Class 12 Electromagnetic Induction: Flux, Units and Lenz's Law Practice
A large magnetic field does not by itself imply an induced EMF. What matters is changing magnetic flux linkage. Separate the magnitude calculation from the direction explanation, then check that both describe the same changing situation.
Curriculum checked 2 October 2026 · original practice for selected skills.
Before you start
You will need: Magnetic field direction, area units and rate of change.
Attempt each task before opening a hint. If you need help, reveal the first hint, then the setup, then compare your written steps with the solution. Finish with the retests without referring back.
Three ideas to keep in view
Flux through a flat loop is BA cos θ, where θ is measured from the area normal, not the plane of the loop.
For N identical turns, average EMF magnitude is N times the magnitude of flux change per turn divided by elapsed time. Convert cm² and milliseconds before substituting.
Lenz's law opposes the change in flux. A decreasing inward field calls for an induced inward field, not an outward one.
A coil viewed face-on. Crosses represent an external magnetic field into the page. Decide whether that field is increasing or decreasing before choosing an induced current direction.
Try the worked example first
P12-EMI-W1Area and time unitsFocused practice
A closed 40-turn coil has area 25 cm² per turn. A uniform field normal to it increases into the page from 0.08 T to 0.20 T in 20 ms. Find average EMF magnitude and induced conventional current direction as viewed on this page.
Show your working
Need a starting hint?
Convert both area and time. The induced field opposes the increase.
Still stuck? Reveal the setup
A=0.0025 m² and Δt=0.020 s.
Show worked solution+
1
Calculate magnitude
Use N A ΔB / Δt.
∣E∣=40(0.0025)(0.12)/0.020=0.6 V
2
Choose direction
The induced field is out of the page, requiring counterclockwise conventional current.
Answer: 0.6 V; counterclockwise current
Before moving on: can you explain why your method works, as well as give the answer?
Your turn: three different checks
Write a method as well as an answer. The tasks change the reasoning, not just the numbers.
P12-EMI-Q1A steady fluxFocused practice
A stationary loop has fixed area and orientation in a steady uniform field. What induced EMF is produced?
Show your working
Need a starting hint?
Ask whether flux changes with time.
Still stuck? Reveal the setup
ΔΦ=0.
Show worked solution+
1
Identify constants
B, A and the angle are all fixed.
2
Apply Faraday's law
Zero rate of change gives zero induced EMF.
Answer: 0 V
Before moving on: can you explain why your method works, as well as give the answer?
P12-EMI-Q2Angle conventionFocused practice
A loop of area 0.020 m² is in a 0.30 T field. The field makes 60° with the loop's normal. Find magnetic flux.
Show your working
Need a starting hint?
Use the angle to the normal directly.
Still stuck? Reveal the setup
cos 60°=1/2.
Show worked solution+
1
Set up
Flux is BA cos θ.
2
Calculate
0.30 × 0.020 × 0.5=0.003 Wb.
Answer: 3.0 × 10⁻³ Wb
Before moving on: can you explain why your method works, as well as give the answer?
P12-EMI-Q3Turn countFocused practice
Flux through each turn of a 100-turn coil changes by 0.004 Wb in 0.20 s. Find average EMF magnitude.
Show your working
Need a starting hint?
The quoted flux is per turn.
Still stuck? Reveal the setup
Multiply its change by N before dividing by time.
Show worked solution+
1
Flux linkage change
100 × 0.004=0.4 Wb-turn.
2
Rate
0.4/0.20=2 V.
Answer: 2 V
Before moving on: can you explain why your method works, as well as give the answer?
Catch a likely mistake
A tempting claim: “The induced field always points opposite the external field.”
It opposes a change. When external flux decreases, the induced field supports its original direction.
Close the examples and try again
These changed questions test whether you can reconstruct the method. If you use a hint, record where you got stuck and retry later. Completing this small set does not establish full chapter mastery.
P12-EMI-R1Decreasing fieldIndependent retest
A closed 20-turn coil of area 50 cm² has a normal inward field decrease from 0.14 T to 0.10 T in 10 ms. Find average EMF magnitude and current direction viewed face-on.
Show your working
Need a starting hint?
The induced field tries to maintain the inward flux.
Still stuck? Reveal the setup
Use A=0.005 m², |ΔB|=0.04 T and Δt=0.010 s.
Show worked solution+
1
Magnitude
20 × 0.005 × 0.04 / 0.010=0.4 V.
2
Direction
An induced inward field requires clockwise conventional current.
Answer: 0.4 V; clockwise
Could you solve this without the earlier example? If not, revisit the first line where you got stuck.
P12-EMI-R2Changed linkageIndependent retest
A 50-turn coil's flux per turn changes by 0.006 Wb in 0.15 s. Find average EMF magnitude.
Show your working
Need a starting hint?
Multiply by the number of turns.
Still stuck? Reveal the setup
50 × 0.006 / 0.15.
Show worked solution+
1
Linkage change
The change is 0.30 Wb-turn.
2
Divide by time
0.30/0.15=2 V.
Answer: 2 V
Could you solve this without the earlier example? If not, revisit the first line where you got stuck.
Choose the next useful step
If a retest exposed the same error, rewrite the first incorrect step and explain its correction aloud. If both were independent, return to a mixed exercise or a missed paper question. A parent can ask what changed in the method rather than only asking for the answer.
Does a calculated EMF always mean a current flows?+
No. Current requires a closed conducting path. An open circuit can have an induced EMF without a sustained conduction current around the loop.
Can I print the questions and worked solutions separately?+
Yes. Use the two print buttons above. The question sheet leaves working space and hides solutions; the worked-solutions option includes the solution steps. Your browser can save either view as a PDF.
PERSONAL SUPPORT
Does the same mistake keep returning?
Bring one attempted question and the step that caused difficulty. Discuss suitable TutorMax support for the learner's current level.
