Fig. 1 shows a simplified d.c. motor in an electric toothbrush. The toothbrush battery is connected to brushes, a split-ring commutator and a coil. The labe...
Question 1 Report
Fig. 1 shows a simplified d.c. motor in an electric toothbrush. The toothbrush battery is connected to brushes, a split-ring commutator and a coil. The labelled arrows show the force on each side of the coil at one instant. These forces turn the axle, which moves the brush head. The casing is sealed to keep water away from the electrical parts. A designer is considering using a stronger permanent magnet, without changing the battery or the resistance of the coil.
(a) What produces the turning effect on the coil? [1]
(b) When does the split-ring commutator change the connections to the coil? [2]
(c) What effect would a stronger magnet have on the force and the motor speed? [2]
Fig. 1 shows the coil and brushes inside a battery-powered hand fan. Two carbon brushes press against the split-ring commutator. This maintains an electrical connection while the coil rotates. The permanent magnets provide a fixed magnetic field. The fan blades are attached to the axle, so a steady turning effect is needed. During a repair, one brush becomes worn and no longer touches the commutator reliably. The fan then runs in short bursts instead of at a constant speed.
(a) What is the function of the carbon brushes? [1]
(b) Which part reverses the current supplied to the rotating coil? [2]
(c) When the fan is operating normally, what energy transfer makes the blades move? [2]
Answer Details
Electric toothbrush motor
(a) Opposite motor-effect forces on opposite sides of the coil produce the turning effect [1].
(b) The split-ring commutator changes the connections every half-turn [1], reversing current so the turning effect remains in the same direction [1]. [2 marks]
(c) A stronger magnet increases the force [1], giving a greater turning effect so the motor can rotate faster [1]. [2 marks]
Hand fan
(a) Carbon brushes provide electrical contact with the rotating commutator [1].
(b) The split-ring commutator reverses the current [1] every half-turn [1]. [2 marks]
(c) Electrical energy is transferred [1] to kinetic or mechanical energy of the rotating blades [1]. [2 marks]
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