2 a) units b) Mains electricity

Question 1

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

Answer 1

Unit of current: ampere (A)
Unit of charge: coulomb (C)
Unit of energy: Joule (J) 
Unit of resistance: ohm (Ω) 
Unit of time: second (s) 
Unit of voltage or potential difference: volt (V)
Unit of Power: watt (W)

Question 2

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

Answer 2

Electricity is very useful, but it can be dangerous if it is not used safely. Broken plugs and frayed wires can expose the metal wires or parts of the plug that are carrying the electricity. Anyone touching these would get an electric shock, so they should be replaced as soon as the damage occurs. Anyone poking a metal object into a socket will also get an electric shock. Cables to electrical appliances should be kept as short as possible to prevent those causing spills. Water can conduct electricity at high voltages, so spilling water onto electrical equipment can be dangerous. Water should also be kept away from sockets and you must never use electrical equipment with wet hands.

Question 3

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

Answer 3

Insulation: Some appliances are cased with insulators like plastic rather than metal to prevent user from receiving shock. This casing is called insulation.

Double Insulation: If all the pars of an appliance are insulated in such a way , so that electric current cannot be touched by the user, the appliance is said to have double insulation.

Earthing: Many appliances have a metal casing. This should be connected to earth wire so that if the live wire becomes frayed or breaks and comes into contact with the casing, the current will pass through the earth wire rather than the user. The current in the earth wire is always large enough to blow the fuse and turning off the circuit. So the user is safe from electric shock.

Fuses: Fuse is a safety device usually in the form of a cylinder or cartridge which contains a thing piece of wire made from a metal that has low melting point. If too large a current flows in the circuit the fuse wire becomes very hot and blows, shutting the circuit off. This prevents you getting a shock and reduces the possibility of an electrical fire. One the fault in the current is corrected, it should be replaced again.

Circuit Breakers: Circuit Breaker is similar to fuses. If too large a current flows in a current a switch opens making the circuit incomplete. Once the fault in the circuit is corrected, the switch is reset, usually by pressing a reset button.

Question 4

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

Answer 4

Normal wiring in the house are said to have low resistance and the current pass through them easily. Heating elements like nichrome wire have high resistance. When current flows through them current cannot pass, and the energy is transferred to heat energy and the element heats up. We use the heating effect of current in electric kettle, iron, filament lamps etc.

Question 5

2.5 know and use the relationship:

Answer 5

power = current × voltage
P = I × V
and apply the relationship to the selection of appropriate fuses

Power is amount that represents how much voltage or energy is converted every second. It is calculated using this equation:
Power, P (in watts) = current, I (in amps) x voltage, V (in volts)
P= I x V

Question 6

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

Answer 6

energy transferred = current × voltage × time

E = I × V × t

The power of an appliance (P) tells you how much energy it converts each second. This means that the total energy (E) converted by an appliances is equal to its power multiplied by the length of time the appliance is being used.

Total energy,
E(in joules) = power, P (in watts) x time, t (in seconds)

E= P x t
Since, P = I x V
E= I x V x t

Question 7

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.

Answer 7

http://shawonnotes.com/IGCSE_Physics/physics_images/alternating-direct-current-graph.jpg

The mains electricity supply provides alternating current (a.c.). Alternating current constantly changes their direction, which is useful in electricity generator and transformers. Battery cell provide direct current (d.c.) where the current is always in the same direction.