Explain induction opposition through energy transfer
Connect mechanical input, electrical transfer and magnetic resistance to movement without creating energy.
Section 4.5.1 · Electromagnetic induction
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Trace energy transfers and conserve energy.
- Distinguish open-circuit voltage from load current.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the unit of transferred energy?
What you will learn
- Connect mechanical input, electrical transfer and magnetic resistance to movement without creating energy.
- Explain why generator output requires an accounted input and why closed-receiver opposition is consistent with conservation.
Movement supplies electrical output
In a generator arrangement with a closed resistive receiver, induced current creates magnetic effects opposing the imposed movement. An external agent must do work to maintain that movement. The mechanical transfer supplies electrical energy that can be dissipated in the receiver or transferred to other loads.
If the induced effect instead continually reinforced the initiating movement while also delivering electrical output without another input, the model would create energy. Opposition to change is consistent with conservation: it does not mean induction destroys energy or that a generator receives electrical energy directly from a permanent magnet’s existence.
Keep the system and path explicit
Under comparable ideal conditions, closing a resistive receiver permits current and a stronger opposing electromagnetic effect than an open receiver. An open receiver can still have induced e.m.f., but no sustained external load current. Real mechanical drag and other losses can remain in both states.
A supplied energy budget must name its interval and inputs. If 12 J of mechanical work gives 9 J delivered to a load, the other 3 J may be specified losses or internal-energy changes. A diagram does not establish a universal efficiency or the absence of every other energy transfer.
Pause and explain
Could induction continually reinforce its own movement and deliver electrical output with no other energy input?
Worked example
A supplied generator budget has 12 J mechanical input and 9 J delivered to a load. Find the remaining accounted energy.
Show the worked solution
- Use the same stated interval and system.
- Remaining energy is 12 − 9 = 3 J.
- Name it according to the supplied losses or storage account.
Answer 3 J remains as the supplied other transfers or internal-energy change.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the transfer chain for maintained movement driving a closed resistive induction receiver.
Give me a hint
Connect imposed movement, induced current and its load.
Compare my reasoning
- An external agent does mechanical work.
- Induction drives current in the closed receiver.
- Electrical transfer increases internal energy in the resistance.
Mechanical work supplies electrical transfer and receiver heating.
Look for these in your work
- Name the input.
- Keep the complete receiver path.
For a supplied interval, mechanical input is 12 J and load transfer 9 J. Find the remaining accounted energy.
Give me a hint
Subtract within the same interval and system.
Compare my reasoning
- Total input is 12 J.
- Delivered load energy is 9 J.
- The remaining account is 3 J.
3 J for the supplied other transfers or storage change.
Look for these in your work
- Use joules consistently.
- Do not infer an unspecified loss mechanism.
Compare open and closed resistive receivers in a supplied moving-magnet model, retaining real drag limits.
Give me a hint
Separate induced voltage from load current and its magnetic reaction.
Compare my reasoning
- Both can have induced e.m.f. during change.
- The closed receiver permits sustained load current and its opposing magnetic effect.
- The open case lacks that load-current effect, but real mechanical drag may remain.
Closing permits load current and its opposing effect; open does not imply no voltage or zero real drag.
Look for these in your work
- Compare the electrical path.
- Retain the real-loss limitation.
Common mistakes
- Opposition does not destroy energy.
- A magnet’s presence is not a continuous free energy source.
- An open receiver can have e.m.f. with no sustained load current.
What can you explain now?
Why must an agent do work to maintain generator movement under a resistive load?
Compare with the explanation
The induced effect opposes the movement.
Mechanical input replaces energy transferred through the induced electrical circuit.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain why generator output requires an accounted input and why closed-receiver opposition is consistent with conservation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.