Page 76 - Physics_Form_2

Physics for Secondary Schools

Electric power Therefore, the lamp transfers 60 J of
In many circuits, it is essential to know electrical energy into heat and light
the rate at which electrical energy is energy each second.
transferred into other forms of energy.
The rate of doing work or the rate at which Power rating of electrical appliances
energy is dissipated is called power. Every electrical appliance should carry
FOR ONLINE READING ONLY
electric energy dissipated a label stating the potential difference
electric power =
time taken for which it has been designed and the
energy transfer power an appliance can convert when
= operating at the stated potential difference.
time taken The rate at which an appliance dissipates

I Rt energy is called the rating of an appliance
2
2
= = IR
t and is usually marked on the body of an
appliance. For example, an appliance
By Ohm’s law, IR V= marked 3000 W, 240 V, dissipates energy
Therefore, at the rate of 3000 joules per second when
P = IV connected to a 240 V supply. Figure 2.38

V shows an electric iron marked 220 - 240
Using, I =
R V, 1000 W, indicating that, the appliance
V 2 dissipates energy at the rate of 1000 joules
P = per second when connected to a 220 - 240
R V supply.
The SI unit of power is joule per It is worth noting that, if the appliance
second ( Js ) or watt (W). The power is is connected to a higher voltage than the
−1
assumed to be 1 watt if 1 ampere current one indicated, it may be damaged, and if
flows against a potential difference of connected to a lower voltage it will not

1 volt, that 1 watt 1 ampere 1 volt  function properly.
. Likewise, if 1 joule of energy is being
spent per second, the power consumed by
an electrical appliance is said to be 1 watt
(1 watt = 1 joule/second).

Example 2.15
If a lamp on a 240 V supply draws a
current of 0.25 A, calculate the power
the lamp uses to transfer electrical
energy into heat and light energy.

Solution
P = IV
P = 240 V × 0.25 A = 60 W Figure 2.38: An electric iron

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Student’s Book Form One

Physics Form 2 Final.indd 70 25/10/2025 10:26