Page 208 - WCS - Electrical

Basic electricity - Electrical power, energy and their units, calculation with
assignments
Electric Power
In mechanical terms we defined power as the rate of doing
work. The unit of power is Watt. In an electrical circuit also
the unit of electrical power is 1 Watt. In mechanical terms
1 Watt is the work done by a force of 1 N to move the body
through 1 metre in one second. In an electrical circuit, the
electromotive force overcomes the resistance and does
work. The rate of doing work depends upon the current
flowing in the circuit in amperes. When an e.m.f of one volt
- Current Intensity (Ampheres) A
i
causes a current of 1 ampere to flow the power is 1 Watt.
P - Power (Watts, Kilowatts) W, kW
Hence Power = Voltage x Current
P= V x l
W - Work, Energy (Watt hour, Kilowatt hour) wh, Kwh
Power in Watts = Voltage in Volts x Current in Amperes
t
- time (hours) h
Electric work, energy
Electrical work or energy is the product of electrical power
and time
Work in Watt seconds
seconds
W = P x t
Since 1 joule represents 1 Watt x 1 sec, which is very
small, larger units such as 1 Watt hour and 1 kilowatt hour

are used.
1 W.h = Power in Watts x time in sec V - Voltage (Volts) V Exercise 1.7.36
= 3600 Watt sec.
1 Kwh = 1000 Wh = 3600000 Watt sec
WORKSHOP CALCULATION & SCIENCE - CITS
Note: The charge for electric consumption is
the energy cost per Kwh and it varies according
to the country and states.
Table of analogies between mechanical and electrical quantities
Table of analogies between mechanical and electrical quantities
Mechanical quantity Unit Electrical quantity Unit
Force 'F’ N Voltage ‘V’ V
Velocity v = Displaceme nt m/s Current I A
Time
Time t seconds Time t seconds
m
Power P = F x v N Power P = V x i W = V x A
sec
Energy = F x v x t j = Nm Energy W = V x i x t j = W x s
W = V I V = I R
= I R W
2
V 2 = I
=
R = WR
V V
R = I =
I R
V 2 W
= =
W V
W W
= =
I 2 R
111
Example
1 Calculate the power rating of the lamp in the circuit, if 0.25 amperes of current flows and the voltage is
240 volts.
P = V x I
V = 240 Volts
I = 0.25 Amperes
Therefore Power = 240 Volts x 0.25 Amperes
= 60 Volts Ampere
But 1 Watt = 1 Volt x 1 Amphere
Therefore Power = 60 Watts
2 A current of 15 amperes flow through a resistance of 10 Ohms. Calculate the power in kilowatts
consumed.
Given that R = 10 and I = 15A
Power = V x I = I x R x I = I2 x R
Therefore Power = 152 x 10 = 2250 Watts = 2.25 kW
3 At a line voltage of 200 Volts a bulb consumes a current of 0.91 ampheres. If the bulb is on for 12 hour
calculate the work in Wh to find the work given that V = 200 Volts.
I = 0.91 Amps.
t = 12 hours
Therefore Power=V x I = 200 Volts x 0.91 Amps = 182 Watts

195

CITS : WCS - Electrical - Exercise 18