Define magnetic flux and state its SI unit. State Faraday's laws of electromagnetic induction and write the induced emf in a coil of N turns.

1. 1 · Define magnetic flux. Φ = BA cos θ, where θ is the angle between B and the normal to the area. Scalar; SI unit weber (1 Wb = 1 T m²).
2. 2 · State the first law. Whenever the flux linked with a circuit changes, an emf is induced; it lasts only while the change goes on.
3. 3 · State the second law. The induced emf equals the rate of change of flux linkage: ε = −N dΦ/dt.
4. 4 · Explain the minus sign. It shows that the induced emf opposes the change in flux (Lenz's law).
5. 5 · Real life. A swiped magnetic stripe card induces emfs in the coil of the reader.
6. 6 · Close in one line. So, the faster the flux changes, the larger the induced emf.

Scoring tip: Write the formula with the minus sign and say what N, Φ and t stand for. Give the unit weber with its equivalent T m².