Magnetism · AP Physics 2 · 6 min
Lenz's law is about resisting a change in magnetic flux, not automatically cancelling the existing magnetic field.
01 The trap
Wrong path
Students point induced fields opposite the given magnetic field in every induction problem.
Why it feels right
The phrase 'opposes the field' is easier to remember than 'opposes the change in flux,' but it is not the law.
02 Correct model
First decide whether magnetic flux through the loop is increasing or decreasing. The induced current creates a field that resists that change.
03 Mini-example
Prompt
A uniform magnetic field into the page through a loop is decreasing. Which way is the induced field?
Common wrong answer
Out of the page because it opposes the original field.
Correct reasoning
Into the page, because the induced field tries to replace the decreasing into-page flux.
Diagnostic cue
If you have not said 'increasing' or 'decreasing,' you have not applied Lenz's law yet.
04 Guided practice
Warm-up isolates the principle. Bridge changes the context. Contest-style requires a complete setup on less familiar geometry.
A magnetic field into the page through a loop is decreasing. What direction is the induced magnetic field through the loop?
The induced field tries to oppose the decrease.
The induced magnetic field is into the page, trying to maintain the decreasing into-page flux.
Trap: Pointing out of the page because the induced field is thought to oppose the original field.
A conducting loop is pulled out of a region with magnetic field into the page. While it leaves, what direction is the induced field inside the loop?
The loop responds to the change in flux through its area.
The induced field is into the page. It tries to oppose the loss of into-page flux as the loop exits the field region.
Trap: Choosing direction from the loop's velocity instead of from the flux change.
If a loop's induced magnetic field points out of the page, is the induced current clockwise or counterclockwise as viewed by you?
Thumb gives field through the loop; curled fingers give current.
The current is counterclockwise. A counterclockwise current produces a magnetic field out of the page through the loop.
Trap: Reversing the field-current right-hand rule.
A uniform magnetic field out of the page through a conducting loop is increasing. Which direction is the induced current as viewed on the page?
The induced field must point into the page.
The induced current is clockwise, producing an into-page field that opposes the increasing out-of-page flux.
Trap: Opposing the original field without first identifying whether the flux is increasing or decreasing.