Questions 3 - Electricity

Sample questions
2.2 - Why should you not touch a mains appliance with wet hands?
Water is a conductor therefore it may cause the electric charge to flow to the water instead of the appliance and you could be electrocuted.

2.3 - The normal amount of current supplied to a device is 2.7 A. Which fuse rating would be suitable for this device?
a) 1A
b) 3A
c) 5A

Answer: B. If it was a 1A fuse, the device wouldn't work -when you turned it on, the fuse would melt and you'd have to replace it. If it was a 5A fuse, if there was an electricity surge, the device would be damaged and fried by excess current under 5A (difference between 2.7 and 5 is too big)

2.5 - Find the power of an appliance that transfers 1000 J in 30 seconds.

• E = 1000J
• T = 30 sec
• P = E / T (note - this equation is not on spec. but you are expected to know it)
• P = 1000 / 30
• P = 33.33333....
• P = 33.3 W (3 s.f.)

2.6 - How much charge flows when a current of 3A runs for 5 mins?

• I = 3A
• T = 5 mins 300 sec
• Q = I x T
• Q = 3 x 300
• Q = 900 C

 Questions (not answered)

• 1. How much energy is transferred when:
• a)  200 C flows through a resistor that has a potential difference of 230 V?
• b) a current of 5A passes through a 12 V lamp for 20 mins?

• 2. State one advantage of a circuit breaker compared with a fuse.

• 3. An appliance is supplied with 3A when it functions normally. 
• a) Should you use a 3A, 4A or 5A fuse?
• b) Explain your choice.