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Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
In the previous section we have discussed mechanical energy. We have seen that it can be classified into two distinct categories : one based on motion, namely kinetic energy; the other on configuration (position), namely potential energy. Energy comes in many a forms which transform into one another in ways which may not often be clear to us.
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Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
We have seen that the frictional force is not a conservative force. However, work is associated with the force of friction, Example 6.5. A block of mass m sliding on a rough horizontal surface with speed v comes to a halt over a distance x0.
The work done by the force of kinetic friction f over x0 is –f x0. By the work-energy theorem \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyBaiaadA % hadaqhaaWcbaaeaaaaaaaaa8qacaWGVbaapaqaaiaaikdaaaGcpeGa % ai4laiaaikdacqGH9aqpcaqGGaGaamOzaiaabccacaqG4bWaaSbaaS % qaaiaad+gaaeqaaaaa!40CA! mv_o^2/2 = {\rm{ }}f{\rm{ }}{{\rm{x}}_o}\).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. We shall see later that the internal energy is associated with the ceaseless, often random, motion of molecules.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 by10°C.
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Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
One of the greatest technical achievements of humankind occurred when we discovered how to ignite and control fire. We learnt to rub two flint stones together (mechanical energy), got them to heat up and to ignite a heap of dry leaves (chemical energy), which then provided sustained warmth. A matchstick ignites into a bright flame when struck against a specially prepared chemical surface.The lighted matchstick, when applied to a firecracker, results in a spectacular display of sound and light.
Chemical energy arises from the fact that the molecules participating in the chemical reaction have different binding energies. A stable chemical compound has less energy than the separated parts. A chemical reaction is basically a rearrangement of atoms. 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. If the reverse is true, heat is absorbed and the reaction is endothermic. Coal consists of carbon and a kilogram of it when burnt releases about 3 \(\times\) 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.
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Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
Various Forms of Energy: Heat, Chemical Energy and Electrical Energy
The flow of electrical current causes bulbs to glow, fans to rotate and bells to ring.There are laws governing the attraction and repulsion of charges and currents, which we shall learn later. Energy is associated with an electric current. An urban Indian household consumes about 200 J of energy per second on an average.
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The Equivalence of Mass and Energy
The Equivalence of Mass and Energy
Till the end of the nineteenth century, physicists believed that in every physical and chemical process, the mass of an isolated system is conserved. Matter might change its phase, e.g. glacial ice could melt into a gushing stream, but matter is neither created nor destroyed; Albert Einstein (1879-1955) however, showed that mass and energy are equivalent and are related by the relation.
E=mc2
where c, the speed of light in vacuum is approximately 3 \(\times\)108 m s–1. Thus, a staggering amount of energy is associated with a mere kilogram of matter
E = 1\(\times\) (3 \(\times\)108)2 J = 9 \(\times\)1016 J.
This is equivalent to the annual electrical output of a large (3000 MW) power generating station.
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Nuclear Energy
Nuclear Energy
The most destructive weapons made by man, the fission and fusion bombs are manifestations of
Table 6.3. Approximate energy associated with various phenomena
the above equivalence of mass and energy [Eq. (6.20)].On the other hand the explanation of the life-nourishing energy output of the sun is also based on the above equation.In this case effectively four light hydrogen nuclei fuse to form a helium nucleus whose mass is less than the sum of the masses of the reactants.This mass difference, called the mass defect \(\Delta\)m is the source of energy (\(\Delta\)m)c2.In fission, a heavy nucleus like uranium \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0raaSqaai % aaiMdacaaIYaaabaGaaGOmaiaaiodacaaI1aaaaOGaamyvaaaa!3ABF! {}_{92}^{235}U\), 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, which can be tapped to provide electrical energy as in nuclear power plants (controlled nuclear fission) or can be employed in making nuclear weapons(uncontrolled nuclear fission). Strictly, the energy \(\Delta\)E released in a chemical reaction can also be related to the mass defect \(\Delta\)m = \(\Delta\)E/c2. However, for a chemical reaction, this mass defect is much smaller than for a nuclear reaction. Table 6.3 lists the total energies for a variety of events and phenomena.
EXAMPLE 10
Examine Tables 6.1-6.3 and express (a) The energy required to break one bond in DNA in eV; (b) The kinetic energy of an air molecule (10—21 J) in eV; (c) The daily intake of a human adult in kilocalories.
ANSWER
(a) Energy required to break one bond of DNA is \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % aIXaGaaGimamaaCaaaleqabaGaeyOeI0IaaGOmaiaaicdaaaGccaWG % kbaabaGaaGymaiaac6cacaaI2aGaey41aqRaaGymaiaaicdadaahaa % WcbeqaaiabgkHiTiaaigdacaaI5aaaaOGaamOsaiaac+cacaWGLbGa % amOvaaaacqWIdjYocaaIWaGaaiOlaiaaicdacaaI2aGaamyzaiaadA % faaaa!4C62! \frac{{{{10}^{ - 20}}J}}{{1.6 \times {{10}^{ - 19}}J/eV}} \simeq 0.06eV\)
Note 0.1 eV = 100 meV (100 millielectron volt).
(b) The kinetic energy of an air molecule is \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % aIXaGaaGimamaaCaaaleqabaGaeyOeI0IaaGOmaiaaigdaaaGccaWG % kbaabaGaaGymaiaac6cacaaI2aGaey41aqRaaGymaiaaicdadaahaa % WcbeqaaiabgkHiTiaaigdacaaI5aaaaOGaamOsaiaac+cacaWGLbGa % amOvaaaacqWIdjYocaaIWaGaaiOlaiaaicdacaaIWaGaaGOnaiaaik % dacaWGLbGaamOvaaaa!4DD9! \frac{{{{10}^{ - 21}}J}}{{1.6 \times {{10}^{ - 19}}J/eV}} \simeq 0.0062eV\)
This is the same as 6.2 meV.
(c) The average human consumption in a day is \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca % aIXaGaaGimamaaCaaaleqabaGaaG4naaaakiaadQeaaeaacaaI0aGa % aiOlaiaaikdacqGHxdaTcaaIXaGaaGimamaaCaaaleqabaGaaG4maa % aakiaadQeacaGGVaGaam4AaiaadogacaWGHbGaamiBaaaacqWIdjYo % caaIYaGaaGinaiaaicdacaaIWaGaam4AaiaadogacaWGHbGaamiBaa % aa!4CED! \frac{{{{10}^7}J}}{{4.2 \times {{10}^3}J/kcal}} \simeq 2400kcal\)
We point out a common misconception created by newspapers and magazines. They mention food values in calories and urge us to restrict diet intake to below 2400 calories. What they should be saying is kilocalories (kcal) and not calories. A person consuming 2400 calories a day will soon starve to death! 1 food calorie is 1 kcal.
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The Principle of Conservation of Energy
The Principle of Conservation of Energy
We have seen that the total mechanical energy of the system is conserved if the forces doing work on it are conservative.If some of 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, as long as one accounts for all forms of energy. Energy may be transformed from one form to another but the total energy of an isolated system remains constant. Energy can neither be created, nor destroyed.
Since the universe as a whole may be viewed as an isolated system, the total energy of the universe is constant.If one part of the universe loses energy, another part must gain an equal amount of energy.
The principle of conservation of energy cannot be proved. However, no violation of this principle has been observed. The concept of conservation and transformation of energy into various forms links together various branches of physics, chemistry and life sciences.It provides a unifying, enduring element in our scientific pursuits. From engineering point of view all electronic, communication and mechanical devices rely on some forms of energy transformation.