Electromagnetism
6 marks · NormalAn electric motor uses electromagnets and is connected to the National Grid.
Describe how an electric current in a coil produces a force, and explain how this makes the coil rotate in an electric motor.
Transformers are used in the National Grid. Explain why step-up transformers are used to transmit electricity from power stations.
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(a) — · must show description of current creating a magnetic field, interaction/overlap with external magnetic field, forces on sides of coil (in opposite directions), causing rotation
(b) — · must show mention of high current causing energy loss, step-up transformer increases voltage, reduces current, lower current reduces energy lost as heat
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes an electric current producing a magnetic field.
M2 (a) · 1 mark · Explains that overlapping fields produce a force on the conductor.
A1 (a) · 1 mark · Correctly states both sides of coil experience opposite forces, causing rotation.
M3 (b) · 1 mark · Mentions that high current in transmission lines increases energy loss (by heating).
M4 (b) · 1 mark · Explains step-up transformer increases the voltage, so current in the wires can be reduced.
A2 (b) · 1 mark · Links reduced current to less energy lost as heat.
Worked solution
a) When current flows in the coil of an electric motor, it creates a magnetic field around the wire. When the coil is placed between the poles of a permanent magnet, the field from the coil and the field from the magnet overlap. This overlap produces a force on each side of the coil (due to the motor effect) in opposite directions. One side is pushed up and the other side down, causing the coil to rotate.
b) When transmitting electricity, energy is lost due to heating in the transmission wires (because of their resistance). Larger current means more heat is lost (energy loss = I^2R). Step-up transformers increase voltage which allows current to be reduced for the same power. Lower current means less energy is lost as heat, improving efficiency.