Edexcel GCSE Physics Topic 13: Electromagnetic Induction Exam Questions

About Edexcel GCSE Physics Topic 13: Electromagnetic Induction

Topic 13 on Paper 2 follows on from the motor effect in Topic 12. It explains how a potential difference is induced across a conductor when it moves relative to a magnetic field or when the field through it changes, and what affects the size and direction of the induced p.d. You apply this to alternators producing a.c., dynamos producing d.c., microphones and loudspeakers, and transformers. The topic finishes with why the National Grid transmits electricity at high voltage. Examiners frequently ask for transformer calculations using the turns ratio equation and the power equation, and for explanations of why step-up transformers reduce energy losses in cables.

What you need to know

• Inducing a p.d. A p.d. is induced when a wire cuts magnetic field lines or the magnetic field through a coil changes. If the circuit is complete, a current flows.
• Size of induced p.d. Increased by moving the magnet or wire faster, using a stronger magnet or using more turns on the coil. Reversing the motion or the magnet reverses the direction.
• Opposing the change The induced current produces its own magnetic field that opposes the change that caused it.
• Generators An alternator rotates a coil in a magnetic field with slip rings to produce a.c.; a dynamo uses a split-ring commutator to produce d.c.
• Microphones and loudspeakers A microphone uses induction: sound vibrates a diaphragm and coil in a magnetic field, inducing a varying p.d. A loudspeaker uses the motor effect to turn a varying current into vibrations.
• Transformers An alternating current in the primary coil produces a changing magnetic field in the iron core, inducing an alternating p.d. in the secondary coil. Vp / Vs = Np / Ns, and for a 100% efficient transformer Vp x Ip = Vs x Is.
• National Grid Step-up transformers increase the p.d. and reduce the current for transmission, which reduces energy lost by heating cables (P = I^2 x R). Step-down transformers reduce the p.d. for safe use in homes.

Common mistakes in Topic 13: Electromagnetic Induction

• Saying transformers work with direct current. They need alternating current to produce a changing magnetic field.
• Saying high voltage reduces losses directly. It is the lower current that reduces heating in the cables.
• Saying the iron core conducts current from the primary to the secondary coil. The core carries the changing magnetic field; there is no electrical connection.
• Mixing up the motor effect and induction. Loudspeakers and motors use the motor effect; microphones and generators use induction.
• Forgetting a p.d. is only induced while there is relative movement or a changing field. A stationary magnet in a coil induces nothing.

Try three Topic 13: Electromagnetic Induction exam-style questions