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GCSE Standard Questions: Electricity in the Home

Q20. The diagram shows some overhead power cables in the National Grid.

(a) Explain the advantage of transmitting electricity at a very high potential difference.

Power is the product of potential difference and current - so to transfer a particular amount of energy in a given time you can vary the p.d. and current to suit your needs. Very high p.d. means you can use very low currents. This reduces power loss during energy transfer as the amount of thermal energy lost to the surroundings depends on the current - so using a smaller current means less of your energy (in the form of thermal energy) is transferred to surroundings which increases the efficiency of power transmission.

[3 marks]

(b) It is dangerous for a person to fly a kite near an overhead power cable. The diagram shows Billy flying a kite.

Billy could still receive a fatal electric shock if the kite was very close to, but not touching the power cable.

Explain why.

The electric field strength near to the power cables is very high. This causes the air to become ionised. The kite becomes charged by induction and the string conducts that charge therough Billy as it flows to earth.

[3 marks]

(c) Doc Brown investigated how the potential difference needed for air to conduct charge varies with the distance between a cable and earth. Here is a graph plotted of his data.

The data in the graph above gives the relationship between potential difference and distance when the air is dry. When the humidity of air increases the air becomes a better conductor of electricity.

Draw a line on the graph to show how the potential difference changes with distance if the humidity of the air increases.

If the air is humid it has lower resistance - therefore current will flow to Earth at a lower p.d.

straight line passing through the origin

line drawn below existing line for all values

[2 marks]

(d) Here is a diagram showing a cross-section through a power cable.

A 1 metre length of a single aluminium wire is a better conductor than a 1 metre length of the steel wire.

The individual wires behave as if they are resistors connected in parallel.

Explain why the current in the steel wire is different to the current in a single aluminium wire.

The potential difference across the wires/cable is the same (and so is their length and cross section) but the resistance of the steel wire is greater and so less current will flow in the steel strand than in each of the aluminium strands.

[2 marks]

(Total 10 marks)