Electric charge + mains electricity

Question 1

Conductors

Answer 1

Allow current to flow easily
e.g. metals

Question 2

Insulators

Answer 2

Do not allow current to flow easily
e.g. plastics, rubber, glass, wood

Question 3

Practical: Charging by Friction

Answer 3

Rub two insulating materials e.g. rod and cloth, friction occurs
Electrons transfer from one material to the other (protons cannot move)
The object gaining the negatively charged electrons becomes negatively charged and vice versa
Proof of charge: Can attract small items/repel water

Question 4

Why do sparks/discharges occur during friction

Answer 4

Electrons jumping through the air

Question 5

Dangers of electrostatic charges

Answer 5

When fuelling aircraft/tankers, static charge can build up from fuel flowing through pipes
Build up of charge can lead to sparks, which may ignite flammable gases
Prevention: Earthing wires are used to safely discharge the pipes

Question 6

Inkjet printers function

Answer 6

Image of document projected onto a positively charged copying plate
Plate loses charge in the light areas and keeps positive charge in dark areas (text)
Negatively charged ink is applied to plate, sticks to parts with positive charge
The ink is then transferred onto a new blank sheet of white paper
The paper is heated to ensure the ink powder sticks

Question 7

Insulation in appliances

Answer 7

Wires are covered with insulating materials like plastic to prevent touching live wires
Stops electric shock by keeping current within wires

Question 8

Double Insulation in appliances

Answer 8

Found in devices with no metal parts exposes e.g. hairdryers
Extra layer of insulation - no need for earth wire

Question 9

Earthing

Answer 9

Earth wire (green) provides a low resistance path to earth/ground
If the live wire touches the metal case, current flows through the earth wire
This causes a large current, blowing the fuse and cutting the current (safe)

Question 10

Fuse

Answer 10

The current is too high (exceeds fuse value)
The fuse melts, breaking the circuit and stopping the current (preventing shock)

Question 11

Circuit breakers

Answer 11

Detects large/sudden increase in current
Switch opens automatically to break the circuit
Can be reset (unlike a fuse which must be replaced)
Faster/more sensitive than a fuse

Question 12

Alternating current (a.c.)

Answer 12

Current that continuously changes its direction (from + to - back and forth)
Two identical terminals that alternate between + and -
Mains electricity: 50Hz, 230V

Question 13

Direct current (d.c.)

Answer 13

Current that constantly flows in the same direction in a circuit (from + to -)
Fixed positive terminal and negative terminal
Cells/Batteries