OCR GCSE Physics A P4.2 Uses of Magnetism Exam Questions

About OCR GCSE Physics A P4.2 Uses of Magnetism

P4.2 completes the P4 Magnetism and magnetic fields chapter of OCR Gateway Physics A. It covers the motor effect, using Fleming's left-hand rule and the equation F = BIl, and explains how electric motors work. Separate physics content goes further with loudspeakers, the generator effect (electromagnetic induction), alternators and dynamos, microphones and transformers. You need to explain the factors that affect the induced potential difference and use the transformer equations. It builds on P4.1 and links to P8.2 Powering Earth, where transformers are used in the National Grid. Examiners frequently set F = BIl calculations, motor explanations and transformer questions.

What you need to know

• Motor effect A current-carrying wire in a magnetic field experiences a force. The force is greatest when the wire is at right angles to the field and zero when parallel.
• Fleming's left-hand rule First finger: field (north to south). Second finger: current. Thumb: force or motion.
• Force equation Force = magnetic flux density x current x length (F = BIl), with flux density in tesla (T).
• Electric motors Forces on opposite sides of a current-carrying coil act in opposite directions, producing rotation. A split-ring commutator reverses the current every half turn.
• Loudspeakers A varying current in a coil in a magnetic field produces a varying force that makes the cone vibrate and produce sound.
• Generator effect A p.d. is induced when a conductor moves relative to a magnetic field or the field changes. Alternators use slip rings to produce a.c.; dynamos use a commutator to produce d.c. Microphones use this effect to turn sound into an electrical signal.
• Transformers An alternating current in the primary coil creates a changing field in the iron core, inducing a p.d. in the secondary. Vp / Vs = Np / Ns and, for 100% efficiency, Vp x Ip = Vs x Is.

Common mistakes in P4.2 Uses of Magnetism

• Using the right hand for the motor effect. Fleming's left-hand rule is used for the force on a current-carrying conductor.
• Saying the commutator produces the force. The force comes from the motor effect; the commutator keeps it turning the coil the same way.
• Mixing up loudspeakers (motor effect) and microphones (generator effect).
• Saying transformers work with d.c. They need a changing current to create a changing magnetic field.
• Forgetting to convert length from cm to m in F = BIl.

Try three P4.2 Uses of Magnetism exam-style questions