Page 140 - Class-11-Physics-Part-1_Neat

126 PHYSICS

unlike the conservative force, W depends on Chemical energy arises from the fact that the
nc
the particular path i to f. t molecules participating in the chemical reaction
have different binding energies. A stable chemical
6.10 VARIOUS FORMS OF ENERGY : THE LAW compound has less energy than the separated parts.
OF CONSERVATION OF ENERGY
A chemical reaction is basically a rearrangement
In the previous section we have discussed of atoms. If the total energy of the reactants is more
mechanical energy. We have seen that it can be than the products of the reaction, heat is released
classified into two distinct categories : one based and the reaction is said to be an exothermic
on motion, namely kinetic energy; the other on reaction. If the reverse is true, heat is absorbed and
configuration (position), namely potential energy. the reaction is endothermic. Coal consists of
Energy comes in many a forms which transform carbon and a kilogram of it when burnt releases
into one another in ways which may not often 7
about 3 × 10 J of energy.
be clear to us.
Chemical energy is associated with the forces
6.10.1 Heat that give rise to the stability of substances. These
forces bind atoms into molecules, molecules into
We have seen that the frictional force is not a
polymeric chains, etc. The chemical energy
conservative force. However, work is associated
arising from the combustion of coal, cooking gas,
with the force of friction, Example 6.5. A block of
mass m sliding on a rough horizontal surface wood and petroleum is indispensable to our daily
existence.
with speed v comes to a halt over a distance x .
0 0
The work done by the force of kinetic friction f
6.10.3 Electrical Energy
over x is –f x . By the work-energy theorem
0 0
The flow of electrical current causes bulbs to
2
m v /2 = f x . If we confine our scope to glow, fans to rotate and bells to ring. There are
0
o
mechanics, we would say that the kinetic energy laws governing the attraction and repulsion of
of the block is ‘lost’ due to the frictional force. charges and currents, which we shall learn
On examination of the block and the table we later. Energy is associated with an electric
would detect a slight increase in their current. An urban Indian household consumes
temperatures. The work done by friction is not about 200 J of energy per second on an average.
‘lost’, but is transferred as heat energy. This
raises the internal energy of the block and the 6.10.4 The Equivalence of Mass and Energy
table. In winter, in order to feel warm, we Till the end of the nineteenth century, physicists
generate heat by vigorously rubbing our palms believed that in every physical and chemical
together. We shall see later that the internal process, the mass of an isolated system is
energy is associated with the ceaseless, often conserved. Matter might change its phase, e.g.
random, motion of molecules. A quantitative idea glacial ice could melt into a gushing stream, but
of the transfer of heat energy is obtained by matter is neither created nor destroyed; Albert
noting that 1 kg of water releases about 42000 J Einstein (1879-1955) however, showed that mass
of energy when it cools by10 °C.
and energy are equivalent and are related by
6.10.2 Chemical Energy the relation
One of the greatest technical achievements of E = m c 2 (6.20)
humankind occurred when we discovered how where c, the speed of light in vacuum is
–1
8
to ignite and control fire. We learnt to rub two approximately 3 ×10 m s . Thus, a staggering
flint stones together (mechanical energy), got amount of energy is associated with a mere
them to heat up and to ignite a heap of dry leaves kilogram of matter
8 2
16
(chemical energy), which then provided E = 1× (3 ×10 ) J = 9 ×10 J.
sustained warmth. A matchstick ignites into a This is equivalent to the annual electrical output
bright flame when struck against a specially of a large (3000 MW) power generating station.
prepared chemical surface. The lighted
6.10.5 Nuclear Energy
matchstick, when applied to a firecracker,
results in a spectacular display of sound and The most destructive weapons made by man, the
light. fission and fusion bombs are manifestations of

2018-19