Faraday's Law: EMF from a Changing Field

1 · Predict

A loop of wire sits in a magnetic field whose strength oscillates in time (the loop's area stays fixed). Does an EMF appear in the loop even though the loop itself isn't moving?

• Yes — Faraday's law says a changing flux induces an EMF, even in a stationary loop, as long as the field itself is changing.
• No — EMF requires physical motion of the loop or field source.
• EMF only appears if the loop is moving through the field.

2 · Set Up

1. Open the changing-flux-b preset and press Reset. The loop's area is fixed at 0.02 m²; the field oscillates as B(t) = 0.6 T × (1 + 0.5 sin(2π × 0.4Hz × t)).
2. Enable the induced-EMF readout.
3. Read the induced EMF at each listed time.

3 · Collect Data

Plot induced EMF (y-axis) against time t (x-axis) for your three readings, and sketch how it oscillates over one full period (2.5 s).

4 · Analyze

1. For one trial, compute EMF = −A·dB/dt = −A·B₀·0.5·ω·cos(ωt) using A = 0.02 m², B₀ = 0.6 T, ω = 2π×0.4 rad/s. Compare to the table.
2. Explain why the induced EMF is proportional to how FAST the field is changing (dB/dt), not to the field's instantaneous value — the EMF can be large even when B itself is near its average value, if B is changing quickly there.

5 · Extend

1. This is the basic principle behind an AC generator: rather than changing B directly, a generator rotates a loop through a fixed field, which changes the flux just as effectively. Explain why generator design usually rotates the coil instead of oscillating the magnet.
2. The negative sign in EMF = −dΦ/dt (Lenz's law) means the induced current opposes the change that caused it. Explain what 'opposing the change' means physically for a loop in an increasing field versus a decreasing field.

The Physics Behind This Experiment

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