Power

Electric power expresses the rate at which an electrical device is converting energy from one form into another. For example a room heater converts electrical energy into heat energy. The rate at which it does this is its power, expressed in watts. The symbol for watt is W.

For example, a 100 W light bulb is much brighter than a 60 W bulb because the higher-wattage bulb converts more electrical energy into light.

An average electric lamp runs at 10 W. A typical two-bar heater is rated at 2000 W, or 2 kilowatts. The power of an electrical power station (converting energy from coal into electrical energy) is measured in megawatts.

It can be shown that the power of a device is proportional to the amount of current flowing through it. It is also proportional to the voltage that is driving the current. The bigger the current and the bigger the driving force, the bigger the power.

A simple mathematical relationship exists among voltage, current, and power:

Power (W) = Voltage (V) × Current (A)

Example: A Study lamp runs on a 12 V supply and takes 0.5A. What is its power?

Power = 12x0.5 = 6 Watts.

Fill in the blanks

1)What is the power dissipated by a resistor for the following voltage and
current values?

i)   V = 10 volts,    I = 3 amperes    P = _______ watt
ii)  V = 100 volts,   I = 5 amperes    P = _______ watt
iii) V = 120 volts,   I = 10 amperes   P = _______ watt

2)What is the power dissipated by a resistor given the following resistance
and current values?

i)    R = 20 ohm,    I = 0.5 ampere   P = _______ watt
ii)   R = 560 ohms,  I = 0.02 ampere  P = _______ watt
iii)  R = 300 ohms,  I = 2 ampere     P = _______ watt

3)What is the power dissipated by a resistor given the following resistance
and current values?

i)  V = 10 volt, R = 20 ohms   P =  _______ watt
ii) V = 20 volt, R = 100 ohms  P =  _______ watt
iii) V = 25 volt,R = 100 ohms  P =  _______ watt

Answers
1)  i) 30    ii) 500   iii) 1200
2)  i) 5      ii) 2.24  iii) 1200
3)  i) 5      ii) 4       iii)  6.25