current Larger
①Current flows only while the magnetic field through the coil is changing
②The faster the magnet moves, the larger the current
③The stronger the magnet, the larger the current
④The more turns the coil has, the larger the current
How a Generator Works
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Generator whose coil turns in a magnetic field, tracing a sine curve of induced voltage
90°
Electromagnetic induction
Magnetic field through the coil changes → induced voltage → induced current
The faster the field changes, the larger the induced voltage. With no change, even a very strong magnet produces no current
💡 Why a Generator Makes Alternating Current
①As the coil turns in the field, the number of field lines through it keeps rising and falling
②So the induced voltage follows a sine curve and reverses every half turn: alternating current
③The change is fastest, and the voltage largest, when the coil's face is parallel to the field (90° and 270°)
④Household electricity reverses 50 or 60 times a second, depending on the country (60 Hz in Korea and the US, 50 Hz in much of Europe)
Lenz's Law
Lenz's law
The induced current flows in the direction that opposes the change in the magnetic field through the coil
When an N pole approaches, the near end of the coil becomes N and pushes it away; when the N pole leaves, it becomes S and pulls it back
Summary
Electromagnetic Induction in One Line
Changing field → induced voltage → induced current
How fast the field changes sets the size of the current; opposing the change sets its direction
🎯 Common Test Points
①An induced current flows only while a magnet or coil moves and the field changes
②Faster motion, a stronger magnet and more turns give a larger current
③Pushing in and pulling out give currents in opposite directions
④Lenz's law: the induced current opposes the change in the field
⑤A generator turns motion into electrical energy; an electric motor turns electrical energy into motion
Examples and Unit Review
Example 1
If a magnet is left still inside a coil, does an induced current flow? And how should the magnet move to make the current larger?
1
An induced current flows only while the number of field lines through the coil is changing.
2
A still magnet causes no change, so no current flows. Pushing it in or pulling it out faster makes the change quicker and the current larger.
▸ No current while it is still; moving the magnet faster makes the current larger
In the diagram above, with the speed at 0 the needle points to zero even when the magnet sits right next to the coil.
Example 2
What kind of current does a generator produce when its coil turns in a magnetic field, and why?
1
As the coil turns, the number of field lines through it keeps rising and falling, so a current is induced.
2
Rising and falling numbers of field lines push current in opposite directions, so the result reverses every half turn: alternating current.
▸ Alternating current, because the number of field lines through the turning coil keeps rising and falling
Turning the angle in the lower diagram from 0° to 360°, you can watch the voltage curve rise and fall once each way.
Unit-test style
Which statement about electromagnetic induction is correct?
①An induced current flows even when the magnet is still inside the coil
②An induced current flows when the number of field lines through the coil changes
③The slower you move the magnet, the larger the induced current
④A generator is a device that turns electrical energy into a magnet
⑤The induced current flows in a direction that aids the change in the magnetic field
▸ ② An induced current flows when the number of field lines through the coil changes
1
Electromagnetic induction is the phenomenon where an induced current flows when the number of field lines through the coil changes.
2
There is no current when the magnet is still (①), the faster it moves the larger the current (③), a generator turns motion into electricity (④), and the induced current flows so as to oppose the change in the magnetic field (⑤, Lenz law).