## Physics Homework #171 Heat And Thermodynamics Quiz

This is an old physics olympiad problem, I think. The answer hinges on the spheres expanding due to heating. Sphere A raises its center of mass some and sphere B lowers its center of mass some. By conservation of energy sphere A is thus slightly colder, since more of its energy went into its gravitational potential.

It's a bit of a silly problem since the effect is extremely small. We can see this because from common experience if you heat a metal sphere \$10 C\$, the change in radius is pretty small - so small you probably won't notice without measuring it or else having something with a different expansion coefficient wrapped around the sphere. Meanwhile, if you drop a fist-sized a metal sphere by \$1cm\$, a distance much larger than such a sphere would expand with a \$10 C\$ change, the temperature change is much, much smaller than \$10 C\$. It's smaller than you can even notice, really. So the gravitational potential change is very small compared to the heat, and the difference in temperatures is minute.

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Specific Heat and Thermodynamics”.

1. The internal energy change in a system that has absorbed 2kcal of heat and done 500 J of work is
a) 6400 J
b) 5400 J
c) 7900 J
d) 8900 J

Explanation: As Q=∆U+W
∆U=Q-W=2×4.2×1000-500
∆U=8400-500=7900 J

2. 110 J of heat is added to a gaseous system, whose internal energy increases by 40 J. Then the amount of external work done is
a) 150 J
b) 70 J
c) 110 J
d) 40 J

Explanation: ∆Q=+110 J,∆U=+4J
∆W=∆Q-∆U=110-40=70J

3. The molar specific heat constant pressure of an ideal gas is 7R/2. The ratio of specific heat at constant pressure to that at constant volume is
a) 9/7
b) 8/7
c) 7/5
d) 5/7

Explanation:
Cp=7R/2
CV=CP-R=7R/2-R=5R/2
r=Cp/Cv =(7R/2)/(5R/2)=7/5

4. The change internal energy in a cyclic process is
a) Zero
b) Infinity
c) Constant
d) Unity

Explanation: The chance in internal energy in a cyclic process is zero because the system returns to its initial state.

5. It is possible that the temperature of the body changes even without giving heat to it or taking heat from it. True or false?
a) True
b) False

Explanation: During an adiabatic compression, temperature increases and in adiabatic expansion, temperature decreases, although no heat is given to or taken from the system in these changes.

6. The mechanical energy can be completely converted into heat energy but the whole of the heat energy cannot be converted into mechanical energy. True or false?
a) True
b) False

Explanation: The whole of mechanical energy can be absorbed by the molecules of the system in the form of their kinetic energy. This kinetic energy gets converted into heat. But the whole of the heat energy cannot be converted into work as a part of it is always retained by the system as its internal energy

7. Which statement is incorrect?
a) All reversible cycles have same efficiency
b) Reversible cycle has more efficiency than an irreversible one
c) Carnot cycle is a reversible one
d) Carnot cycle has the maximum efficiency of an the cycles

Explanation: Work done per cycle = Area of the loop representing the cycle
As different reversible cycles may have different loop areas, their efficiencies will also be different.

8. Which is an intensive property?
a) Volume
b) Mass
c) Refractive index
d) Weight

Explanation: An intensive property is that which does not depend on the quality of matter of the system. Refractive index is an intensive property. Volume, mass and weight are extensive properties.

9. The latent heat of vaporisation of water is 2,240 J. If the work done in the process of vaporisation of 1g is 168 J, then the increase in internal energy is
a) 2408 J
b) 2240 J
c) 2072 J
d) 1904 J

Explanation: From the first law of thermodynamics,
dQ = mL = dU + dW
dU = m L – dW = 1×2240-168
dU = 2072 J

10. If the amount of heat given to a system is 35 J and the amount of work done by the system is -15J and the amount of work done by the system is -15J, then the change in the internal energy of the system is
a) -50J
b) 20J
c) 30
d) 50J

Explanation:
∆Q=∆U+∆W
35=∆U+15
∆U=35+15=50J

11. Assertion: Reversible systems are difficult to find in real world
Reason: Most process is dissipative in nature
a) Both assertion and reason are true and reason is correct explanation of the assertion
b) Both assertion and reason are true but reason is not correct explanation of the assertion
c) Assertion is true but reason is false
d) Both assertion and reason are false

Explanation: Both the assertion and reason are true. The energy consumed is doing work against dissipative forces cannot be recovered.

12. The change in internal energy, when a gas is cooled from 927° to 27°, is
a) 100%
b) 300%
c) 200%
d) 75%

Explanation: U=nCv T
∆U/U×100=∆T/T×100
∆U/U×100=(1200-300)/1200×100=9/12×100=75%

13. During adiabatic compression of a gas, its temperature
a) Falls
b) Remains constant
c) Rises
d) Becomes zero