Topic 2 - Electricity

Question 1

Charge equation

Answer 1

Charge flow(C) = Current(A) • Time(s)

Question 2

Potential difference equation

Answer 2

Potential difference(V) = Current(A) • Resistance(O)

Question 3

Total resistance equation

Answer 3

Total resistance= resistance of component 1 + resistance of component 2

Question 4

Power equations

Answer 4

1. Power(W)= current(A) • potential difference(V)
2. Power(W)= (Current(A))^2 • resistance(O)

Question 5

Energy transferred equations

Answer 5

1. Energy transferred(J)= Power(W) • Time(s)
2. Energy transferred(J)= Charge flow(C) • Potential difference(V)

Question 6

What is meant by potential difference and resistance in a circuit?

Answer 6

Potential difference is the driving force that pushes charge round the circuit, resistance is anything that slows flow down.

Question 7

What is a ohmic conductor?

Answer 7

A conductor that obeys Ohm’s Law, e.g a resistor

Question 8

Practical:
Explain how you would investigate how the length of a wire affects it’s resistance.

Answer 8

1. Attach a crocodile clip to the wire, level with 0 cm on the ruler.
2. Attach the second crocodile clip to the wire 10 cm away from the first clip. Write down the length of the wire between the clips
3. Close the switch a, then record the current and potential difference using a ammeter and voltmeter
4. Open the switch, then move the second crocodile clip another 10 cm. Close the switch, then record the new length, current and potential difference.
5. Repeat
6. Use your measurements for current and potential difference to calculate the resistance (V=IR)
7. Plot a graph of resistance against wore length and draw a line of best fit.

Question 9

Name a linear component and non-linear component.

Answer 9

Resistor and filament lamp

Question 10

Explain how the resistance of a LDR varies with light intensity.

Answer 10

When light increases, resistance decreases and when light decreases, resistance increases.

Question 11

What happens to the resistance of a thermistor as it gets colder?

Answer 11

It increases.

Question 12

True or False? Potential difference is shared between components in a series circuit.

Answer 12

True

Question 13

True or False? The current is constant in a series circuit.

Answer 13

True

Question 14

True or False? The potential difference across each component connected in parallel is different.

Answer 14

False

Question 15

Explain why adding resistors in parallel decreases the total resistance of a circuit, but adding them in series increases the total resistance.

Answer 15

In series, the resistance adds up. In parallel, if you have 2 resistors, their total resistance is more than the resistance of the smallest resistor, so adding more resistors decreases the total resistance.

Question 16

True or False? Mains supply is an alternating current.

Answer 16

True

Question 17

What is the potential difference and frequency of the UK mains supply?

Answer 17

230V and 50Hz

Question 18

Name and give the colours of the 3 wires in a 3-core cable.

Answer 18

Earth-Yellow and green (Safety wire)
Live-Brown (provides voltage)
Neutral-Blue (completes the circuit)

Question 19

Give the potential difference for the 3 wires in the 3-core cable.

Answer 19

Earth-0V
Live-230V
Neutral-0V

Question 20

Explain why touching a live wire is dangerous.

Answer 20

Your body is at 0 V, and the wire is at 230V. This means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you. This causes a large electric shock, which can be fatal.

Question 21

What is the power rating of an appliance?

Answer 21

The maximum amount of energy transferred from one energy store to another per second when the appliance is used (W)

Question 22

Explain why electricity is transferred by the national grid at a high potential difference but low current.

Answer 22

For a given power, increasing the potential difference decreases the current, which decreases energy lost by heating the wires and surroundings. This makes the national grid efficient.

Question 23

What are the functions of a step-up and step-down transformer?

Answer 23

Potential difference is increased using a step-up transformer for efficient transmission. It’s then reduced again for domestic use in homes using a step-down transformer.

Question 24

How does rubbing of materials cause static electricity to build up?

Answer 24

Electrons are scarped off one material and dumped on the other material. This leaves the materials electrically charged, with a positive static charge on one material, and a negative static charge on the other material.

Question 25

True or False? Two positive charges attract each other.

Answer 25

False

Question 26

In which direction do the arrows on electric field lines point?

Answer 26

If the electric field is positive, the lines point away from the centre, if the electric field is negative, the lines point towards the centre.

Question 27

Using the concept of electric fields, explain how a build up of static electricity can cause a spark.

Answer 27

Sparks are caused when there is a high enough potential difference between a charged object and the earth. A high potential difference causes a strong electric field between the charged and earthed object. The strong electric field causes electrons in the air particles to be removed (ionisation). Air is usually a insulator, but when ionised, it is much more conductive, so a current can flow through it. This is the spark.