P5- Electricity In The Home

Question 1

What are the two types of electricity supplies?

Answer 1

Alternating current (ac) and direct current (dc)

Question 2

Direction of current in ac

Answer 2

The current is constantly changing direction

Question 3

How are alternating currents produced?

Answer 3

Alternating voltages in which the positive and negative ends keep alternating

Question 4

Is mains supply ac or dc?

Answer 4

ac supply at around 230v

Question 5

What is the frequency of the ac mains supply?

Answer 5

50 Hz

Question 6

What direction is direct current?

Answer 6

Current that is always flowing in the same direction

Question 7

How is direct current created?

Answer 7

Its created by a direct voltage

Question 8

What colour is the live wire?

Answer 8

Brown

Question 9

What colour is the neutral wire?

Answer 9

Blue

Question 10

What colour is the earth wire?

Answer 10

Green and yellow

Question 11

Live wire

Answer 11

The live wire provides the alternating potential difference from the mains supply

Question 12

What type of current do cells and batteries supply?

Answer 12

Direct current (dc)

Question 13

Neutral wire

Answer 13

The neutral wire completes the circuit and carries away current- electricity normally flows in through the live wire and out through the neutral wire. It is around 0V.

Question 14

Earth wire

Answer 14

It is for protecting the wiring and for safety- it stops the appliance casing from becoming live. It doesn’t usually carry a current- only when there’s a fault. Its also at 0V.

Question 15

Which type of wire can give you an electric shock?

Answer 15

Live wire

Question 16

How can a live wire give you an electric shock?

Answer 16

Your body (just like the earth) is at 0 V. 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 could injure or even kill you.

Question 17

Can you still get an electric shock from a live wire even if a plug socket or a light switch is turned off.

Answer 17

Yes, there is still a danger of an electric shock. A current is not flowing but there is still a pd in the live wire.

Question 18

Why can any connection between live and earth be dangerous?

Answer 18

If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire

Question 19

How is energy transferred to electrical appliances?

Answer 19

Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle.
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor.

Question 20

What does the total energy transferred by an appliance depend on?

Answer 20

How long the appliance is on for and its power

Question 21

What is the power of an appliance?

Answer 21

The energy that it transfers per second.(therefore the more energy it transfers in a given time, the higher its power.

Question 22

Power equation (in electrical appliances)

Answer 22

Energy transferred / Time

Question 23

3000 Joules of energy are transferred to a kettle in 2 mins. What is the power of the appliance?

Answer 23

2 mins = 120s
3000/120 = 25W

Question 24

What is an appliances power rating?

Answer 24

The maximum safe power that they can operate at (maximum operating power)

Question 25

How does a power rating help customers when buying electrical appliances?

Answer 25

The lower the power rating, the less electrcicity an appliance uses in a given time and so the cheaper it is to run

Question 26

What is the national grid?

Answer 26

A giant system of cables and transformers that covers the UK and connects power stations to consumers.

Question 27

When does demand for electricity increase?

Answer 27

When people get up in the morning, come home from school/work and when it gets dark and cold outside. Additionally popular events such as a sporting final on tv could also cause a peak in demand.

Question 28

Why do power stations run well below their maximum power output?

Answer 28

So there's spare capacity to cope with a high demand, even if there's an unexpected shut down of another station

Question 29

What does the national grid use to transmit the huge amount of power?

Answer 29

A high pd and a low current

Question 30

Why does the national grid use a high pd and a low current?

Answer 30

A low current is used because with a high current you lose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings. Its much cheaper to boost the pd up really high and keep the current as low as possible

Question 31

What do transformers do?

Answer 31

Step the pd up at one end (step up transformers) for efficient transmission Bring the pd down to safe, usable levels (step down transformers)

Question 32

What does a step up transfomer do?

Answer 32

The pd is increased

Question 33

What does a step down transformer do?

Answer 33

It decreases the pd for domestic use

Question 34

What is a build up of static electricity caused by?

Answer 34

Friction - When certain insulating materials are rubbed together negatuvely charged electrons will be scraped off one and dumped on the other. This will leave the materials electrically charged, with a positive static charge on one and an equal negative charge on the other.

Question 35

What happens when a polythene rod is rubbed with a cloth duster?

Answer 35

Electrons are transferred from the cloth duster to the polythene rod leaving the rod negatively charged and the cloth positively charged

Question 36

What happens when an acetate rod is rubbed with a cloth duster?

Answer 36

Electrons are transferred from the acetate rod to the cloth duster leaving the cloth duster negatively charged and the acetate rod positively charged

Question 37

How does static electricity cause sparks?

Answer 37

When an electric charge builds on an object, the potential difference between the object and the earth (0V) increases. If the pd gets large enough electrons can jump across the gap between a charged object and the earth - this is the spark.

Question 38

What do like charges do?

Answer 38

Repel

Question 39

What do opposite charges do?

Answer 39

Attract

Question 40

What happens when two charged objects move apart?

Answer 40

The electrostatic forces of either attraction or repulsion get weaker

Question 41

What is an electric field?

Answer 41

An electric field is created around any electrically charged object The closer you are to the object the stronger the field

Question 42

How can you show an electric field around an object?

Answer 42

Field lines

Question 43

Electric field lines always go from...

Answer 43

Positive to negative