P5 - Electricity in the home

Question 1

Describe the properties of the domestic electricity supply:

Answer 1

-50Hz AC
-230 V

This is for the UK, it may be different for other countries

Question 2

What is AC?

Answer 2

-direction of current/polarity swaps back and forth at a consistent rate
-produced when an alternating PD is used

Question 3

What is DC?

Answer 3

-constant direction of current/polarity
-produced when a direct PD is used

Question 4

What does an oscilloscope do?

Answer 4

measures PD in a circuit, and produces a PD vs time graph

Question 5

What is a 3 core cable?

Answer 5

insulated cable with 3 individually insulated cables inside it (live, neutral, earth) that have copper wires inside each

Question 6

What is the colour is the insulation on each of the 3 core cable’s wires?

Answer 6

live - brown
neutral - blue
earth - green and yellow stripes

Question 7

Describe the 3 wires in a 3 core cable:

Answer 7

-live, 230V, carries the alternating PD from mains supply

-neutral, 0V, completes the circuit by carrying away current

-earth, 0V (when there isn’t a fault), safety wire connected to appliance casing, stops it from becoming live

Question 8

When does the earth wire have to be used? Describe what happens after:

Answer 8

-live wire touches metal casing

-large current flows to the earth rather than a person
-causes fuse to melt, cuts off current
-prevents electric shock

Question 9

Where (give examples) might a 2 core cable be used?

A 2 core cable is one that only has a live and neutral wire

Answer 9

appliances with plastic/double insulated casing (hairdryers, phone chargers, radios)

Question 10

What can cause a short circuit?

Answer 10

-if a live wire inside the appliance touches a neutral wire, a large current flows between them at the point of contact

-fuse blows, current is cut off

Question 11

Why would you get an electric shock from touching a live wire?

Answer 11

-potential of live wire is 230V
-potential of a person is 0V
-there is a large PD between live wire and person
-current passes through body easily

Same reason why current from live can flow across to neutral/earth

Question 12

Describe the structure of a 3 pin plug:

Answer 12

-cable grip stops 3 core cable being pulled out

-earth (top), longest pin so goes in first
-neutral (left)
-live (right)

-fuse protects the circuit, between live wire and pin

Question 13

What is a surge, and when might one happen?

Answer 13

sudden increase in supplied current that can damage an appliance (causing fire/electric shock)

-turning an appliance on/off, or if there is a fault

Question 14

What is power, and give its equations:

Answer 14

P = E/t (and P=IV, P = I²R, P=V²/R)

rate of energy transferred in joules per second (W)

Energy transferred, or work done - either is fine

Question 15

What is current, and give its equation:

Answer 15

Rate of flow of electric charge supplied by electrons

(Q=It, or I = Q/t to show the definition)

Question 16

What is charge, and give its equations:

Answer 16

measure of the total current that flowed in a certain period of time

(Q=It, E=QV)

Question 17

Why do resistors get hot?

Answer 17

-as charge flows through a component, work is done against its resistance
-electrons collide with ions in the lattice of the resistor
-as the electrons bump into the ions, energy is transferred causing the ions to vibrate
-some energy dissipated as thermal energy, so its temperature increases

Question 18

What is the National Grid?

Answer 18

network of transformers, wires and pylons across the UK to distribute electrical power from power stations to consumers

Question 19

What is a step-up transformer and why are they used?

Answer 19

-increases PD and decreases current at same time to maintain same power (P=IV)

-decreasing current minimises energy loss by heat during transmission (from resistance in wires), and increases the efficiency of the transmission

P=I²R is the power lost by heating, and decreases the current will decrease the energy lost by its square

Question 20

What is a step-down transformer and why are they used?

Answer 20

-decreases PD and increases current

-lower voltages are safer and wouldn’t blow our devices

Question 21

What is the equation for efficiency?

Answer 21

useful power out / total power in

Question 22

What is an electrical appliance?

Answer 22

something that transfers electrical energy to another energy store

Question 23

How does a fuse stop the wires overheating?

Answer 23

-current goes above appliance rating
-fuse wire melts
-cuts off current flowing in circuit of appliance
-stops current increasing further, reducing heat loss

Question 24

What is double insulation?

Answer 24

-outer casing is plastic
-wires inside are insulated in plastic

Question 25

Why might someone flying a kite near a electricity pylon be shocked?

Answer 25

-electric field strength is very high
-causes air to become ionised, and can conduct charge
-the kite’s string would conduct the charge to the person/earth

From November 2021, Paper 1, Q10.2