Page 21 - 10-phy-14 Current Electricity

14. Current Electricity eLearn.Punjab

14.11 ELECTRIC POWER

The amount of energy supplied by current in unit time is
known as electric power.

Hence power P can be determined by the formula Remembering power formula
Electric power P = electrical energy/time = W/t

where W is the electrical energy given by
W= QV

Therefore, above equation becomes
Electric power P = QV/t =IV=I2 R

When current I is passing through a resistor R, the electric power
that generates heat in the resistance is given by I2 R. The unit of
electric power is watt which is equal to one joule per second
(1 Js-1 ). It is represented by the symbol W. Electric bulbs commonly
used in houses consume 25 W, 40 W, 60 W, 75 W and 100 W of
electric power.

Do you know? Example 14.7: The resistance of an electric bulb is 500 Ω.
Find the power consumed by the bulb when a potential
Although the light intensity from a 60 difference of 250 V is applied across its ends.
W incandescent light bulb appears to Solution: Given that, R= 500 Ω, V= 250 V
be constant, the current in the bulb Using the formula, I = V/R
fluctuates 50 times each second between I = 250 V/ 500 Ω = 0.5 A
-0.71 A and 0.71 A. The light appears to be and
steady because the fluctuations are too Power P = I2 R = (0.5 A)2 × 500 Ω = 125 W
rapid for our eyes to perceive.

Kilowatt-Hour

Electric energy is commonly consumed in very large quantity for the measurement of which joule
is a very small unit. Hence, a very large unit of electric energy is needed which is called kilowatt-
hour. It is defined as

The amount of energy delivered by a power of one kilowatt in one
hour is called kilowatt-hour.

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