Question

A body was supplied 1000 kcal of heat. Express this heat in megajoules.
A. 2.2 MJ
B. 3.2 MJ
C. 4.2 MJ
D. 5.2 MJ

Answer 1

Hint: Earlier it is believed that energy and work are two different phenomena. Hence earlier, scientists used to express energy (heat energy) in the form of calories and work in the form of Joules. But after many experiments by various scientists, it is shown that energy and work are the same phenomenon and energy is defined as the capacity of doing work.

Complete step by step answer:
After it is shown that both work and energy represents the same quantity, scientists discovered an equivalent factor called joule’s mechanical equivalent.
Hence 1 calorie is defined as 4.186 Joules i.e. 1 cal = 4.186 J
Now, 1kcal = 1000 cal
$\implies 1000kcal = 10^6 cal$
And since 1 cal = 4.186 J
Thus $1000 \ kcal = 4.186 \times 10^6 \approx 4.2 MJ$
Hence, option C. is correct.

Additional information:
Work: It is defined as the amount of energy used to displace a body by a force by some distance. It's S.I. unit is Joule.
Energy: Energy of a system is defined as the amount of work done in assembly of the system. Its S.I. unit is Joule.
Power: It is defined as the rate of doing work or the rate of dissipation of heat. It's S.I. unit is Watt.
Heat: Heat is also a form of energy which sets the molecules of a medium into vibration and hence gets increased in entropy and hence the object feels warm.

Note:
Physics is all about concepts. But to solve complex problems, we must have the knowledge of conversion from one unit to another. Students are advised to focus on concepts as well as try to memorize the things while practicing the subject. The basic conversions and formulas are always good to be remembered.