A7LAM CSCA ACADEMYCSCA Academy / Physics / Electromagnetism
CSCA Physics · Lesson 10 of 16Electromagnetic induction
Induction électromagnétique · الحث الكهرومغناطيسي
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A changing magnetic flux through a circuit creates a voltage. This is how generators and transformers work.
Key ideas
- Flux Φ = BS (weber, Wb) when B is perpendicular to the area S.
- Faraday's law: E = NΔΦ/Δt; only a changing flux induces an EMF.
- Lenz's law: the induced current opposes the change that caused it.
- A rod of length L moving at v across B: E = BLv.
Formulas to know
- E = NΔΦ / Δt
- E = BLv
- Transformer: U₁/U₂ = n₁/n₂
Worked example
A 100-turn coil: the flux changes by 0.02 Wb in 0.1 s. Induced EMF?
- Formula: E = NΔΦ/Δt
- Replace: E = 100 × 0.02 / 0.1
- Compute: E = 2 / 0.1
Answer: E = 20 V
Practice questions
1. A 0.5 m rod moves at 4 m/s across B = 0.2 T. EMF?
0.4 V ✓ · 2 V · 10 V · 0.1 V
E = BLv = 0.2 × 0.5 × 4 = 0.4 V.
2. A transformer has 100 turns (input) and 500 turns (output). 220 V in gives:
44 V · 1,100 V ✓ · 220 V · 500 V
220 × 500/100 = 1,100 V.
3. A magnet rests inside a coil without moving. Induced current?
Maximum · Zero ✓ · Constant, not zero · It reverses
No change in flux, no EMF.