Home Electric Circuits

Electric energy required to operate home electric appliances is obtained from the 

national electric grid. Electric energy generated by electric power stations are 

raised to high voltages such as 132 kV or 220 kV using step-up transformers and 

distributed throughout the island. In distribution sub-centres, these high voltages 

are lowered to voltages such as 33 kV or 11 kV and ultimately they are lowered 

down to 230 V or 110 V before supplying to households. Electricity provided to houses is in 

the form of an alternating current with a frequency of 50 Hz for 230 V and 60 Hz for 110 V

A – Distribution wire    B – Supply cable

C – Overload circuit breaker (or Service fuse)  D – Electric meter

E – Isolator (or Main fuse with main switch)  F – Residual current circuit 

breaker or trip switch (RCCB)

G – miniature circuit breakers or fuses (MCB) H – Switch

I – plug socket    J  – Earth wire

K – Light bulb

Efficiency of electric appliances and conserving power

In many instances, the same purpose can be achieved using various different 

appliances. In order to get illumination we can use filament bulbs, fluorescent 

light tubes or CFL lights (compact fluorescent lights). Choosing a more efficient 

appliance helps us to save energy. A few different types of bulbs giving the same 

illumination, their power output and life times are given in Table

According to Table, it is advantageous to use LED bulbs as light sources. 

However, the use of good quality LED bulbs is limited due to their high initial cost.

Similarly, the efficiencies of cookers used to prepare food are different from one to 

another due to varying amounts of heat wastage. Old cookers that use heating coils 

are the lowest in efficiency. Emersion heater is highly efficient for heating water. 

The reason is that, all the heat generated in the heater is transferred to water when 

using it. Heaters which contains hot plates such as rice cooker is more efficient 

because heat loss is less from them. Although microwave ovens cannot be used 

to cook all types of food, they are very efficient since they produce heat inside the 

food items. In addition to these, induction cookers with high efficiencies are now 

available in the market. In these devices, the variable magnetic field emitted by the 

cooker generates heat only at the bottom of the cooking utensil.

Power output of an electric appliance

We know that power is the work done in a unit time. 

Just like in mechanical work, power is the rate of doing work in work done by 

electricity too. That is, power is the work done in a unit time or the energy consumed 

in a unit time.

Therefore the rate of energy consumption P by an electric appliance operated with 

a voltage V and drawing a current I is given by the following equation.

Power = voltage ×current

P = VI

When the voltage, V is measured in Volts (V) and the current, I in Amperes (A) the 

power, Pis given in Watts (W). 

Example 1

When a filament bulb is connected across a voltage difference of 12 V, a current of 

2 A flows through it. What is the power of the bulb?

Power  P= VI

= 12× 2 W

P= 24 W

Power of the bulb is 24 W.

Power and Energy of Electric Appliances

We use electric energy to do various jobs in our daily lives conveniently. On all 

these occasions we convert electric energy into another form of energy to suit our 

requirements. This energy conversion takes place in various electric appliances. 

The instruments used for these energy conversions are known as electric appliances. 

The main forms of energy conversions that take place in some electric appliances 

used in daily life are given in the Figure