USES OF ELECTRICITY The main uses of electricity are in: q Electric - - PowerPoint PPT Presentation

GRADE 11

• PR

PRACTICAL EL ELECTR ECTRIC IC CIR CIRCU CUITR ITRY

USES OF ELECTRICITY

• The main uses of electricity are in:

qElectric heating qElectric lighting qElectric 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

light up object used electric current to light up. The ‘heating element’ in a light bulb is a fine wire or filament

• f 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

• Transducers

that are used to convert electrical energy to light energy are lamps.

• There

are two types

• f

lamps namely: vIncandescent lamps vFluorescent lamps

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

• xidation of the tungsten metal at high temperatures.
• Advantage:

ØThey are very cheap

• Disadvantage:

ØOnly 10% of electrical energy is converted to light energy, 90% lost as heat in the surroundings. ØThese 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

Power

• Using the definition of power

which is the rate of doing work

• r rate of energy converted.

Formula for power

• Mathematically we have
• Power = !"#$ %"&'

()*' (+$'&………………(1)

• P = ,

(

• r P = -

(……….(2)

• 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.

• V = !

"……..(3)

• W = Q V……(4)
• Substituting (4) in (2)
• P = " #

$ ……(5)

• Since current (I) is defined as the rate of flow of

charges, i.e,

• I = "

$…….(6)

• Substituting (6) in (5)
• P = ("

$) V

• P = I V……………(7)
• By Ohm’s law we have
• V = I R……………….(8)
• I = .

/……………………(9)

• R = .

0……………….(10)

• Substituting (8) in (7)
• P = I (I R) = I² R……..(11)
• Substituting (9) in (7)
• P = V²

𝑺 ………(12)

ENER ENERGY

• Recalling power = !"!#$%

&'(!

• P =

) & …………from (2) earlier

• Substituting (7) in (2)
• I V = )

&

• E = I V t………………..(13)
• Substituting (11) in (2) we have
• I ²R =

) &

• E = I ²R t…………..(14)
• Substituting (12) in (2) we have
• V²

* = ) &

• E = V²

𝑺 t………………………(15)

• Equation (14) and (15) are

used to calculate energy dissipated

• Equation

(13) is used to calculate energy developed

• r supplied by cell or battery.

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 cost

• f

running an airconditioner with an average power

• f

2000W for 8 hours?

• Solution
• P = 2000/1000 = 2kW
• E = Pt = 2kW x 8h = 16kWh
• Cost = 0.90 x 16 = Rs 14.4

EXAMPLE 2

• Example

2: An electric heater is rated at 3 kW.

• Electrical energy costs 20

cents per kW h. What is the cost

• f

using the heater for 30 minutes?

• solution
• E = Pt = 3kW x 0.5h = 1.5kWh
• Cost = 20 x 1.5 = 30 cts

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.