AQA GCSE Physics 4.7.2 The Motor Effect Exam Questions

About AQA GCSE Physics 4.7.2 The Motor Effect

This AQA GCSE Physics section covers the magnetic effect of a current and the force on a current-carrying conductor in a magnetic field. All students learn about the field around a wire and a solenoid and how an electromagnet works. Higher tier only content covers the motor effect, Fleming's left-hand rule, F = B I l and how a rotating electric motor works. Loudspeakers and headphones are physics only and Higher tier only. It is examined on Paper 2. Examiners test field diagrams, applying the left-hand rule and F = B I l calculations.

What you need to know

• Magnetic field of a wire When a current flows through a conducting wire a magnetic field is produced around the wire, in concentric circles. The strength of the field depends on the current and the distance from the wire.
• Solenoids and electromagnets Shaping a wire into a solenoid increases the strength of the magnetic field. The field inside a solenoid is strong and uniform, and outside it is similar to a bar magnet. Adding an iron core increases the field strength. A solenoid with an iron core is an electromagnet, which can be switched on and off.
• Motor effect (Higher tier only) When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other. This is the motor effect.
• Fleming's left-hand rule (Higher tier only) First finger: field (north to south). Second finger: current (positive to negative). Thumb: motion (force). The force is greatest when the conductor is at right angles to the field.
• F = B I l (Higher tier only) F = B I l, where F is force in N, B is magnetic flux density in tesla (T), I is current in A and l is length of conductor in the field in m. On the equation sheet.
• Electric motors (Higher tier only) A coil of wire carrying a current in a magnetic field tends to rotate, because forces act in opposite directions on the two sides of the coil. A split-ring commutator reverses the current every half turn so the coil keeps rotating in the same direction.
• Loudspeakers (physics only, Higher tier only) Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves: an alternating current in a coil in a magnetic field makes the cone vibrate.

Common mistakes in 4.7.2 The Motor Effect

• Using the right hand for the motor effect. Fleming's left-hand rule is for motors (force on a conductor).
• Saying there is a force when the wire is parallel to the field. There is no force when the conductor is parallel to the magnetic field.
• Saying the commutator reverses the magnetic field. It reverses the direction of the current in the coil every half turn.
• Leaving length in cm or current in mA in F = B I l.

Try three 4.7.2 The Motor Effect exam-style questions