Page 141 - Class-11-Physics-Part-1_Neat

WORK, ENERGY AND POWER 127

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 t Example 6.10 Examine Tables 6.1-6.3
and express (a) The energy required to
life-nourishing energy output of the sun is also break one bond in DNA in eV; (b) The
based on the above equation. In this case kinetic energy of an air molecule (10 —21 J)
effectively four light hydrogen nuclei fuse to form in eV; (c) The daily intake of a human adult
a helium nucleus whose mass is less than the in kilocalories.
sum of the masses of the reactants. This mass
difference, called the mass defect ∆m is the
Answer (a) Energy required to break one bond
2
source of energy (∆m)c . In fission, a heavy
of DNA is
nucleus like uranium 235 U , is split by a neutron
92 − 20
10 J
into lighter nuclei. Once again the final mass is 1.6 10 − 19 J/eV ~ 0.06 eV
×
less than the initial mass and the mass difference
translates into energy, which can be tapped to
provide electrical energy as in nuclear power Note 0.1 eV = 100 meV (100 millielectron volt).
plants (controlled nuclear fission) or can be (b) The kinetic energy of an air molecule is
employed in making nuclear weapons 10 − 21 J
(uncontrolled nuclear fission). Strictly, the energy − 19 ~ 0.0062 eV
×
∆E released in a chemical reaction can also be 1.6 10 J/eV
related to the mass defect ∆m = ∆E/c . However, This is the same as 6.2 meV.
2
for a chemical reaction, this mass defect is much (c) The average human consumption in a day is
smaller than for a nuclear reaction. Table 6.3 7
lists the total energies for a variety of events and 10 J ~ 2400 kcal
phenomena. 4.2×10 3 J/kcal

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