Topic 2 - Electricity

Question 1

Define current and state the equation that links current, charge and time with units for each.

Answer 1

Current-The flow of electrons through a circuit

Charge(Q)=Current(I) x Time(s)

Question 2

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

Answer 2

Potential difference - the driving force that pushes the charge round

Resistance - anything that slows the flow down

Question 3

What is the equation that links potential difference, current and resistance

Answer 3

V=I x R

Question 4

Explain how you would investigate how the length of a wire affects its resistance?

Answer 4

1. Attach a crocodile clip to the wire level with 0 cm on the ruler
2. Attach the second crocodile clip to the wire e.g. 10 cm apart
3. Close the switch, then record the current through the wire and the pd across it
4. Open the switch, then move the second crocodile clip, e.g. another 10 cm along the wore. Close the switch again and record the new length, current and pd
5. Use the formula V=I x R to work out the resistance
6. You should find that the as you increase the distance the more resistance there is. (There is a directly proportional relationship)

Question 5

What is an ohmic conductor?

Answer 5

An ohmic conductor is a component at which the current passing through it is directly proportional to the potential difference

Question 6

Name one linear component and one non-linear component

Answer 6

Linear component:
-Resistor
Non-linear component:
- Diodes

Question 7

Explain how the resistance of an LDR varies with light intensity?

Answer 7

In an LDR the resistance decreases as light intensity get higher

Question 8

What happens to the resistance of a thermistors as it gets hotter

Answer 8

In a thermistor the resistance decreases as the temperature gets hotter

Question 9

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

Answer 9

True

Question 10

How does the current through each component vary in a series circuit?

Answer 10

Current is the same everywhere in a series circuit

Question 11

How does potential difference vary between components connected in parallel?

Answer 11

Potential difference is the same across all components in a parallel circuit i.e. V1=V2

Question 12

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

Answer 12

By adding resistors in a series circuit, the resistors have to share voltage. So by adding more resistors more pd has to be shared leading to a lower current through each resistor. Current is the same everywhere in a series therefore total resistance will be higher

When adding resistors in parallel you are adding more branches meaning more pathways for the current to go. By adding branches the current increases. By using R=V/I the bigger the current the less the resistance thus adding more resistors in parallel decreases total resistance.

Question 13

True or False? Mains supply electricity is an alternating current?

Answer 13

True

Question 14

What is the pd and the frequency of the UK mains supply?

Answer 14

230V, 50Hz

Question 15

Name and give the colours of the three wires in a three-core cable. Why are they colour coded

Answer 15

Live - Brown
Earth - Green/Yellow
Neutral - Blue

Question 16

Give the pd for the three wires in a three-core mains cable

Answer 16

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

Question 17

Explain why touching a live wire is dangerous?

Answer 17

Your body is at 0V and the live wire is at 230V. If you have contact with the earth current can flow through your body creating a large electric shock

Question 18

State three equations that can be used to calculate electrical power

Answer 18

P=I x V
P=E/t
P=I² x R

Question 19

What is the power rating of an appliance?

Answer 19

The maximum amount of energy transferred between stores per second when the appliance is in use

Question 20

Explain why electricity is transferred by the national grid at a high pd but low current?

Answer 20

To minimize energy loss (High current makes a large amount of thermal energy)

Question 21

What are the functions of step-up and step-down transformers?

Answer 21

Step-up transformers - Where the pd is increased (“stepped up”)

Step-down transformers - Where the pd is decreased (“stepped down”)

Question 22

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

Answer 22

Electrons are scrapped off from one material to the other leaving one material with positive static charge(without electrons) and one with negative static charge(with electrons)

Question 23

True or False? Two positive charges attract each other

Answer 23

False - Opposite charges attract each other