Current, potential difference and resistance

Question 1

What is the National grid?

Answer 1

The National Grid is used to distribute electricity around the country from power stations to consumers. It is the network of pylons and wires you see high above you across the country.

Question 2

Current is a measurement of…….

Answer 2

……..how quickly electrical charge flows.

Question 3

A step-up transformer steps the voltage up from 20,000 V to 400,000 V. If the current going in is 25,000 A, what is the output current?

Answer 3

50,000 A Double the voltage, double the current as long as the resistance stays the same.

Question 4

Explain why transformers are used in the National Grid.

Answer 4

Transformers allow the power to stay high because a high voltage means the current can be lowered. This stops the wire getting warm and losing energy to the surroundings.

Question 5

What is AC/DC apart from an Australian rock band?

Answer 5

DC = Direct current AC = Alternating Current

Question 6

What is the voltage and frequency of the mains?

Answer 6

In the UK, the frequency of mains electricity is 50 hertz. The voltage of mains electricity supplied to UK homes is 230 V.

Question 7

What current flows through a resistor of 40 ohms when connected to a power supply of 12 volts?

Answer 7

I = V/R = 12/40 = 0.3 Amps

Question 8

What is the name and job of the blue wire?

Answer 8

The live wire

Question 9

How would you calculate power?

Answer 9

power = current x voltage

Question 10

What is the rule for voltage in series circuits?

Answer 10

The voltage from a battery is shared between all the components in a series circuit.

Question 11

What is the rule for current in series circuits?

Answer 11

The current has only one route in a series circuit and all ammeters would show identical current.

Question 12

How does the length of a wire affect its resistance?

Answer 12

As the length increases the resistance also increases.

Question 13

Why does an increase in temperature in a wire increase its resistance?

Answer 13

As a metal heats up their resistance increases because their positive metal ions vibrate more and so block the flow of electrons.

Question 14

Follow the steps from the National Grid from your home or school

Answer 14

Electricity from a power station goes to: - Step-up transformers (you will find out about those next) - High-voltage transmission lines/wires - Step-down transformers - Consumers (i.e. your home, school, supermarket)

Question 15

What is the difference between direct current (d.c.), and alternating current (a.c.)?

Answer 15

Direct current (d.c.) is an electric current that always flows in one direction. Alternating current (a.c.) is an electric current that is constantly changing direction.

Question 16

Resistance is……..

Answer 16

..a measure of how much a conductor prevents the flow of electrons.

Question 17

The voltage of a battery or cell is a measure of …….

Answer 17

….the energy given to the electrons so that they can flow around the circuit.

Question 18

How would you calculate electrical energy?

Answer 18

electrical energy = power x time

Question 19

Why do some metals have a high resistance?

Answer 19

Some metals have a high resistance because their positive metal ions are larger and block the flow of electrons.

Question 20

What is the equation to find charge?

Answer 20

Charge = current x time Q = It

Question 21

What is the name and job of the green and yellow wire?

Answer 21

The earth wire is a safety wire that is needed to earth appliances with a metal case. This makes it safer to touch the appliance if it develops a fault.

Question 22

What is the name and job of the brown wire?

Answer 22

The live wire carries a current that alternates between a negative and positive voltage.

Question 23

Which has a higher resistance a thick or thin wire?

Answer 23

A thin wire has a higher resistance.

Question 24

Why are the wires are made of copper?

Answer 24

Copper is a good electrical conductor.

Question 25

What is meant by a step-up and step-down transformer?

Answer 25

Step-up transformers increase voltage and step-down transformer decrease voltage?

Question 26

What is a transformer?

Answer 26

A transformer is a device to change voltage in a circuit.

Question 27

Current is measured in…..

Answer 27

…….Amps (short for Amperes).

Question 28

Electrical charge is measured in…..

Answer 28

Coulombs

Question 29

What is the name and job of the blue wire?

Answer 29

The neutral wire completes the circuit. It is kept at a zero voltage by the electricity company.

Question 30

What is the rule for current in parallel circuits?

Answer 30

The current is shared between all the components in a parallel circuit because there are different routes that the current can take .

Question 31

What equation would you use to calculate voltage?

Answer 31

voltage = current x resistance

Question 32

Give an example of a source of a.c. and a source of d.c.

Answer 32

Direct current is produced by cells and batteries. Alternating current is produced by most generators and is used in mains electricity

Question 33

What is the rule for voltage in parallel circuits?

Answer 33

The voltage from a battery is identical for all the components in a parallel circuit.

Question 34

Give 2 equations to calculate electrical energy?

Answer 34

electrical energy = power x time electrical energy = charge x voltage

Question 35

A phone is left charging for 45 minutes. While it charges a current of 0.25 Amps flows through it. How much charge flows through the phone in this time?

Answer 35

Q = It Q = 0.25 x (45 x 60) so that the time is in seconds rather than minutes Q = 675 Coulombs

Question 36

What is the name and job of the brown wire?

Answer 36

The neutral wire