P5 - Electricity in the home

Question 1

What is an alternating current?

Answer 1

Electric current in a circuit that repeatedly reverses its direction

Question 2

What is a direct current?

Answer 2

Electric current in a circuit that is in one direction only

Question 3

What is a fuse?

Answer 3

A fuse contains a thin wire that melts and cuts the current off if too much current passes through it

Question 4

What is an oscilloscope?

Answer 4

A device used to display the shape of an electrical wave

Question 5

What are step down transformers?

Answer 5

Electrical device used to step down the size of an alternating potential difference

Question 6

What are step-up transformers?

Answer 6

Electrical device used to step-up the size of an alternating potential difference

Question 7

What is electrical power?

Answer 7

The rate of energy transfer by each electrical component

Question 8

What are the 3 equation for power

Answer 8

• Power = Energy / time
• Power = Current x Potential Difference
• Power = Current² x Resistance

Question 9

What is the unit for power?

Answer 9

Watt

Question 10

What are the 4 types of domestic fuses?

Answer 10

• 1A
• 3A
• 5A
• 13A

Question 11

How do you determine what fuse to use?

Answer 11

Electrical power / potential difference = fuse rating (current) [You always round up to the fuse rating]

Question 12

Why is it a bad idea to use an incorrect fuse?

Answer 12

• A fuse that is too low will fail almost immediately, breaking the circuit
• A fuse that is too big will fail to break the circuit if the current is too large, this can cause damage to the circuit such as burning the insulation or melting the wire. It could even start a fire.

Question 13

What fuse would you use if the current through a circuit is 2.2A and why?

Answer 13

You would use a 3A fuse because it would not melt when the correct current is flowing through the circuit, however if the current becomes too high it would fail, protecting the house

Question 14

What is the potential difference of the mains supply?

Answer 14

230 Volts

Question 15

What fuse would you use if the current in a circuit was 9.1A?

Answer 15

You would use a 13A fuse as the fuse would not melt if the correct current if flowing through the circuit, however it would fail and protect the house if the current is too high

Question 16

What are the 2 equations for voltage?

Answer 16

• V = IR

* V = E / Q

Question 17

What forces charge around a circuit?

Answer 17

Potential difference

Question 18

What is charge carried around the circuit by?

Answer 18

electrons

Question 19

What is the equation for charge flow?

Answer 19

Charge flow = current x time

Question 20

What is 1 coulomb?

Answer 20

6.242 x 10^18 protons or electrons

Question 21

What is the current is 31.21 x 10^18 electrons flow each second?

Answer 21

5A

Question 22

Describe how resistors are heated

Answer 22

Electrons flowing through the resistor constantly collide with the vibrating metal ions in the resistor, transferring energy to the metal ions. The metal ions gain energy to their kinetic energy stores and vibrate more, this causes the resistor to get hotter. The resistor heats the surroundings, dissipating the energy supplied to the surroundings

Question 23

What is the rule for potential difference in series circuits?

Answer 23

Potential difference is shared across components in series

Question 24

Describe the energy store changes in a circuit containing a bulb

Answer 24

The chemical energy store of the battery decreases as it transfers energy by an electric current, increasing the thermal energy store of the bulb and variable resistor. Energy is then transferred to the surroundings as heat and light

Question 25

What is energy efficiency?

Answer 25

The ratio of useful energy output from a device to total energy input of a device

Question 26

What is the equation for energy efficiency?

Answer 26

Energy efficiency = useful output energy / total input energy Energy efficiency = useful output power/ total input power

Question 27

What are the units of efficiency?

Answer 27

There are no units as it is a ratio

Question 28

Why can no device be more than 100% efficient?

Answer 28

Because the law of conservation of energy states that energy cannot be created or destroyed

Question 29

Why is it useful to know what type of energy is being transferred and in what amounts by a device?

Answer 29

To discover how efficient the device is

Question 30

What is the equation for useful power?

Answer 30

useful power = efficiency x input power

Question 31

What is the equation for kilowatt hours?

Answer 31

(Power [Watts] x time [hours]) / 1000

Question 32

What is 1kWh in Joules?

Answer 32

3600000 J

Question 33

What is 1 kWh?

Answer 33

1 kWh is the electrical energy converted by a 1 kW appliance used for 1 hour.

Question 34

What useful energy does a lightbulb produce?

Answer 34

Light radiation

Question 35

What is the role of step-up transformers?

Answer 35

Step-up transformers are used at power stations to transfer electricity to the national grid. These transformers are used to make the size of the alternating potential difference much larger, typically from 25 000 V to 132 000 V

Question 36

What is the role of step-down transformers?

Answer 36

Step-down transformers are sued to supply electricity from the national grid to consumers. Homes and offices in the UK are supplied with mains electricity that provides the same power as a 230V DC supply. Factories use much more power than homes so are supplied with a high p.d

Question 37

What is the purpose of transformers in the national grid?

Answer 37

By making the grid potential difference very large, much less current is needed to transfer the same amount of power (high p.d, low current). So the power loss due to resistance heating in the cables is much reduced as lots of energy is lost when there is a high current. This means that the national grid is an efficient way to transfer power

Question 38

What equation can be used to calculate the energy transferred to a component by a flow of charge?

Answer 38

E = V x Q

Question 39

What is the total energy transferred by the supply equal to?

Answer 39

The sum of the energies transferred to each component of the circuit

Question 40

Describe how electrical appliances waste energy?

Answer 40

Electrical appliances waste energy because the current in the wires and component of the appliance has a heating effect due to the resistance of the wires and the components. So they transfer energy by heating to the surroundings.

Question 41

How much energy dose each coulomb of charge carry in a 12V battery/

Answer 41

12J

Question 42

If the voltmeter reads 10V, what is the energy transfer to the variable resistor per second if the current is 1A

Answer 42

2J

Question 43

How is a charge supplied with energy?

Answer 43

Energy is supplied to the charge at the power source, increasing the potential difference

Question 44

What does it mean if a battery has a larger potential difference?

Answer 44

It supplies more energy to the circuit for every coulomb of charge that which flows round it

Question 45

What is a live wire?

Answer 45

a wire carrying an electric current