What is Energy
Definition: Energy is the ability to do work. The amount of energy possessed by a body is equal to the amount of work it can do when its energy is released. Thus, energy is defined as the capacity of doing work. Energy is a scalar quantity and it exists in various forms.
Units of energy: The units of energy are the same as that of work. In SI system, the unit of energy is joule (J). In CGS system, the unit of energy is erg.
- 1 Joule = 107 ergs
- Other units of energy in common use are watt-hour and kilowatt hour.
- 1 watt-hour = 1 watt × 1 hour
= 1 watt × 60 × 60 sec
= 3600 J
- 1 kilowatt-hour (kWh) = 3.6 × 106 Joule
- Heat energy is usually measured in calorie or kilocalorie such that
1 calorie = 4.18 J
- A very small unit of energy is electron volt(eV).
1 eV = 1.6 × 10-19 J
Principle of Conservation of Energy
- Principle of conservation of energy states that energy cannot be created or destroyed but can be changed from one form to another.
- The total energy of the universe is constant. The total energy of an isolated system is constant.
- Figure shows the transformation of energy from one form to another.
Principle of Conservation of Energy Experiment
Aim: To study the principle of conservation of energy.
Materials: Ticker tape, a polystyrene sheet, string, a 300 g slotted mass, a pulley, cellophane tape
Apparatus: Ticker timer, a.c. power supply, two retort stands with clamps, trolley, electronic balance, plane
- A friction-compensated plane is arranged as shown in Figure.
- The mass of the trolley, m1 is measured with an electronic balance.
- A slotted mass of mass, m2 = 300 g is tied to one end of a non-elastic string.
- The other end of the string is tied to one end of the trolley.
- The string is placed over the pulley and held at 0.5 m above the polystyrene sheet.
- The ticker timer is switched on and the slotted mass is released so that it falls downward, pulling the trolley down the runway.
- The ticker timer is analysed to determine the final velocity of the trolley.
- The plane is friction-compensated to minimise energy loss due to friction.
- When the slotted mass drops, it loses gravitational potential energy. The trolley and slotted mass gain kinetic energy.
- Ideally the gravitational potential energy loss, Ep equals to the kinetic energy gain, Ek. This is in accordance to the principle of conservation of energy.
- However, the experimental results shows that Ek is slightly less than Ep. This is because of unavoidable energy loss due to friction of the trolley as well as air friction.