9. Figure 13 shows part of a mains electricity lighting circuit in a house.
i- A fault in the switch caused a householder to receive a mild electric shock before a safety device switched the circuit off.
The mean power transfer to the person was 5.75 W.
The potential difference across the person was 230 V.
Calculate the resistance of the person.
[5 marks]
SOLUTION
Using power formula to find current in the circuit.
Now, using the V=IR, to find resistance.
ii- An electrician replaced the switch.
The electrician would have received an electric shock unless the circuit was disconnected from the mains supply.
Explain why.
[3 marks]
ANSWER
Since one of the wire is live, and the electrician is at earth potential (0V). Therefore, in case of he touches the live wire, the electrician would get an electric shock as there exist a huge potantial difference betweena live wire and an electrician.
iii- The current from an electric shock causes a person’s muscles to contract.
The person cannot let go of the electrical circuit if the current is too high.
Figure 14 shows how the maximum current at which a person can let go depends on the frequency of the electricity supply.
The UK mains frequency is 50 Hz.
Explain why it would be safer if the UK mains frequency was not 50 Hz.
[2 marks]
ANSWER
The graph shows that 50 Hz has the lowest let go current. Whereas at higher frequency, the let go-current is higher. Therefore people will be able to let go at a greater current.
(Usually students gets confused at this part and questions me that why don’t we than change the frequency from 50Hz if the different value would be more safer. So here is the answer. Remember, 50 Hz is an international standard that is not only low enough to minimise energy losses and reduce electomagnetic interferneces, but also high enough to effiuciently power electrical appliances.)
