The principle of conservation of energy states that energy cannot be created or destroyed, i.e. in an isolated system, the total energy before transformation is equal to the total energy transformation. A few examples of energy transformation are given below:
1. In an electric bulb, electrical energy is converted into light and heat energy.
2. In a microphone, sound energy is converted into electrical energy, while in a loudspeaker electrical energy is converted into sound energy.
3. In a hydroelectric plant,, water falls from a height on to a turbine causing it to turn. The turbine turns a coil in a magnetic field, thereby generating a electric current. Therefore, potential energy of the water is converted into kinetic energy of the turbine, which is converted into electrical energy.
4. In a cell, e.g. a Leclanche cell, chemical energy is converted into electrical energy. If the cell is used to light a bulb, then the electrical energy is converted into light energy.
5. In the sun, nuclear energy is converted into light and heat energy. The same occurs in all nuclear reactions in stars.
6. When a substance is heated, heat energy is converted into kinetic energy of the molecules. Part of the energy is used to do work during expansion.
7. If a tungsten filament is heated, it emits electrons. Thus, heat energy is converted into electrical energy. The effect is called thermionic emission.
8. When a current passes through a resistance, heat is generated. Thus, electrical energy is converted into heat energy, e.g. toasters, irons, geysers, etc.
9. In a bicycle pump, mechanical energy is converted into heat energy. Thus the pump gets hot. Of a gas is compressed, the mechanical work done gets converted into heat energy. Alternatively, is a gas is allowed to expand, it does work and its temperature falls as its energy is used up. If we rub our hands, heat is produced due to friction.
10. In an electric motor, electrical energy is changed into mechanical energy.
11. In a generator or dynamo, mechanical energy is converted into electrical energy.
12. In places where there are strong winds, the winds turn the blades of a wind mill, the shaft of which turns a coil in a magnetic field, generating an electric current. Thus the energy of motion of the wind is converted into mechanical energy of the wind mill, which is converted into electrical energy as a coil is made to turn in a magnetic field.
13. Energy changes for freely falling bodies and the pendulum are discussed in the next subsection. Many of the above examples will only be clearly understood after the student has read the relevant chapters.
Note: It must be noted that this principle no longer holds as mass can be converted into energy as shown by Einstein’s mass energy relation.
E = mc2
Where E = energy produced,
M = mass converted to energy, called mass defect
And c = Velocity of light.
Nuclear energy arises due to a mass defect.
The energy from the sun is called solar energy. This is due to nuclear fusion reactions.
Thus it will be more appropriate to have a mass-energy principle which states that mass-energy is always converted in the universe.