GRADE 11 • PR PRACTICAL EL ELECTR ECTRIC IC CIR CIRCU CUITR ITRY
USES OF ELECTRICITY • The main uses of electricity are in: q Electric heating q Electric lighting q Electric motors
ELECTRIC HEATING • Electricity is very important in the process of heating, specially in household appliances like electric kettles, electric irons, toasters etc. • These appliances have heating elements. When a current is made to pass through the elements, they get heated. • The elements are usually made up of metals of very high resistivity and they have high melting points also. • The wire used for making elements is NICHROME
EL ELECTRIC LIGHTING NG • Filament lamp, tube light and other • Transducers that are used to light up object used electric current convert electrical energy to light to light up. The ‘heating element’ in energy are lamps. a light bulb is a fine wire or filament of tungsten which becomes white • There are two types of lamps hot when a suitable current is namely: passed through. Tungsten is used v Incandescent lamps because it has a very high melting temperature (3400◦C) and can be v Fluorescent lamps kept white hot without melting
INCANDESCENT LAMPS • Incandescent lamps are basically filament lamps. • These lamps consist of a heating filament normally tungsten and are filled up with inert gases so as to prevent oxidation of the tungsten metal at high temperatures. • Advantage: Ø They are very cheap • Disadvantage: Ø Only 10% of electrical energy is converted to light energy, 90% lost as heat in the surroundings. Ø These lamps cast a lot of shadows and are not advisable to be used in working places (schools or offices)
FLUORESCENT LAMPS • Fluorescent lamps do not have a filament but contains two electrodes. When current passes through the electrodes, the mercury vapour inside the glass tube emits ultra-violet light mixed with visible light. This UV light is converted into visible light by the fluorescent powder found inside the tube. • Advantage: Ø They are three times more efficient than filament lamps. Ø They are economical. Ø They have a long life approximately 3000 hours. Ø They cast soft shadows. Disadvantage: Ø They are expensive.
ELECTRIC MOTORS • An electric motor uses electrical energy to produce mechanical energy, very typically through the interaction of magnetic fields and current-carrying conductors . • Electric motors that are generally used in household tasks are electric fans, washing machines, mixers, hair dryers etc.
ELECTRICAL P POWER A AND E ENERGY Formula for power Power • Mathematically we have • Power = !"#$ %"&' • Using the definition of power ()*' (+$'& ………………(1) which is the rate of doing work • P = , or P = - ( ……….(2) or rate of energy converted. ( • Where w = work done E = energy • P = power • t = time •
POWER IN TERMS OF ELECTRICAL QUANTITIES Recalling potential difference is work done per • unit charge. • By Ohm’s law we have • V = ! " ……..(3) • V = I R……………….(8) • I = . • W = Q V……(4) / ……………………(9) Substituting (4) in (2) • • R = . 0 ……………….(10) • P = " # $ ……(5) • Substituting (8) in (7) Since current (I) is defined as the rate of flow of • charges, i.e, • P = I (I R) = I² R……..(11) • I = " $ …….(6) • Substituting (9) in (7) Substituting (6) in (5) • • P = V ² 𝑺 ………(12) • P = ( " $ ) V • P = I V……………(7)
ENER ENERGY Recalling power = !"!#$% • &'(! ) P = & …………from (2) earlier • Substituting (7) in (2) • I V = ) • • Equation (14) and (15) are & • E = I V t………………..(13) used to calculate energy dissipated Substituting (11) in (2) we have • ) I ²R = • Equation (13) is used to • & calculate energy developed • E = I ²R t…………..(14) or supplied by cell or battery. Substituting (12) in (2) we have • V² ) * = • & • E = V² 𝑺 t………………………(15)
CAL CALCUL CULATI TION O OF E ENERGY C Y CONSUM UMPTI TION • To calculate the cost of electricity consumption, the number of kilowatthours (KWh) of energy used should be known. • The kilowatthour is defined as the energy used by an appliance at a rate of 1000 watt in one hour. • 1 KWh = (1000 W) x (1 hour) = 1000 x (60 x 60 s) • = 3600000 Ws • = 3600000 J • = 3.6 Mega Joule (MJ) •
EXAMPLE 1 • Example 1: If electrical energy costs Rs 0.90 per unit, what is the • Solution cost of running an • P = 2000/1000 = 2kW airconditioner with an • E = Pt = 2kW x 8h = 16kWh average power of 2000W for 8 hours? • Cost = 0.90 x 16 = Rs 14.4
EXAMPLE 2 • Example 2: An electric heater is rated at 3 kW. • solution • Electrical energy costs 20 • E = Pt = 3kW x 0.5h = 1.5kWh cents per kW h. What is • Cost = 20 x 1.5 = 30 cts the cost of using the heater for 30 minutes?
HAZARDS 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. • It may also occur whenever there is twisting or bending of the wires.
HAZARDS OF ELECTRICITY (CONT) • 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 starts a fire. • Short circuit refers to the contact of live wire to the neutral wire.
HAZARDS OF ELECTRICITY (CONT) • 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.
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