Determine induced-current directions and connect flux change to alternating output.
Method and worked examples
Lenz’s law states that an induced current produces a magnetic field opposing the change in flux that caused it. First identify the external flux direction and whether its magnitude increases or decreases. Choose the induced field that resists that change, then use the loop right-hand rule to find current direction. Opposition to change is different from always opposing the external field.
If a north pole approaches a coil, the near face becomes north and repels the approach; if it recedes, the near face becomes south and attracts it. Mechanical work is needed to maintain the motion, consistent with energy conservation. Rotating coils change their flux periodically and generate alternating emf. Eddy currents in bulk conductors likewise oppose motion-related flux changes, converting mechanical energy into internal energy during electromagnetic braking.
Example 1
A field into the page increases through a loop. Determine induced current as viewed from the front.
The loop produces an out-of-page field to oppose the increase. The right-hand rule gives anticlockwise current.
Example 2
A coil rotates uniformly in a uniform field. Why does the emf alternate?
Flux varies as BA cosωt. Its rate of change reverses every half-turn, so the induced emf reverses direction.
Watch out:It opposes the change: if external flux decreases, the induced field reinforces the original direction.
11. Lenz’s law, eddy currents and generators | Year 12 Physics | Ascent Arcade