Figure 13 shows some overhead power cables in the National Grid.
i- Explain the advantage of transmitting electricity at a very high potential difference.
[3 marks]
ANSWER
Transmitting electricity at a high potential difference so that the flow of current is low, reducing the energy dissipated to the surroundings. As a result, there is an increase in efficiency of power transmission.
ii- It is dangerous for a person to fly a kite near an overhead power cable.
Figure 14 shows a person flying a kite
The person could receive a fatal electric shock if the kite was very close to, but not touching the power cable.
Explain why.
[3 marks]
ANSWER
The electric field around the cable is very high that means that air could becomes a conductor and string may conduct the charge from air to the person holding the string of kite.
A scientist investigated how the potential difference needed for air to conduct charge varies with the distance between a cable and earth.
Figure 15 shows the results
iii- The data in Figure 15 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 Figure 15 to show how the potential difference changes with distance if the humidity of the air increases.
[2 marks]
ANSWER
With an increase in humidity, potential difference is expected to decrease for the same values distance. This is because the resistance decreases.so we need to sketch the line which is less steeper.
iv- Figure 16 shows 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.
[2 marks]
ANSWER
The voltage across all wires is same but the resistance of steel wire is greater than aluminium. Therefore, less current flows through a steel wire.
