Physics - electricity 2

What is direct current

The flow of a current in one direction. Can be provided by cells or batteries.

What is alternating current

The rapid change in the direction of current. Provided by generators. The electrons are oscillating back and forth but still transfer energy.

What is the UK mains frequency? How many times does this mean the current changes direction per second?

50Hz → 100 changes/second

What is the mains voltage? What is the peak voltage?

230V a.c. ; 325V

True or False: the direction of current is taken from + to - in a battery

True - even through the electrons are actually moving the opposite way

Complete the labels: (image)

1 = earth wire 2 = neutral wire 3 = cable grip 4 = fuse 5 = live wire 6 = outer insulator

What colour is the earth wire

Yellow and green stripes

What colour is the neutral wire

blue

What colour is the live wire

red

How to tell the time period of an alternating current on an oscilloscope trace

Count the squares from peak to peak or trough to trough

What are the wires of a plug made of

copper

Why are the wires made of copper

Very good insulator, affordable, flexible

What are the pins made of

brass

Why are the pins made of brass

Strong and less malleable, does not corrode easily, still a good conductor

What is the cable insulation made of

Flexible plastic

Why is the cable insulation made of flexible plastic

Is an insulator, must be flexible so the cables can bend

What is the case made of

Stiff plastic,

Why is the case made of stiff plastic

Strong and a good insulator

What does the earth wire do? What circumstances would it conduct electricity?

It conducts electricity when there is a fault in a system. For instance, if a live wire has come loose and is touching a metal part of the appliance that the user might touch, the earth wire conducts this electricity, carrying it away from the appliance

The earth wire has a very high/low resistance. Why?

Low resistance, so it can quickly carry the voltage away from the appliance. This means that the current of the circuit will increase, melting the fuse or tripping the circuit breaker, therefore breaking the circuit

What does the fuse do

It melts when there is a high current, breaking the circuit. This means it helps to prevent fires, as it prevents the cable from overheating

Why might some appliances be double insulated?

They only have a two wire lead, and do not need an earth wire as the appliance itself is made of insulating material, so it cannot become live. It is wrapped it an extra layer of insulation to ensure it is safe

Sockets are connected in series/parallel to the mains circuit. Why?

Parallel, so they can be switched on and off without affecting each other

The switch of a socket is always in series/parallel with the live wire. Why

So switching ti off immediately cuts of the current

What is a circuit breaker and what does it do

A device which compares the current between the neutral wire and the live wire. If there is any difference, (which could be caused by leakable, or current going somewhere it shouldn’t), then the circuit breaker flips a switch, turning the circuit off.

Compare fuses and circuit breakers

Fuses can take time to melt, while circuit breakers can break the circuit instantly, which makes circuit breakers safer If a fuse melts, it must be replaced, which is a difficult job while a circuit breaker tripping only flips a switch Fuses are smaller and cheaper, so can be fitted onto every appliance, while circuit breakers can only be fitted by the metre

What are the possible fuse sizes

3A, 5A, 13A

How to choose the right fuse for an appliance

Find the current, and then move up to the next fuse size

What should the fuse be for a 6.5 amp tumble drier

13

What should the fuse be for a 0.5 amp kettle

3

A portable fan needs a 5amp fuse. What would happen if you used a 13 amp fuse

The wires could overheat to a dangerous level, potentially starting a fire, and the fuse would not melt to break the circuit

A portable fan needs a 5amp fuse. What would happen if you used a 3 amp fuse

The fuse would melt instantly, when there is no danger

image

1 power station 2 step up transformer 3 step down transformer 4 consumer 5 transmission cables

What does a step up transformer do and why

Increases the voltage and decreases the current. As the electrical current is being carried very far, a high current would mean that the wires would heat up and dissipate a lot of energy to the environment, which would be wasteful. By increasing the voltage and decreasing the current, energy is conserved

What does a step down transformer do and why

Decreases the voltage and increases the current. This is because a large current would be incredibly dangerous and cause explosions, while a higher current would help appliances to function

When electricity is travelling through overhead cables, describe the voltage and current

Current is low, voltage is high

How do fossil fuels produce energy for use

Burnt to produce steam Steam spins a turbine Turbine drives a generator Magnets and coils of wire in the generator produce electricity as they spin Electricity is distributed around the country though the national grid

Image( /\/\/\/). What type of current is this, what could have produced it

Alternating, mains

Image (-----) What type of current is this, what could have produced it

Direct, cell