Edexcel GCSE Physics Topic 12: Magnetism and the Motor Effect Exam Questions

About Edexcel GCSE Physics Topic 12: Magnetism and the Motor Effect

Topic 12 on Paper 2 starts with permanent and induced magnets and how to plot magnetic field patterns. It then covers the magnetic field around a current-carrying wire and a solenoid, and how electromagnets are made stronger. The motor effect is the key idea: a current-carrying conductor in a magnetic field experiences a force, whose direction is given by Fleming's left-hand rule and whose size is calculated with F = B x I x l on the higher tier. This explains how a d.c. motor rotates. The topic leads directly into Topic 13 on electromagnetic induction. Examiners test field diagrams, Fleming's rule and the force equation.

What you need to know

• Magnets Like poles repel, unlike poles attract. Permanent magnets keep their magnetism; induced magnets are magnetic only when in a magnetic field. Iron, steel, nickel and cobalt are magnetic materials.
• Magnetic fields Field lines go from north to south and are closest where the field is strongest. A uniform field between flat poles has parallel, equally spaced lines. A compass points along field lines, showing the Earth has a magnetic field.
• Field around a wire A current-carrying wire produces circular field lines around it. The field is stronger with a larger current and closer to the wire; its direction is found with the right-hand grip rule.
• Solenoids and electromagnets The field inside a solenoid is strong and uniform. Adding an iron core, more turns or a larger current makes the electromagnet stronger.
• Motor effect A current-carrying conductor at right angles to a magnetic field experiences a force. Fleming's left-hand rule: first finger field, second finger current, thumb motion.
• Force equation Higher tier: F = B x I x l, where B is magnetic flux density in tesla (T), I is current and l is the length of conductor in the field.
• Electric motors Forces on opposite sides of a coil act in opposite directions, causing rotation. A split-ring commutator reverses the current every half turn so the coil keeps turning the same way.

Common mistakes in Topic 12: Magnetism and the Motor Effect

• Drawing field lines going from south to north outside a magnet. Outside a magnet, lines go from north to south.
• Mixing up the fingers in Fleming's left-hand rule. First finger is Field, seCond finger is Current, thuMb is Motion.
• Saying copper or aluminium are magnetic. Only iron, steel, nickel and cobalt (and their alloys) are magnetic materials.
• Forgetting that there is no force when the wire is parallel to the field. The force is greatest when the wire is at right angles to the field.
• Saying the commutator stops the motor. It reverses the current each half turn to keep the motor rotating in one direction.

Try three Topic 12: Magnetism and the Motor Effect exam-style questions