Topic 2 - Electricity - Electricity in the home and electric fields

Question 1

What are the two types of electricity supplies?

Answer 1

Alternatiing curent (ac) and direct current (dc)

Question 2

What is ac?

Answer 2

1. In ac supplies the current is constantly changing direction
2. Alternating currents are produced by alternating voltages

Question 3

What is dc?

Answer 3

1. Direct cirrent is a current that is always flowing in the same direction
2. It is created by a direct voltage

Question 4

What is the UK mains supply?

Answer 4

1. The UK mains supply is the electricity in your home
2. It is ac supply at around at around 230V

Question 5

What is the frequency of the UK mains supply?

Answer 5

50Hz

Question 6

What type of electricity supply are batteries?

Answer 6

Direct current

Question 7

What colour is the live wire?

Answer 7

Brown

Question 8

What does the live wire do?

Answer 8

The live wire provides the alternating potential differnce at around 230V from the mains supply

Question 9

What colour is the neutral wire?

Answer 9

Blue

Question 10

What does the neutral wire do?

Answer 10

1. The neutral wire completes the circuit - when the appliance is operating normally, current flows through the live and neutral wires
2. It is around 0V

Question 11

What colour is the earth wire?

Answer 11

Green and yellow

Question 12

What does the earth wire do?

Answer 12

1. It is for protecting the wiring and for safety - it stops the appliance casing from becoming live
2. It doesn’t usually carry a current - only when there is a fault
3. It is also 0V
4. The earth wire provides a path for electric current to flow safely into the ground if a fault occurs, ensuring that metal parts of an appliance are earthed and do not become dangerously live
5. such as when a live wire accidentally touches the metal casing of an appliance

Question 13

Why is touching the live wire dangerous?

Answer 13

1. The live wire is 230V and you are 0V. This means if you touch the live wire a large potential difference is produced and current flows through you
2. This causes a large electric shock and could injure or even kill you
3. Touching the live wire gives electricity a path through you to the earth so it flows through you

Question 14

What is the national grid?

Answer 14

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

Question 15

Why does the national grid use high voltages?

Answer 15

1. For a given power, increasing the pd decreases the current, which decreases the energy lost by heating
2. This is because the higher the current the hotter the wires get and the more energy is transferred to the thermal energy store of the surroundings

Question 16

Structure of transformers?

Answer 16

Transformers all have two coils, a primary coil and a secondary coil joined with an iron core

Question 17

What does a step-up transformer do?

Answer 17

1. Step-up transformers increase the voltage of electricity
2. They have more turns on the secondary coil than the primary coil
3. As the pd is increased by the transformer, the current is decreased

Question 18

What does a step-down transformer do?

Answer 18

1. The voltage is decreased so that it is safe for homes
2. The current is increased
3. The primary coil has more turns than the secondary coil

Question 19

Formula for transformers?

Answer 19

Pd across secondary coil x current in secondary coil = pd across primary coil x current across in primary coil

Question 20

Static electricity 4 marker?

Answer 20

1. When two insulating materials are rubbed together, electrons are transferred from one material to the other
2. One material has a greater affinity for electrons, meaning it attracts and gains electrons and becomes negatively charged
3. The other material loses electrons and becomes positively charged
4. This creates a static charge as the electrons are transferred and not the positive charges
5. These opposite charges attract each other

Question 21

How does too much static cause sparks?

Answer 21

1. As electric charge builds on an object, the pd between the object and the earth increases
2. If the pd gets large enough, electrons can jump across the gap between the charged object and the earth - this is the spark
3. The air between a spark source and a conductor become ionised, creating a conductive path for the spark to follow
4. They can jump to any earthed conductor that is nearby

Question 22

What is an electric field?

Answer 22

1. An electric field is created around an electrically charged object
2. The closer to the object you get, the strong the field is

Question 23

What happens when a charged object is placed in an electrica field?

Answer 23

1. When a charged object is placed in the electric field of another object, it feels a force
2. This force causes the attraction or repulsion between charges
3. The forces is caused by the electric fields of each charged object interacting with each other

Question 24

What is static electricity?

Answer 24

The buildup of charge on insulating materials