Question

When 20 cal of heat is supplied to a system, the increase in internal energy is 50 J. If the external work done is 35 J, the mechanical equivalent of heat is:

• A. 4.25 J/cal
• B. 1.26 J/cal
• C. 4.92 J/cal
• D. 2.1 J/cal

Answer 1

Answer: (A)

4.25 J/cal

According to first law of thermodynamics $J\Delta Q=\Delta W+\Delta U$, where $J$ is the mechanical equivalent of heat.
$J\ast 20=50+35$
$J=4.25J/cal$