2.7 understand the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery.

There are two different types of electricity: alternating current and direct current.

Mains electricity is alternating current (AC). Alternating current changes from one direction to another rapidly. Direct current is electricity supplied by cells and batteries, it flows in ones direction only.

2.6 use the relationship between energy transferred, current, voltage and time

2. energy transferred = current × voltage × time
   
   E=I×V×t 
   

(as power = IxV, E=IxVxt is the same as E=Pxt)

2.5 know and use the relationship between power, current and voltage and apply the relationship to the selection of appropriate fuses

2. The power of an electrical appliance can be calculated from the current that flows through it and the potential difference (voltage) across it.
   You can work out power using this equation:
   P = I × V
   P is the power (in watts, W)
   I is the current (in amperes/amps, A)
   V is the potential differences (in volts, V)
   
   
   Example
   • What is the power of a 5 A 1.5 V lamp?
   • Power = 5 × 1.5 = 7.5 W
   
   

   Working out the best fuse to use

   
   
   The equation P = I × V can be rearranged to find the current if the power and potential difference (voltage)are known:
   I = P ÷ V
   
   
   Example
   What current flows through a 1.15 kW electric fire at a potential difference of 230 V? (Remember that 1.15 kW is 1,150 W)
   Current = 1150 ÷ 230 = 5A
   
   
   Fuses come in standard ratings of 3 A, 5 A or 13 A.
   The best fuse to use in this example would be the 13A fuse. The 3A and 5A fuses would blow even when the fire was working normally.

2.4 understand that 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

Resistors work by slowing down the movement of electrons and the kinetic energy that was moving them is converted into thermal (heat) energy . This can be used to heat objects, for example, in hairdryers and heaters.

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

Insulation
All insulation is is covering a live wire (a wire that is conducting electricity) with a material that is not an electrical conductor. This means the wire is safe to touch as the current is contained within the wiring as it can't pass through the layer of insulation.


Earthing

Many electrical appliances, for example cookers, washing machines and fridges have metal cases. The earth wire creates a safe route for the current to flow through if the live wire touches the metal casing (otherwise the current would flow through the case and it is likely you would get an electric shock).

However, the earth wire is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.

Double insulation
Some appliances, for example vacuum cleaners and electric drills, do not have an earth wire. This is because they have plastic casings, or they have been designed so that the live wire cannot touch the casing. As a result, the casing cannot give an electric shock, even if the wires inside becomes loose and touched the casing.

Fuses

If a fault in the circuit causes too much current to flow, the fuse breaks the circuit. The fuse contains a piece of wire which melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

Fuses in plugs are made in standard ratings. The most common are 3 A, 5 A and 13 A. The fuse should be rated at a slightly higher current than the device needs. Foe example, if the device works at 3 A, use a 5 A fuse, if the device works at 10 A, use a 13 A fuse.

Circuit Breakers
Circuit breakers contain an electromagnet that activates if the current goes above a certain limit (different limits for different appliances). Should the current go above the limit of an appliance, the electromagnet pulls an iron switch towards it, this opens the switch, consequently breaking the circuit.

Residual current circuit breakers (RCCBs) protect some circuits. They detect a difference in the current between the live and neutral wires. RCCBs work much faster than fuses do. (For exams, you do not need to know how they work, just what they do)

2.2 understand and identify the hazards of electricity including frayed cables, long cables, damaged plugs, water around sockets, and pushing metal objects into sockets

Frayed cables
When a cable is frayed, the insulation around the wire(s) has worn down, this exposes live wires which would potentially conduct electricity, if you come into contact with a live wire you could get electrocuted.
Long cables
Long cables are easily tangled which increases resistance, they also pose a trip hazard. Also, as they are longer, they have a higher risk of overheating (you do not need to know why, you just need to know that they do)
Damaged plugs
If a plug is damaged, some of the safety features may be broken. This is hazardous.
Water around sockets
Water is a very good conductor of electricity. If energy from the circuit gets into the water it can flow through the water creating an electrocution risk
Pushing metal into sockets
Metal, like water, is also a conductor of electricity Therefore, is energy from the current touches metal the energy will travel through the metal, anyone touching/holding the metal with the current running through it may be electrocuted.