Oppose the change, not always the field
You will be able to: Use changing external flux to find induced field and current direction.
Which way does induced current circulate?
An outward magnetic field through a loop is getting stronger. The induced current creates an inward field to oppose that increase. If the outward field instead weakens, induced current creates an outward field to oppose the decrease.
A useful starting point: Changing flux produces an emf →
Words and symbols before equations
- External flux
- Flux due to the specified source field before considering the loop’s response.
- Induced field
- Field produced by the induced current in a closed loop.
- Clockwise/counterclockwise
- Directions as viewed from the front of the page; always state the viewing side.
- Lenz’s law
- The induced response opposes the change in magnetic flux.
What this picture assumes
Fixed closed resistive loop viewed from the front; outward flux positive. Induced-current direction from the external flux change. No self-inductance or persistent-current effects.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- External B outward, magnitude growing. Induced B inward; current clockwise viewed from front.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Use three steps: determine the external flux change, choose an induced field that opposes that change, and use the loop right-hand rule to find current circulation. Counterclockwise viewed from the front creates outward field; clockwise creates inward field.
An increasing outward flux gives inward induced field and clockwise current. A decreasing outward flux gives outward induced field and counterclockwise current. Thus the induced field can point in the same direction as the external field when that external flux is decreasing.
This response is consistent with energy conservation: the induced effect resists the change driving it. For constant flux there is no emf from Faraday induction in our resistive model. We neglect self-inductance and transients beyond the specified flux change.
| External outward flux | Induced B | Induced current |
|---|---|---|
| Increasing | Inward | Clockwise |
| Decreasing | Outward | Counterclockwise |
| Steady | No induced response in this model | Zero in resistive model |
A worked example, step by step
External flux into the page is increasing in magnitude through a closed resistive loop. Find induced field and current.
- Choose outward as positive, so the external flux is becoming more negative.
- The change is inward.
- The induced field must point outward to oppose that increase in inward flux.
- Counterclockwise current as viewed from the front produces outward field.
“Opposes the magnetic field” is incomplete. The induced field opposes the change in flux, which can require reinforcing a decreasing field.
Outward field weakens. Must induced B point inward?
Compare with an explanation
No. It points outward to oppose the loss of outward flux.
Predict. Change one thing. Explain.
Choose outward or inward external field, then growing, steady or shrinking magnitude. Explain all three reasoning steps before reading the induced direction.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
External B outward, magnitude growing. Induced B inward; current clockwise viewed from front.
Fixed closed resistive loop viewed from the front; outward flux positive. Induced-current direction from the external flux change. No self-inductance or persistent-current effects.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant field, force, flux or energy relationship to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionAn inward external field through a fixed loop weakens. (a) Describe the signed change with outward positive. (b) State induced field direction. (c) State front-view current direction. (d) Explain why this can reinforce the existing field.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Negative flux becomes less negative, so its change is outward/positive.
- 1 point: Induced B points inward.
- 1 point: Clockwise viewed from the front.
- 1 point: It opposes the loss of inward flux, consistent with Lenz’s law.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What does the induced response oppose?
The change in external magnetic flux.
RECALL 2Why state the viewing side?
Clockwise and counterclockwise reverse when viewed from the other side.
RECALL 3What if flux is steady in this resistive model?
There is no Faraday-induced emf or current.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Oppose the change, not always the field
- First identify dΦ_external/dt, including its sign.
- Outward induced field ↔ counterclockwise front-view current.
- Inward induced field ↔ clockwise front-view current.
Remember: “Opposes the magnetic field” is incomplete. The induced field opposes the change in flux, which can require reinforcing a decreasing field.
Conditions: Fixed closed resistive loop viewed from the front; outward flux positive. Induced-current direction from the external flux change. No self-inductance or persistent-current effects.
Refresh Kid · AP Physics 2 Unit 4 (official Unit 12) · Objectives 12.4.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 12.4, objectives 12.4.A. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. Refresh Kid calls this the fourth AP Physics 2 unit; College Board numbers it Unit 12; the first unit in this course is official Unit 9. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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