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Work, Power and Energy 1. Work : Work is said to be done, when a force or a component of a force causes displacement in its own direction. 2. Erg : When a force of one dyne, causes a displacement of 1 cm, in its own direction, the work done is said to be of one erg. It is a CGS unit of work. 3. Joule : When a force of one newton, causes a displacement of 1 m, in its own direction, the work done is said to be of one joule. It is a SI unit of work. 4. Relation between joule and erg : 1 J = (1 N) (1 m) = (105 dyne) (100 cm) = (107 dyne cm) = 107 ergs. 5. Bigger units of work : 1 kilojoule (kJ) = 103 J; 1 megajoule (M.J.) = 106 J; 1 gigajoule = 109 J 6. Power : The rate of doing work is called power. or The product of force and velocity is called power. 7. Watt (J/s) : When a work of 1 J is done in one second, the power is said to be of one watt. It is the SI unit of power. Class-X 1 Key points 8. Bigger units of power : (a) 1 kilowatt (kW) = 103 W (b) 1 megawatt (MW) = 106 W (c) 1 gigawatt (GW) = 109 W (d) 1 horsepower (HP) = 746 W (approx) 9. Energy : The capacity of doing work is called energy. Energy has same units as work. 10. Special Units of Energy : (a) Units of heat energy (1) Calorie : The amount of heat energy required to raise the temperature of 1 g of water through 1 C. 1 calorie = 4.2 J (2) Kilocalorie : The amount of heat energy required to raise the temperature of 1 kg of water through 1 C. 1 kilocalorie = 4200 J (b) Units of electric energy (1) Watt-hour (Wh) : When the electric power of one watt flows through a conductor for one hour, then the electric energy flowing through the conductor is one watt hour. 1 Wh = 3600 J = 3.6 kJ Class-X 2 Key points (2) Kilowatt hour (kWh) : When the electric power of one kilowatt flows through a conductor for one hour, then the electric energy flowing through the conductor is one kilowatt hour. 1 kWh = 3600000 J = 3.6 MJ (c) Units of energy for moving electrons in an electric field. Electron volt (eV) : It is the electric work done, when an electron moves through an electric field at a potential difference of 1 volt. 1 (eV) = 1.6 10 19 J. 11. Kinetic energy : The energy possessed by a body on account of its motion is called kinetic energy. 12. Potential energy : The energy possessed by a body on account of its position or configuration is called potential energy. 13. Law of conservation of energy : The energy can neither be created, nor destroyed. It may be transformed from one form to another form, but total energy of the system remains constant. 14. Dissipation of energy : The phenomenon of transformation of energy, from useful form to useless form, is known as dissipation of energy. Class-X 3 Key points
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