2 Electricity units and safety
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What is the unit of current?
ampere (A)
What is the unit of charge?
coulomb (C)
What is the unit of energy?
joule (J)
Wht is the unit of resistance?
ohm (Ω)
What is the unit of voltage?
volt (V)
What is the unit of power?
watt (W)
What is alternating current?
Where the direction of the current goes backwards then forwards - changing direction rapidly
What is direct current?
Current that flows only in one direction
Is mains electricity alternating current (a.c.) or direct current (d.c.)
mains electricity is alternating
Does a battery / cell supply an alternating current (a.c.) or a direct current (d.c.)?
Batteries supply a direct current
Wht is the difference between an electrical conductor and an electrical insulator?
An electrical insulator is very difficult for current to pass through (it has a very hig resistance) whereas an electrical conductor like a metal is very easy for a current to pass through.
Poorly manufactured or old electrical devices could cause an electric shock. What would have to happen for you to get an electric shock from an electrical device such as a hair dryer?
The live wire would have to become displaced and touch a metal part of the device making that metal part of the device ‘live’.
Poorly manufactured or old electrical devices could cause a fire. Explain how a faulty electrical device can cause a fire.
A short circuit - where wires are damaged and can touch each other - allows a very high current to flow, which makes the wires very hot, and can start a fire.
Explain how a fuse protects electric devices (3 parts to this answer)
Too high current causes the fuse to melt and break the circuit
Explain how a circuit breaker protects electrical devices
Too high current, causes an electromagnet to break the circuit
What problem do fuses and circuit breakers protect against?
Too high currents
What is the difference between a fuse and a circuit breaker?
The whole fuse has to be replaced but a circuit breaker can just be switched to allow current to flow again.
What does it mean if an appliance is doubled insulated?
The live wire and electrical parts are separated from the user by at least 2 layers of insulating material such as plastic. Any metal parts - such as the beater on an electric mixer will fit into a plastic slot so they are also double insulated.
How does double insulation protect the user of an electrical device
There are 2 layers of insulation that would have to get broken for the user to touch the live wire which is very unlikely.
What does an earth wire connect to at each end?
One end connects to the outer casing of an electrical appliance and the other end connects to a metal pole stuck into the ground.
How does an earth wire protect the user of an iron?
If the live wire is damaged and can touch the metal part of the iron, this would become live too and touching it woul cause an electric shock. The earth wire is connected to the metal part of the iron and grounds the metal part so that it does not cause a shock. (This should then cause the fuse to melt and break the circuit)
What do earth wires and double insulation protect against?
The live wire touching the outer casing of a device
What 2 things can go wrong in circuits?
• Live wire touches casing causing possible electric shock • Short circuit causing very high current and heat - damages the circuit and can cause fire