2 Electricity
(a) Units
(b) Mains electricity
(c) Energy and voltage in circuits
(d) Electric charge
(a) Units
2.1 use the following units:
● ampere (A)
● coulomb ©
● joule (J)
● ohm (Ω)
● second (s)
● volt (V)
● watt (W)
(b) Mains electricity
2.2 understand how the use of insulation, double insulation, earthing, fuses and circuit
breakers protects the device or user in a range of domestic appliances
The brain controls the body via electrical impulses which are carried in the nerves. On top of
this the body is 70% water and electricity flows through water more easily than through air.
So even small electrical impulses can damage the nerves (connecting brain to muscle) and
this can kill you (heartbeat is controlled electrically).
It is not the size of voltage that matters it is the size of the current. But the higher the voltage
the higher the chance of the current jumping across a gap to a person nearby.
Earthing
In a socket and plug there are three
wires: the live, neutral and earth wire.
The live wire contains the a.c. going to
the appliance. The neutral wire simply
completes this circuit.
, ● The cable grip/clamp is there for mechanical stability.
● The pins are made of brass as it is both cheap and an electrical conductor.
● The plug case is made of plastic so it won’t conduct as plastic is an electrical
insulator.
The earth wire protects users from electrical shock when appliances develop faults and the
part you touch could become live. If the live wire becomes loose and touches the metal case
of the appliance a larger current flows through the earth wire into the device (instead of
user). This blows the fuse breaking the circuit.
Fuses
A fuse contains a thin wire which melts if the current is too high.
Because it is connected to the live wire this then breaks the circuit. So
no electricity can flow through the appliance so it stops working and
danger is averted.
Insulation of wires
The insulator (rubber or plastic) stops current from
coming in contact with human.
Double Insulation
A double insulation appliance has a plastic case,
without any wire connected to the case. This
means that the case cannot become live,
because plastic does not conduct electricity → no
need for an earth wire ← this is the symbol
Circuit breakers (RCCBs - residual current circuit breakers)
A circuit breaker uses electromagnets (or bimetallic strips) and performs a similar job to a
fuse. If the current flowing through a device is too large then a switch opens making the
circuit incomplete. The switch can be reset instead of needing replacing like a fuse.
2.3 understand why a current in a resistor results in the electrical transfer of energy and
an increase in temperature, and how this can be used in a variety of domestic contexts
Current in metals increases temperature
● Atoms are always vibrating, the hotter the atom the more vibrations
, ● As the free, delocalised electrons move through the wire due to current they collide
with the ions and with each collisons some energy is transferred to the ions so they
vibrate more
● Therefore wire gets hotter
→ more current = more heat
Things that affect heat of wire
1. Current
2. Voltage
3. Type of wire
Low resistance means more current is going through something. (link to parallel circuits //
current choses easiest path) this is worth a lot as in P = I^2R it is squared so it has a big
impact → thus more energy transferred so better heater.
For a good heater you need
1. Low resistance
2. High current
3. → High power
Experiment : how does temperature change with current
Independent : current
Dependent : temperature
Control : type of wire, room temperature, time period
Method
1. Set up circuit
2. Turn on power pack at low voltage
3. Record current and temperature in results table
4. Increase voltage and obtain more recordings
2.4 know and use the relationship between power, current and voltage: power = current ×
voltage P = I × V and apply the relationship to the selection of appropriate fuses
Power is the rate of transferring energy, appliances transfer energy at different rates.
(a) Units
(b) Mains electricity
(c) Energy and voltage in circuits
(d) Electric charge
(a) Units
2.1 use the following units:
● ampere (A)
● coulomb ©
● joule (J)
● ohm (Ω)
● second (s)
● volt (V)
● watt (W)
(b) Mains electricity
2.2 understand how the use of insulation, double insulation, earthing, fuses and circuit
breakers protects the device or user in a range of domestic appliances
The brain controls the body via electrical impulses which are carried in the nerves. On top of
this the body is 70% water and electricity flows through water more easily than through air.
So even small electrical impulses can damage the nerves (connecting brain to muscle) and
this can kill you (heartbeat is controlled electrically).
It is not the size of voltage that matters it is the size of the current. But the higher the voltage
the higher the chance of the current jumping across a gap to a person nearby.
Earthing
In a socket and plug there are three
wires: the live, neutral and earth wire.
The live wire contains the a.c. going to
the appliance. The neutral wire simply
completes this circuit.
, ● The cable grip/clamp is there for mechanical stability.
● The pins are made of brass as it is both cheap and an electrical conductor.
● The plug case is made of plastic so it won’t conduct as plastic is an electrical
insulator.
The earth wire protects users from electrical shock when appliances develop faults and the
part you touch could become live. If the live wire becomes loose and touches the metal case
of the appliance a larger current flows through the earth wire into the device (instead of
user). This blows the fuse breaking the circuit.
Fuses
A fuse contains a thin wire which melts if the current is too high.
Because it is connected to the live wire this then breaks the circuit. So
no electricity can flow through the appliance so it stops working and
danger is averted.
Insulation of wires
The insulator (rubber or plastic) stops current from
coming in contact with human.
Double Insulation
A double insulation appliance has a plastic case,
without any wire connected to the case. This
means that the case cannot become live,
because plastic does not conduct electricity → no
need for an earth wire ← this is the symbol
Circuit breakers (RCCBs - residual current circuit breakers)
A circuit breaker uses electromagnets (or bimetallic strips) and performs a similar job to a
fuse. If the current flowing through a device is too large then a switch opens making the
circuit incomplete. The switch can be reset instead of needing replacing like a fuse.
2.3 understand why a current in a resistor results in the electrical transfer of energy and
an increase in temperature, and how this can be used in a variety of domestic contexts
Current in metals increases temperature
● Atoms are always vibrating, the hotter the atom the more vibrations
, ● As the free, delocalised electrons move through the wire due to current they collide
with the ions and with each collisons some energy is transferred to the ions so they
vibrate more
● Therefore wire gets hotter
→ more current = more heat
Things that affect heat of wire
1. Current
2. Voltage
3. Type of wire
Low resistance means more current is going through something. (link to parallel circuits //
current choses easiest path) this is worth a lot as in P = I^2R it is squared so it has a big
impact → thus more energy transferred so better heater.
For a good heater you need
1. Low resistance
2. High current
3. → High power
Experiment : how does temperature change with current
Independent : current
Dependent : temperature
Control : type of wire, room temperature, time period
Method
1. Set up circuit
2. Turn on power pack at low voltage
3. Record current and temperature in results table
4. Increase voltage and obtain more recordings
2.4 know and use the relationship between power, current and voltage: power = current ×
voltage P = I × V and apply the relationship to the selection of appropriate fuses
Power is the rate of transferring energy, appliances transfer energy at different rates.
