Electricity Past Paper PDF

11.8 ELECTRIC POWER You have studied in your earlier Class that the rate of doing work is power. This is also the rate of consumption of energy. Equation (11.21) gives the rate at which electric energy is dissipated or consumed in an electric circuit. This is also termed as elect...

11.8 ELECTRIC POWER You have studied in your earlier Class that the rate of doing work is power. This is also the rate of consumption of energy. Equation (11.21) gives the rate at which electric energy is dissipated or consumed in an electric circuit. This is also termed as electric power. The power P is given by P = VI Or P = I2R = V2/R (11.22) The SI unit of electric power is watt (W). It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. Thus, 1 W = 1 volt × 1 ampere = 1 V A (11.23) The unit ‘watt’ is very small. Therefore, in actual practice we use a much larger unit called ‘kilowatt’. It is equal to 1000 watts. Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (W h). One watt hour is the energy consumed when 1 watt of power is used for 1 hour. The commercial unit of electric energy is kilowatt hour (kW h), commonly known as ‘unit’. 1 kW h = 1000 watt × 3600 second = 3.6 × 106 watt second = 3.6 × 106 joule (J) More to Know! Many people think that electrons are consumed in an electric circuit. This is wrong! We pay the electricity board or electric company to provide energy to move electrons through the electric gadgets like electric bulb, fan and engines. We pay for the energy that we use. Example 11.12 An electric bulb is connected to a 220 V generator. The current is 0.50 A. What is the power of the bulb? Solution P = VI = 220 V × 0.50 A = 110 J/s = 110 W. Example 11.13 An electric refrigerator rated 400 W operates 8 hour/day. What is the cost of the energy to operate it for 30 days at Rs 3.00 per kW h? Electricity 191 2024-25