PHY051 · Magnetism · Class 12 + College
Magnetic flux
Objective: To understand magnetic flux through a loop and how it changes as the loop turns.
🧰 Apparatus
- Wire loop on an axle
- Uniform field (Helmholtz coils or magnet pair)
- Flux meter or search coil with data logger
📐 Formulas and SI units
Φ = BA cos θ (Wb = T m²)
💡 Theory
Magnetic flux measures how much field passes through an area: Φ = BA cos θ, where θ is the angle between the field and the normal to the loop. It is largest when the loop faces the field and zero when the loop is edge-on. A turning loop has changing flux, which is how generators work.
🪜 Procedure
- Place the loop in the uniform field facing it (θ = 0).
- Measure the flux.
- Rotate the loop in 30° steps and record the flux.
- Plot Φ against θ: a cosine curve.
📋 Observations Simulated values
✅ Result and conclusion
0.2 T through 0.05 m² at 60° gives 5 mWb; the flux is zero at 90°.
🔬 In a real lab
Real fields are only uniform near the centre of the coils; search coils measure the change in flux, not flux itself.
⚠️ Safety
Strong magnets pinch fingers and wipe cards; keep them apart and away from devices.
❓ Quick check
1. Flux is zero when the loop is…
2. Unit of magnetic flux?
3. Turning a loop in a field changes the flux, which induces…
🏋️ Practice
EasyB = 0.5 T, A = 0.1 m², θ = 0. Φ?
Answer: 0.05 Wb
MediumSame at 60°?
Answer: 0.025 Wb
ChallengeΦ for θ = 180°?
Answer: −BA (reversed)