P5 - Mains Electricity

Question 1

What is direct current and what are some sources of it

Answer 1

• Current that only flows in one direction
• Supplied by batteries or cells

Question 2

What is alternating current and what are some sources of it

Answer 2

• The current repeatedly changes direction
• This changes the direction of the voltage too
• You can get it from plug sockets (mains electricity)

Question 3

What is the potential difference and frequency of UK mains electricity

Answer 3

230V and 50Hz

Question 4

3 Types of wires in mains electricity and their corresponding colours

Answer 4

• Live wire - brown
• Neutral wire - blue
• Earth wire - yellow and green stripes

Question 5

Role of the live wire

Answer 5

Carries the alternating potentail difference from the supply

Question 6

Role of the neutral wire

Answer 6

Completes the circuit and carries current away from the appliance

Question 7

Role of the earth wire

Answer 7

• Has a voltage of 0V and only carries a current if there is a fault (safety feature)

Question 8

Equation for frequency

Answer 8

• frequency = 1/time period

Question 9

Role of step up transformers and why they are needed

Answer 9

• Increases the voltage
• This reduces the current needed to carry the same amount of electrical power
• When current flows through a wire, it heats it up
• The lower the current, the less energy is wasted as less heat energy is dissipated to the surroundings
• This increases the efficiency of the National Grid system

Question 10

Role of step down transformers and why are they needed

Answer 10

• Dcreases the voltage to 230V
• This is a safer voltage suitable for use in homes

Question 11

Role of a fuse

Answer 11

• Thin wire that melts if the current is too large
• This stops any more currnet from flowing (safety feature)
• They are connected to the live wire

They also have a rating e.g. 3A, 5A, 13A

Question 12

Advantages of circuit breakers over fuses

Answer 12

• Can be used many times whilst once a fuse melts, it cannot be used again
• Stops current flow even faster than a fuse

Question 13

Why is it dangerous for a metal case to touch the live wire

Answer 13

• Metals are good electrical conductors
• If anyone touches the casing, the potential difference will flow through them and into the ground (path for charge to flow)
• This could cause a fatal electric shock due to the large difference in potential difference

Question 14

How are metal appliances made safe

Answer 14

• The appliance is connected to the earth wire
• If the live wire touches the casing, the current will flow through ther low resistance earth wire, taking the current away
• This will break the fuse and stop any more current from flowing
• This stops anyone from getting an electric shock

Question 15

Why is it dangerous for the earth wire and the live wire to touch

Answer 15

• This would remove the safety feature of the earth wire
• As they have a large potential difference, it would cause a big current o flow which could make the fuse explode

Question 16

Why can the live wire be dangerous even if the switch is open

Answer 16

• It may still hold some potential difference as part of the wire up to the switch is still 230V

Question 17

Why will a spark jump between a negatively charged object and an earthed conductor

Answer 17

• There is a large potential difference between the two objects
• Electrons move trhough the air
• From the negatively charged object to the earthed conductor