Mains Electricity

a) Units
2.1 use the following units: ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V), watt (W)


b) Mains electricity
2.2 recall the hazards of electricity including frayed cables, long cables, damaged plugs, water around sockets, and push metal objects into sockets

frayed cables-damaged insulation:

electrical appliances can only work if they have wires to carry electricity from the voltage supply. These wires are usually insulated with rubber and they are wound together to form a cable enclosed by PVC or rubber. Insulating materials become worn with time and use. For e.g. the electrical cables of the hair dryer and electric iron usually get bent and twisted because of the way they are used. This might cause the electrical insulation to crack and break, thus exposing the conducting wires inside. The exposed live wire can cause severe electric shock to the user if it is touched. In this case, it can lead to serious injury or even death. 

long cables: http://www.enotes.com/science/q-and-a/an-electric-circuit-why-dangerous-use-long-cables-188815

damaged plugs: It can cause a spark, causing fires. The exposed wires could also expose you to an electric shock. 

water around sockets: water can conduct electricity and it provides a conducting path for a large current to flow, it could electrocute you if you were to touch it. 

pushing metal objects into sockets: bad idea, metal conducts electricity, so it'd be a suicide mission to electrocution. 

2.3 describe the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances

• insulation: non-conducting material that provides electric isolation of 2 parts. e.g. a frying pan has a wooden handle
• double insulation: is a safety feature in an electrical appliance that can substitute the Earth wire. Devices that have double insulation normally use a 2-pin plug as only the live and neutral wires are required for the appliance. This safety feature provides 2 levels of insulation: 1) the electric cable is insulated from the internal components of the appliance. 2) the internal components are also insulated from the external casing. Appliances with this feature normally have non-metallic casings. domestic appliance: hairdryer, mains radio, fan   Symbol:   (A simple way of think of it is that the individual live, neutral and earth wires are insulated, then they are insulated as a bundle together again. So that's double insulation for you. If you were to remove some insulation of your wires at home, you'll find the 3 wires inside are insulated individually too.)
• Earthing: the Earth wire (green and yellow) is a low-resistance wire. It is usually connected to the metal casing of the appliance. If there is a fault-e.g. the live wire is not properly connected and it touches the metal casing of the appliance-the user could get an electric shock. Earthing prevents this from happening. The large current that flows from the live wire through the metal casing to the earth wire will blow the fuse and cut off the supply to the appliance.  domestic appliance: washing machine, electric stove
• Fuses: is a safety device included in an electrical circuit to prevent or stop (it breaks circuit) excessive current flow. A fuse consists of a short, thin piece of wire which becomes hot and melts when the current flowing through it is greater than its rated value. a) Fuses should have a current rating just slightly higher than the current an electrical appliance will use under normal conditions b) a fuse should be connected to the live wire. c) before you change a fuse, always switch off the mains power supply. domestic appliance: connected to lighting circuit
• circuit breakers: safety devices that can switch off the electrical supply in a circuit when there is an overflow of current. Without the circuit breaker, this surge of current can cause damage to home appliances or even start a fire.

2.4 know some of the different ways in which electrical heating is used in a variety of domestic contexts

electrical heating in 'domestic contexts':

• boil water, cook food
• iron clothes
• electric radiator
• kettle, oven, iron, electric hotplate (for cooking)...

2.5 understand that a current in a resistor results in the electrical transfer of energy and an increase in temperature 

2.6 recall and use the relationship:

power= current x voltage

P = I x V

2.7 use the relationship between energy transferred, current, voltage and time:

energy transferred = current x voltage x time

E= I x V x t

2.8 recall that mains electricity is alternating current (a.c.) and understand the difference between this and the direct current (d.c.) supplied by a cell or battery

A.C. (alternating current)- current changes its direction of flow periodically. So the same end becomes positive terminal for one cycle, then becomes the negative terminal in the second cycle. So the current flows in forward direction, and again in reverse direction.

D.C (direct current)- the current flows in the same direction

They are represented in circuits with these symbols:
The DC one below shows a battery, as you can see there are two cells connected. (a battery is basically two or more cells connected), the AC shows a squiggly line in a circle-that just shows the power supply is AC current. So like what the specification says-it's from the mains electricity, like what your household appliances would be connected to in the sockets.
[So when people say smth is connected to the mains-it's basically your wall socket--that supplies AC current.]

As you can see, current in a DC circuit goes one way, it AC current, it goes in both directions. Watch video below to get a better understanding. :)