D.4 · IB Physics
Electromagnetic induction
Move a magnet through a coil and watch the flux change and the EMF it induces. Faster motion, more turns or a stronger magnet all raise the EMF; a stationary magnet induces nothing.
Predict first
In Auto mode with the default settings, you raise the magnet speed from 0.50 to 1.00 m s⁻¹. What happens to the peak EMF?
EMF = −N dΦ/dt. The flux changes with position in the same way, but the magnet crosses each position twice as fast, so dΦ/dt and the EMF double.
Check it in the model: Move the Magnet speed (peak) slider from 0.50 to 1.00 m s⁻¹ and compare the Peak EMF readout after a few seconds.
flux Φ induced EMF
Φ = Φ₀ ÷ (1 + (x/R)²)^1.5, Φ₀ = μ₀μ ÷ 2R
R = 2.5 cm, μ₀ = 4π × 10⁻⁷ H m⁻¹
no change in flux, no EMF
Try this
- In Auto mode, double the speed from 0.25 to 0.50 m s⁻¹. How does the peak EMF change, and how does the time axis of the graphs change?
- Switch to Drag mode. Hold the magnet still inside the coil, then move it out slowly and then quickly. Compare the EMF and the meter.
- Find the position where the flux is largest. Is the EMF zero or at its maximum there? Match this to the gradient of the Φ graph.
Exam tip: EMF = −N dΦ/dt is the gradient of the flux graph, so the EMF is zero where Φ is at a maximum and largest where Φ changes fastest; the minus sign (Lenz) means the induced current opposes the change.