Electromagnetic induction (A-level only): Simple experimental phenomena.

The idea

Electromagnetic induction occurs when magnetic flux linkage through a circuit changes. Simple experimental phenomena include moving a bar magnet into or out of a coil, moving a second coil carrying direct current, rotating a coil in a magnetic field, and switching a current on or off in a nearby coil. In each case, a changing flux linkage induces an e.m.f. and, in a closed circuit, a current. The magnitude depends on the rate of change of flux linkage and the number of turns; the direction opposes the change, as described by Lenz's law. These experiments let students observe induced voltage on a galvanometer or oscilloscope and relate observations to Faraday's law.

What earns the marks

• Describe at least two simple experimental arrangements that demonstrate electromagnetic induction, such as a magnet moving relative to a coil or one coil connected to a changing current placed near another coil.
• State the observation that an induced e.m.f. or current occurs only while flux linkage is changing, and that steady flux linkage produces no induced effect.
• Relate the observed induced e.m.f. to the rate of change of flux linkage and the number of turns, using Faraday's law qualitatively or quantitatively.
• Explain the direction of the induced current using Lenz's law, noting that the induced effect opposes the change producing it, for example the coil face becomes a like pole as a magnet approaches.
• Identify control variables and measurement methods in these experiments, such as using a sensitive galvanometer, data logger or oscilloscope, and changing one factor at a time.

Common mistakes and how to put them right

• Believing that a steady magnetic field through a stationary coil induces a current.

  Put it right by stating that flux linkage must change, so movement, rotation or a changing current is required.

• Confusing the direction of induced current with the direction of the applied field.

  Put it right by applying Lenz's law and identifying the pole induced on the coil face nearest the approaching or receding magnet.

• Assuming the induced e.m.f. depends only on the strength of the magnet.

  Put it right by explaining that the rate of change of flux linkage and the number of turns also determine the magnitude.

Exam tips

• Describe the experimental set-up, the change made, and the resulting observation in a clear sequence, using terms such as flux linkage, induced e.m.f. and galvanometer deflection.
• When explaining direction, name the law (Lenz's law) and state the opposing change explicitly rather than simply saying the current goes the other way.
• Use a sketch of the field lines or coil face to support your explanation, and keep the distinction between flux, flux linkage and induced e.m.f. clear.