Download P1a Topic 9 Producing and Measuring Electricity Topic 10 You`re in

P1a Topic 9 Producing and Measuring Electricity
Topic 10 You’re in Charge
REVISION
Cell = 2 electrodes + 1 electrolyte. Converts chemical energy into electrical
energy.
In a dry cell the electrolyte is a paste which won’t leak out
Battery = 2 or more cells
Rechargeable batteries contain chemicals which are hazardous so should be
recycled.
Solar cells convert light energy into electrical energy.
Voltage, symbol V, is a measure of the energy transfer in units called Volts
It’s measured with a Voltmeter connected under things
V
The Voltage of a standard laboratory cell is 1.5V
Current, symbol I, is a measure of the rate of flow of electrons in units called
Amps. It’s measured with an Ammeter connected next to things
A
The Current in a standard laboratory lamp is 0.2A
There are two types of current : direct ( d.c. ) and alternating ( a.c. )
Cells produce d.c. : Electrons flow from the negative terminal to the positive
The mains supply is a.c. : Electrons flow first one way then the other.
The Current in a series circuit is the same everywhere
The supply Voltage is shared out amongst components
3V battery
0.2A
0.2A
1.5V
1.5V
The supply Current in a parallel circuit is shared amongst components
The Voltage is the same everywhere
1.5V cell
0.2A
0.2A
0.1A
1.5V
0.1A
1.5V
Voltage and Current can be varied using a variable resistor
Voltage
Lamp
Wire
Diode
Current
For a wire the Voltage and Current are directly proportional
If the Voltage is doubled the Current is doubled
Voltage = a constant x Current. The constant is called Resistance, symbol R
Resistance = Voltage and has units Ohms, Ω
Current
V
R I
Circuits can be controlled automatically with Light Dependent Resistors
( LDRs ) or Thermistors
The capacity of a battery gives us an indication of how long it will last.
It’s measured in Amp-hours, Ah
A 20Ah battery will supply 20A for 1hour or 10A for 2 hours or
1A for
hours.
A superconductor is a material which, when it is cooled to a very low
temperature, has virtually no resistance. The discovery of superconductors
led to the invention of Maglev ( Magnetic levitation ) trains. These go very fast (
500km/h ) but use very little electricity.
An electric motor changes electrical energy into kinetic energy ( movement )
A coil of wire carrying a current has to be in a magnetic field :-
Split ring commutator swaps the direction of current every half turn to keep
the motor rotating.
A motor can be made to turn faster by :1. increasing the current
2. increasing the strength of the magnets
3. having a coil with more turns
The direction of rotation can be changed by :1. swapping the connections to the battery
2. swapping the magnets over
An electric generator generates electricity from kinetic energy.
A coil of wire has to be moving in a magnetic field.
The amount of electricity can be increased by :1. turning the coil faster
2. increasing the strength of the magnets
3. having a coil with more turns
A dynamo generates electricity from kinetic energy.
A magnet moves inside a coil.
Power, symbol P, is a measure of the rate of energy transfer, measured in Watts
1 Watt = 1 Joule of energy transfer per second
P
Electric Power = Current x voltage
I V
So a typical electric kettle running from the 240V mains supply with a current of
10A has a power of 2400W.
Efficiency = Useful output power x 100%
Total input power
An electric light bulb uses 60W of electric power to produce 3W of light.
Its efficiency is 3 x 100% = 5%
60
Most electrical devices are high powered so usually we see kW written on them.
The kettle was 2.4kW. Electricity suppliers have a standard cost per kWh of
electricity, eg 5p/kWh. To work out the cost of a device use the formula :Cost = Power in kW x Time in h x Cost per kWh
eg : the kettle used for 2 minutes
Cost = 2.4 x 2 x 5p = 0.4p
60
a 60W light bulb used for 4 hours
Cost = 0.06 x 4 x 5p = 1.2p
Electrical devices in our homes are supplied with electricity from the mains via
plugs :-
The live wire is brown and is 240V higher than the blue neutral wire so is more
dangerous. Protective devices such as switches and fuses are always connected to
the live wire.
A fuse is just a thin wire which melts when too much current is in it.
Fuses are typically 3A, 5A or 13A. A kettle would have a 13A fuse in its plug.
The earth wire is green and yellow. It is connected to any metal inside a device.
Usually no current flows in this wire but if there’s a fault current is taken safely to
Earth ( and the fuse melts )
A Residual Current Circuit Breaker ( RCCB ) compares the current in the live and
neutral wires and switches off the electricity if there’s a difference caused by a
fault. RCCBs are very quick to act so are used on electric lawn mowers.
Most power stations in Britain burn coal ( a fossil fuel ) to get heat to boil water to
get steam to turn the turbines and generators. Burning coal produces carbon
dioxide which is a greenhouse gas and sulphur dioxide which leads to acid rain.
An alternative is to use nuclear power stations which don’t burn fuel but these
produce radioactive waste.
Light energy from the Sun ( Solar energy ), Wind, Waves, Tides are all called
Renewable Energy Sources because they’re always available to us. Generators
which use them produce no waste but they are inefficient so lots of them have to
be used taking up lots of land.