# Various Forms of Energy : the Law of Conservation of Energy

#### Topics

• Heat
• Chemical Energy
• Electrical Energy
• The Equivalence of Mass and Energy
• Nuclear Energy
• The Principle of Conservation of Energy

## Notes

#### Various forms of energy

Heat:

• A block of mass m sliding on a rough horizontal surface with speed v0 comes to a halt over a distance x0.
The work done by the force of kinetic friction f over x0 is –fx0.
By the work-energy theorem, (mv_o^2)/2 = fx_0.

• If we confine our scope to mechanics, we would say that the kinetic energy of the block is ‘lost’ due to the frictional force.
On examination of the block and the table we would detect a slight increase in their temperatures.

• The work done by friction is not ‘lost’, but is transferred as heat energy.
This raises the internal energy of the block and the table.

• In winter, in order to feel warm, we generate heat by vigorously rubbing our palms together.

• A quantitative idea of the transfer of heat energy is obtained by noting that 1 kg of water releases about 42000 J of energy when it cools by 10°C.

Chemical Energy:

• Chemical energy arises from the fact that the molecules participating in the chemical reaction have different binding energies.

• If the total energy of the reactants is more than the products of the reaction, heat is released and the reaction is said to be an exothermic reaction.
Example- When you freeze water you remove energy from water to lower its temperature and its phase is changed to ice, so it is a exothermic process.

• If the reverse is true, heat is absorbed and the reaction is endothermic.
Example- While melting the ice you provide energy to the ice to increase its temperature and change its phase to water, so it is a endothermic process.

• Coal consists of carbon and a kilogram of it when burnt releases about 3 × 107 J of energy.

• Chemical energy is associated with the forces that give rise to the stability of substances. These forces bind atoms into molecules, molecules into polymeric chains, etc.

• The chemical energy arising from the combustion of coal, cooking gas, wood and petroleum is indispensable to our daily existence.

The Equivalence of Mass and Energy

• Physicists believed that in every physical and chemical process, the mass of an isolated system is conserved till Albert Einstein show the relation, E = mc2 where c, the speed of light in vacuum is approximately 3 ×108 m s–1

• This equation showed that mass and energy are equivalent and are related by E = m c2.

• If there is a difference between the sum of reactants and products that difference, dm, is called mass defect.

• In case of chemical reactions the mass defect is very small and can be neglected, but in the case of nuclear reactions this becomes significant.

Nuclear Energy:

• The energy released from the nuclear reactions, either fission or fusion, is called as nuclear energy.

• Nuclear fusion and fission are manifestations of the equivalence of mass and energy.

• In fusion light atom nuclei like Hydrogen fuse to form a bigger nucleus whose mass is less than the sum of the masses of the reactants.

• In fission, a heavy nucleus like uranium 235U92, is split by a neutron into lighter nuclei. Once again the final mass is less than the initial mass and the mass difference translates into energy.

Strictly, the energy ∆E released in a chemical reaction can also be related to the mass defect ∆m = (∆E)/(c^2). However, for a chemical reaction, this mass defect is much smaller than for a nuclear reaction. Table below lists the total energies for a variety of events and phenomena.

Approximate energy associated with various phenomena

 Description Energy (J) Big Bang 1068 Radio energy emitted by the galaxy during its lifetime 1055 Rotational energy of the Milky Way 1052 Energy released in a supernova explosion 1044 Oceans's hydrogen in fusion 1034 Rotational energy of the earth 1029 Annual solar energy incident on the earth 5 × 1021 Annual wind energy dissipated near earth's surface 1022 Annual global energy usage by human 3 × 1020 Annual energy dissipated by the tides 1020 Energy release of 15-megaton fusion bomb 1017 Annual electrical output of large generating plant 1016 Thunderstorm 1015 Energy released in burning 1000 kg of coal 3 × 1010 Kinetic energy of a large jet aircraft 109 Energy released in burning 1 litre of gasoline 3 × 107 Daily food intake of a human adult 107 Work done by a human heart per beat 0.5 Turning this page 10-3 Flea hop 10-7 Discharge of a single neuron 10-10 Typical energy of a proton in a nucleus 10-13 Typical energy of an electron in an atom 10-18 Energy to break one bond in DNA 10-20

Electrical Energy:

• The flow of electrical current causes bulbs to glow, fans to rotate and bells to ring.

• Energy is associated with an electric current.

• There are laws governing the attraction and repulsion of charges and currents, which we shall learn later.

• An urban Indian household consumes about 200 J of energy per second on an average.

Principle of Conservation of Energy

• If the forces involved are non-conservative, part of the mechanical energy may get transformed into other forms such as heat, light and sound.

• However, the total energy of an isolated system does not change.

• Since the universe as a whole may be viewed as an isolated system, the total energy of the universe is constant.

• The sum of all kinds of energies in an isolated system remains constant at all times.

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