Practical Electricity

Recap… 5 important formulae:

1.

2.

3.

4.

5.

ItQ

VQW

IRV

PtE

VIP

Q = Charge (Coulomb)I = Current (Ampere)t = time (second)V = Voltage or potential difference (volt)W = Work done (Joule)R = Resistance (ohm Ω)E = Energy (Joule)P = Power (Watt)

QuestionThe heating element of an electric kettle with a resistance of 38.4 Ω is connected across a 240 V power supply. Calculate(a) the current flowing through the element;(b) the amount of heat energy produced in one minute.

Solution:(a) R = 38.4 Ω and V = 240 V.

.

V I R

AI

I

R

VI

25.64.38

240

(b) E = P t

P = V IP = 240 × 6.25P = 1500 W

E = P tE = 1500 × 1 × 60E = 90 000 JE = 90 kJ

CURRENT MONTH CHARGES Usage Rate ($) Amount ($) Total ($)

Electricity Services Reading estimated on 16-01-2010 Electricity

161 kWh

162 kWh

0.2169

0.2287

34.92

37.05 71.97

Cost of one kWh

Measuring Electricity Consumption The cost of electricity consumed is calculated

based on the number of kilowatt-hours (kWh) of electrical energy used.

One kilowatt-hour (kWh) is the amount of electrical energy used by 1 kW device in one hour.

Example of PUB bill

Example

A 1.5 kW electrical heated is used to heat a large container of water for 2 hours.

(a) Calculate the amount of electrical energy used by the heater in kWh,

(b) The cost of the electricity if the cost of 1 kWh is 22¢

Solution:

Given P = 1.5 kW, t = 2 hrs

(a) Energy used E (in kWh) = P × t

= 1.5 × 2 = 3.0 kWh

(b) The cost = 3 x 22 = 66 ¢

The figure below shows a utilities bill of a household for one month. Calculate the total cost of electricity?

The heating elements used in kettles, irons, ovens and heaters are usually made of nichrome wire because of its high resistivity and ability to withstand high temperatures.

When current flows, the wire heats up. The thermal energy is spread evenly over a large metal base.

A thermostat controls the temperature by turning the current on or off

Electric kettle

Electric iron

Electric radiator

The Uses of Electricity: Heating Elements

The filament is made of tungsten because of its high resistivity and high melting point.

The filament is very thin, giving it a higher resistance than the rest of the circuit (R=l ∕A)

When an electric current flows through the filament, the tungsten gets extremely hot (2500 oC) and generates light.

A filament lamp

The Uses of Electricity:Electrical lighting – Filament Lamp

The Dangers of Electricity

• Electric shock – touching wet electrical appliances, and – use of wire with damaged insulation

• Fire from overheating of wires due to short circuits and overloading of extension cords

Safe Use of Electricity at HomeSafety features installed at home for safe

use of electricity:1. Circuit breakers2. Three-pin plug3. Fuses4. Correct placement of switch in the circuit5. Earth wire

1. Circuit Breakers

These are safety devices that can switch off the electrical supply when there is excessive current in the circuit.

Circuit breaker

2. Three-Pin Plugs Live wire:

brown insulation Neutral wire:

blue insulation Earth wire:

green-and-yellowyellow or yellowyellow insulation

3. Fuses A fuse is a safety device that

protects electrical equipment and circuit from excessive current.

It consists of a short thin piece of wire which becomes hot and melts when the current flowing through it is greater than its rated value.

Fuse rating = 3 A, 5 A, 13 A Choose fuse with rating slightly

higher than the maximum allowable current though an electrical appliance when it is working properly.

It is connected to the live wire

A glass cartridge fuse contains a thin metal wire which melts when excessive current flows through it.

Calculate the current taken by an 800 W vacuum cleaner connected to a 240 V supply. Suggest a suitable rating for a fuse to be used to prevent this vacuum cleaner from overheating.

A33.3240

800

I

I

V

PI

VIP

Hence, the fuse rating used is 5 A

4. Switches

Switches are used to turn an electrical appliance on or off

Fuse and switches are always insert along ‘live’ wires of circuits.

Which is a correct circuit connection?

5. Earth Wire The earth wire (green and yellow) is a low-

resistance wire and is usually connected to the metal casing of the appliance.

Earthing prevents users from getting an electric shock if the live wire is not properly connected and touches the metal casing of the appliance.