- Chemical Energy
- Electrical Energy
- The Equivalence of Mass and Energy
- Nuclear Energy
- The Principle of Conservation of Energy
Various forms of energy
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 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.
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
|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|
|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|
|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|
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.
Answer the following:
The casing of a rocket in flight burns up due to friction. At whose expense is the heat energy required for burning obtained? The rocket or the atmosphere?
A bolt of mass 0.3 kg falls from the ceiling of an elevator moving down with an uniform speed of 7 m s–1. It hits the floor of the elevator (length of the elevator = 3 m) and does not rebound. What is the heat produced by the impact? Would your answer be different if the elevator were stationary?
A rain drop of radius 2 mm falls from a height of 500 m above the ground. It falls with decreasing acceleration (due to viscous resistance of the air) until at half its original height, it attains its maximum (terminal) speed, and moves with uniform speed thereafter. What is the work done by the gravitational force on the drop in the first and second half of its journey? What is the work done by the resistive force in the entire journey if its speed on reaching the ground is 10 m s–1?