Electric Generator

Faraday's Law of electromagnetic induction states that a current is only induced when the magnetic flux through the conductor is changing. A stationary magnet inside a stationary coil produces no change in flux.

Since both the magnet and the coil are stationary, the magnetic field through the coil is constant. There is no $\Delta \Phi$, so by Faraday's Law $\mathcal{E}=-N\frac{\Delta \Phi }{\Delta t}=0$.

Because the flux is not changing, no EMF is induced and therefore no current flows. The correct answer is that no current is induced.

The question asks what happens to the current when a stationary magnet sits inside a stationary coil — a test of the conditions required for electromagnetic induction.

'A constant DC current flows because the magnet creates a steady field' is wrong. A steady (unchanging) magnetic field does not induce a current; only a changing field does.

'An alternating current flows because the magnet has two poles' is incorrect. The presence of two poles does not create AC on its own; the magnet must be moving relative to the coil to induce any current.

'A current flows briefly then stops due to resistance' is incorrect. Resistance affects the size of a current once induced, but if there is no changing flux, no current is induced in the first place.

'No current is induced because the magnetic flux is not changing' is correct. This directly reflects the core condition for electromagnetic induction: flux must change.

The question asks specifically about an AC generator and which component maintains the electrical connection between the rotating coil and the external circuit.

In an AC generator, slip rings are continuous metal rings attached to the ends of the coil. Brushes are stationary contacts that press against the slip rings, allowing current to flow from the rotating coil to the external circuit without tangling the wires.

Slip rings and brushes are the correct components for an AC generator. A split-ring commutator serves a similar connection function but is used in DC generators, not AC generators.

The question asks which component in an AC generator connects the rotating coil to the external circuit.

'Split-ring commutator' is used in a DC generator, not an AC generator. It switches connections every half-turn to keep current flowing in one direction.

'Iron core armature' refers to the coil structure and iron core that concentrates the magnetic field. It is not the component that makes the rotating electrical connection.

'Permanent magnet poles' provide the magnetic field that the coil rotates within. They are stationary and do not connect the coil to the external circuit.

'Slip rings and brushes' is the correct answer. Slip rings rotate with the coil while brushes remain stationary, maintaining a continuous electrical connection.