Practical electricity

use of electricity in heating, lighting and motors Electric kettles, cookers, electric irons and immersion heaters are appliances which make use of the heating effect of an electric current. There is a heating element inside all these appliances. This element is usually made of metal wire with a higher resistance. When electrons pass through the metal wire, they give some of their energy to the atoms in the wire and make them vibrate more, so the wire gets hotter. For the same current flowing in a circuit, the greater the resistance, the hotter it becomes – this is why an electric fire glows red-hot while the connecting wires stay cool.

Lighting Filament lamp, tube light and other light up object used electric current to light up. The ‘heating element’ in a light bulb is a fine wire or filament of tungsten which becomes white hot when a suitable current is passed through. Tungsten is used because it has a very high melting temperature (3400◦C) and can be kept white hot without melting.

motors An electric motor uses electrical energy to produce mechanical energy, very typically through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on vehicles often perform both tasks. Many types of electric motors can be run as generators, and vice versa. Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives.

Electrical power and energy The e.m.f (voltage) of a supply tell us about how much energy is transfer to charges flowing around the circuit. The greater the current flowing around the circuit, the faster that energy is transferred. Hence the rate at which the energy transferred in the circuit ( the power P) depends on both e.m.f V of the supply and the current I that it pushes round the circuit. The following equation shows how to calculate power. Power (w) = current (A) x p.d. (V) in symbol P = V x I

energy transformed (J) = current (I) x p.d. (V) x time (t) Since energy transferred = power x time, we can modify the equation P = VI to give an equation for energy transformed E: energy transformed (J) = current (I) x p.d. (V) x time (t) In symbol E = VIt Example: An electric fan runs from the 230V mains. The current flowing through it is 0.4A. (a) At what rate is electrical energy transformed by the fan? P = VI = 230 x 0.4 = 92W (b) How much energy is transformed in 1 minute? E = VIt = P x t = 92 x 60 = 5520J

Cost of electricity Joule is the very small unit of energy, the energy consumed in household appliances is measured in kilowatt-hours (kWh). One kilowatt-hour is the amount of energy that is used by 1000W appliance in one hour. If you look at the electricity meter at home, you will find that it is marked in kilowatt-hour (kWh). It is therefore called kilowatt-hour meter. E = P(kW) x t(h) = kWh

What is the cost of using the heater for 30 minutes? Example 1: If electrical energy costs $0.90 per unit, what is the cost of running an air-conditioner with an average power of 2000W for 8 hours? P = 2000/1000 = 2kW E = Pt = 2kW x 8h = 16kWh Cost = 0.90 x 16 = $14.4 Example 2: An electric heater is rated at 3 kW. Electrical energy costs 20 cents per kW h. What is the cost of using the heater for 30 minutes? E = Pt = 3kW x 0.5h = 1.5kWh Cost = 20 x 1.5 = 30 cents

Dangers of electricity Damaged insulation Normally, the wires of electrical cables are insulated in rubber. However, the insulating materials deteriorate with time and use. The wires may be exposed which can cause electric shock to the user if the user touches the wires accidentally.

Overheating of cables The overheating of cables refers to the unusually large current flowing in the conducting wires under certain conditions such as a short circuit or overloading. The large amount of heat generated can melt the insulation and star a fire.

Damp conditions Many electrical accidents occur in damp conditions such as those in a wet bathroom. Since the humidity in the bathroom is higher especially when you are taking bath, the water (moisture) provides a conducting path for a large current to flow through the human body in the bathroom. The body’s resistance depends on whether the skin is wet or dry. With dry skin, the typical body resistance is around 200kW. However, it may drop below 1kW if the skin wet. With a such low resistance, the mains voltage (230V) can result deadly current of 0.23A flowing through the body.

Safe use of electricity in the home Fuses A fuse is a safety device which is made of a piece of copper wire. If a fault occurs and too much current flows, the fuse wire melts before anything else is damaged or starts a fire. Fuse must be connected to the live wire of a circuit. When the fuse is blown, the electrical appliance is disconnected from high voltage (live wire). We can then safely touch the exposed live wire on the appliance. Fuse rating The fuse rating is the maximum current that the fuse can carry without melting. Only certain values are available e.g. 5A, 13A, 15A and 30A.

Example: An electrical fan has a rating 0f 460W on 230V mains what rate of fuse should be fitted in the plug? P = VI I = P / V = 460 / 230 = 2A Therefore 3A fuse should be fitted. The fuse rating should be slightly larger than the working current of an appliance under normal operations. Switches A switch is used to disconnect or connect an electrical appliance to the main supply. In the case of leakage of current or exposed live wire, a switch can be used as a safety device to cut off the current. Switch should be installed on the live wire so that the electrical appliance is disconnected from high voltage (live wire) when the switch is open.

Circuit breakers Nowadays, many households are fitted with circuit breakers rather than fuses. They are thermal or electromagnetic switches which cut off the current when it exceeds a certain limit. If a fault in the circuit occurs, the circuit breakers automatically switches off the appliance. It can be reset only after the fault has been fixed. Some modern houses are installed with Earth Leakage Circuit Breakers (ELCB). They are very sensitive electromagnetic switches. They cut off current in the live wire when a very small leakage of current (typically 30mA) is detected in the earth wire.

Double insulation Double insulation means that user is protected from electric shock by two independent layers of insulation. Some electrical appliances are double insulated by using plastic or other insulating materials. They only have two pin plug. The cable only consists of two wires that is live and neutral wire. Earth wire is not needed because its body is insulated and protected from flowing current into its body.

Ear thing metal cases Some electrical appliances body are made up of metal casing. If there is a leakage of electricity, the user may get electric shock if live wire is connected into metal casing. So Earth wire should connect to the metal casing of electrical appliance. The earth wire allows the current to flow through it to the ground. It can protect the user from fatal electric shock. Therefore we have to use the three pin plugs to connect the metal casing electrical appliances and three wire live, neutral and earth.

Wiring a mains plug Colour codes for the wires from the mains: LIVE: BROWN NEUTRAL: BLUE EARTH: GREEN AND YELLOW

The correct wiring of mains plug shown below The correct wiring of mains plug shown below. Connect the brown wire into live terminal, blue wire into neutral terminal and yellow/green wire into earth terminal and tighten the terminal screws. Then put the cover back on.

A wire connected to a source of voltage Live wire A wire connected to a source of voltage   Neutral wire A wire which completes the circuit, although current passes through the wire it remains zero potential. Earth wire It is a safety wire which connects the metal body of electrical appliance to the earth.