mains electricity (topic 2) Flashcards
describe what an A.C. supply is (2)
- alternating current
- current reverses direction every half cycle
- current oscillates at a frequency of 50 Hz in the UK
in UK plugs, the live wire is the colour
brown
the neutral wire is
blue
the earth wire is
green and yellow stripe
which wire should the fuse be connected to
live
- a kettle is rated a 2 kW, 230 V
- calculate the operating current, 1 dp (power = voltage x current)
- the operating current of the kettle is higher than the fuse rating
- this causes the fuse wire to heat up
- and melt
- no current can flow once the fuse has melted
state which fuse the student should have fitted to the kettle and explain why (3 A, 10 A and 13 A) (2)
- 10 A fuse should be fitted
- 3 A is below the operating current
- 13 A could cause excessive current to flow through the kettle and damage it
- fuse selected should be the next available fuse above the operating current
explain why metal appliances need to be earthed (2)
- if the live wire touches the casing it becomes live
- earthing the case prevents the user getting an electric shock in the event of a fault
- all current would flow to ground instead of through the user
(current flowing through fan is 0.15A) (2)
a fault occurs in the oven and the live wire touches the metal case of the oven, explain how the safety features of the plug work to prevent the user getting a dangerous electric shock (5)
state the operating voltage and frequency of the UK mains supply
- 230 V
- 50 Hz
(power rating of 800W)
the oven is fitted with a 3 A fuse, which heating element should the homeowner avoid using
- heating element C should be avoided
- the operating current of heating element C exceeds the fuse rating of the oven
the oven is turned on for 40 minutes, calculate how much extra energy heating element uses compared to heating element A
state the name and purpose of part E (2)
- part E is the cable grip
- hold the cable in place to prevent wires becoming loose
state the name of the three wires A, B and C, and describe their purpose
- A - Earth wire - safety wire designed to direct all the current to ground in the event of a fault
- B - Neutral wire - held at 0 V and allows current to flow, completing the circuit
- C - Live wire - held at 230 V and allows current to flow through the fuse before entering the device
a fuse is a safety device used to prevent dangerously high currents flowing, explain how a fuse works to keep electrical circuits safe (3)
- when a current flows through the fuse, it causes it to heat up
- when the current exceeds the fuse rating, the fuse wire melts
- and breaks the circuit, so current is able to flow
circuit breakers are another common safety device used to cut off circuits in the event of an electrical fault, describe the advantages and disadvantages of using circuit breakers and fuses (4)
- fuses are cheap
- slower to respond than circuit breakers
- circuit breakers are quick to respond in the event of an electrical fault
- circuit breakers more expensive than fuses
the hairdryer contains a heating element, an electric fan and a plastic case, the hairdryer is not earthed, explain why the hairdryer is still safe to use despite not being earthed (4)
- in the event of a fault, the live wire would touch the plastic casing
- plastic is a poor conductor of electricity
- no current would flow through the casing
- and so the user would not experience an electric shock
the power of the hairdryer is 1.2 kW, explain what is meant by 1.2 kW (1)
- 1,200 Joules (1.2 KJ) of energy is transferred per second
calculate the current flowing through the heating element, give your answer to 2 decimal places
a fault develops in the coffee machine and the live wire touches the metal casing, explain how earthing and a fuse prevent the user from getting an electric shock
