Physics - Domestic electricity Flashcards
What is a kilowatt hour (kWh)
The amount of energy that an appliance uses depends upon it’s power and how long it is switched on for
This is measured in kilowatt hours.
If an appliance uses 2 kilowatts of power and is on for 3 hour then it will use 6 kWh of energy (i.e. 2 x 3).
Electricity companies charge households according to how many kWh’s they use. They sometimes call these ‘units of electricity’ instead of kWh’s.
An electricity company charges 15 pence for every unit of electricity used.
An electric heater uses 3kW of power.
It is switched on for 4 hours.
How much does the electricity cost.
The heater uses 3 x 4 = 12 kWh’s of electricity.
12 kWh’s of electricity = 12 units
The cost is 15p x 12 = £1.80
A television set needs 250 Watts of power.
It is switched on for 30 minutes.
If each unit (kWh) costs 16p, how much does it cost to run the TV?
<em>Remember, you need to convert the power to kW and the time to hours.</em>
250 Watts = 0.250 kW
30 minutes = 0.5 hours
Units (kWh) used = 0.250 kW × 0.5 h = 0.125 kWh
Cost = 0.125 units × 16p = 2p
What is alternating current (a.c.) and give an example of a source of alternating current
Alternating current is one which constantly changes direction.
Mains electricity is an a.c. supply.
The UK mains supply is about 230 V. It has a frequency of 50 Hz, which means that it changes direction and back again 50 times a second.
What is direct current (d.c.) and give an example
Direct current is where current flows in only one direction constantly.
Batteries and solar cells supply d.c. electricity.
Why do we have fuses and circuit breakers
If a fault occurs in an appliance it can cause too much current to flow.
A fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts. This breaks the circuit and protects the appliance.
Once a fuse has ‘blown’ it needs to be replaced. Circuit breakers are like fuses, but can be reset.
How do you calculate the right sized fuse for an appliance?
Use the power equation, to find out how many amps of current the appliance needs.
e.g. for an appliance needing 2.5kW of power:
I (current) = Power divided by voltage
I = 2,500 (watts) divided by 230 (volts)
I = 10.9 amps
Fuses for plugs come in 3, 5 and 13 amps, so select the next one above, which is a 13 amp fuse.
Fuses,
minature circuit breakers (MCBs)
and residual current circuit breakers (RCCBs)
all have similar functions, but there are differences.
Describe how MCBs and RCCBs are different from fuses
- *MCBs**
- can be reset
- use an electromagnet to open a switch if the current goes above a certain value
- much faster than a fuse
RCCBs
- switches off the circuit when there is a difference between the currents in the live and neutral wires of the appliance.
- even more sensitive than MCBs
Electrical sockets in houses are connected together on a
ring main
Describe the features of a ring main
Each socket has 3 wires - a live, a neutral and an earth
The sockets are connected in parallel - that ensures they all get 230V, and each can be switched on and off without affecting the others
The 3 wires go in a loop (a ‘ring’) from the consumer unit (where the electricity comes into the house) around the house and back to the consumer unit.
What is the live wire on a ring main for?
The live wire carries current to the appliance at a high voltage.
Switches and fuses are always placed in the live wire.
What is the neutral wire in a ring main for?
The neutral wire carries current away from the appliance at low/zero voltage.
What is the earth wire in a ring main for?
The earth wire carries current safely into the ground if a fault develops in a metal framed appliance.
If a live wire inside an appliance, such as a cooker, comes loose and touches the metal casing it could cause an electric shock. The earth terminal is connected to the metal casing, so in that instance the current would go through the earth wire instead of causing an electric shock.
What is the voltage and frequency of mains electricity in the UK?
The voltage is 230 volts
The frequency is 50 hertz, which means the current alternates at a rate of 50 times per second
