2 Overview of the Chapter AC and DCMains supplyUses of ElectricityDanger of Using main ElectricitySteps taken to protect users from electric shockPower
3 7.1: AC and DCAlternating Current: Alternating current means that the flow of electricity is constantly changing direction.Example: generators provide alternating current. So, mains supply is alternating current.Direct Current: Direct current means the current flows in one direction.Example: battery and cells provide direct current.
5 7.2: Mains Supply In our room electricity comes from mains supply. It is alternating current.Live Wire: Live wire provides the path along which the electrical energy from the power station travel. This wire is alternately positive and negative causing alternating current to flow along it.Neutral Wire: Neutral wire completes the circuit.
6 7.2.1: Ring Main CircuitEarth wire is to protect the user if an appliance develops a fault.
7 7.2.2: Difference between mains supply and battery. Mains supply provides ac (alternating current) but battery provides dc (direct current)Main supply has high voltage but battery provide low voltage.(collection of cells is called battery)
8 7.2.2: Difference between mains supply and battery. Mains supply provides ac (alternating current) but battery provides dc (direct current)Main supply has high voltage but battery provide low voltage.(collection of cells is called battery)
11 7.3: SafetyBroken 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 them causing spills.Never use electrical equipment with wet hands.
13 7.4.1: Safety Devices: FuseA fuse is usually in the form of a cylinder or cartridge which contains a thin piece of wire made from a metal that has a low melting point. If too large current flows in the circuit the fuse wire becomes very hot and melts. The fuse blows, shutting the circuit off. This prevents us getting a shock and reduces the possibility of an electrical fire.The most common fuse for domestic appliances are 3A, 5A, 7A, 9A and 13A.How to choose correct fuse:The correct fuse should be just above the working current but blows if the current is a little larger.Advantage of fuse: CheaperDisadvantages of fuse: need to replace again if blown
14 7.4.2: Safety Devices: Circuit breaker It is a modern safety device which makes the circuit open if large currents flows. Inside there is an electromagnet which controls the operation of the switch.Advantages:no need to replacecan be reset easily after fault is repaired by using a reset button.Disadvantage: costly
16 7.4.3: Safety Devices: Earth Wire If any appliance is made by using metal casing this should be connected to the earth wire.If the live wire becomes frayed or breaks and comes into contact with the casing the earth wire provides low-resistance path for the current. This current is likely to be large enough to blow the fuse and turn the circuit off. Prevents the user from getting electric shock.
18 7.4.4: Safety Devices: Double Insulation If the casing is made from an insulator such as plastic, all the electrical parts of the appliance are insulated, so that they cannot be touched by the user. The appliance is said double insulated.Appliance that have double insulation use a two-wire flex.There is no need for an earth wire.
20 7.4.5: Safety Devices: Switch Switches in mains circuits should always be included in the live wire so that when the switch is open no electrical energy can reach an appliance.If the switch is included in the neutral wire, electrical current can still enter an appliance, and could possibly cause an electric shock.
21 7.5: Heating Effect of Current: Heating elements in kettles or toasters have a high resistance.As the current passes through the heating element which has high resistance, electrical energy is transferred to heat energy and the heating element heats up.
22 7.6: Electrical Power: Power = Current × Voltage In symbol: P = IV Unit: Js-1 (joule per second) or W (watt)Any appliance is marked with power rating in Watt (or VA)A bulb rated 100 W means the bulb converts 100 J of electrical energy into 100 J of heat and light energy every second.
23 7.7: Electrical Energy converted by Appliance: Energy = Power × timeIn symbol, E= PtAs, Power = Current × VoltageSo, Energy = Current × Voltage × timeIn symbol: E = IVtUnit: J (joule)