Power dissipation
The power dissipated in a circuit element is the rate at which it transforms electrical energy
into other forms, usually thermal energy. The power dissipated by a circuit element can be
calculated by multiplying the voltage drop across the device by the current flowing through
it.
P = VI
The unit for measuring power is the watt (W).
Sample problem
What is the power dissipated by a resistor if it has a voltage drop of 5.6V and a current of
550 mA.
Solution
P = VI
P = 5.6 V × 0.55 A
P = 3.08 W
Rounding to 2 significant figures gives an answer of 3.1 W.
Additional question
Calculate the power dissipated in the following cases.
(a) A heating element has a current of 12 A when connected to a 240 V supply.
(2.9 × 103 W)
(b) A television set on standby (ready to be activated by a remote control) draws a current
of 70 mA when connected to a 240 V supply. (17 W)
(c) A diode has a current of 120 mA when forward biased with a DC voltage of 0.70 V.
(8.4 × 10–2 W)
(d) A zener diode has a current of 400 mA with a reverse bias (breakdown) voltage of
35 V. (14 W)
Jacaranda Physics 2, 2nd edition
© John Wiley & Sons Australia, Ltd 2004
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