CBSE Class 12 Physics Heat and Thermodynamics Assignment

CBSE Class 12 Physics Heat and Thermodynamics Assignment. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Comprehension-1

In fluids heat transfer takes place and molecules of the medium takes very active part. The molecules take energy from high temperature zone and move towards low temperature zone. This method is known as convection, when we require heat transfer with fast pase, we use some mechanism to make the flow of fluid on the body fast. The rate of loss of heat is proportional to velocity of fluid (v), and temperature difference (ΔT) between the body and fluid, of course more the surface area of body more the rate of loss of heat. We can write rate of loss of heat

dQ/dt = KAvΔT where K is positive constant. Now answer the following questions

1. A body is being cooled with fluid. When we increase the velocity of fluid 4 times and decrease the temperature difference 1/2 time, the rate of loss of heat increases
(a) four times (b) two times (c) six times (d) no change

2. In the above question if mass of the body increased two times, without change in any of the other parameters, the rate of cooling
(a) decreases (b) increases (c) no effect of change of mass (d) none of these

Comprehension-2

When a body at temperature T is kept in a enclosure of temperature T0, rate of cooling is given by dT− dt = K (T – T₀). This is known as Newton’s law of cooling.A block is kept in a room at temperature 25°C. Block is heated and its temperature rises at the rate of 1°C/sec when heater is switched on and falls at the rate of 0.1°C/sec just after heater is switched off. Assuming Newton’s law of cooling to be valid. (Heat capacity of body is 50 J/°C)

3. Power of the heater is
(a) 100 W (b) 50 W (c) 40 W (d) 80 W

4. What power radiated by block just after heater is switched off
(a) 5 J/sec (b) 10 J/sec. (c) 2.5 J/sec. (d) 6 J/sec.

5. What is the power radiated by the block when temperature of block is 30° assuming its temperature was 35° when heater was switched off.
(a) 3 W (b) 5 W (c) 4 W (d) 2.5 W

Comprehension-3

When a thermodynamic process is shown on P-V diagram, area under curve represents work done during process. During cyclic process work done is area enclosed. The P-V graph for a thermodynamical system is shown in figure.

6. The work done by the system in the process A to B is
(a) 90 J (b) 60 J (c) 0 J (d) 30 J

7. The work done in the process B to C is
(a) – 90 J (b) – 60 J (c) 0 J (d) – 30 J

8. The work done in the complete cycle ABCA is
(a) 90 J (b) 60 J (c) 0 J (d) 30 J

Each of the questions given below consists of two statements, an assertion (A) and reason (R).
Select the number corresponding to the appropriate alternative as follows
(a) If both A and R are true and R is the correct explanation of A
(b) If both A and R are true but R is not the correct explanation of A
(c) If A is true but R is false
(d) If A is false but R is true

9. A: The forces between the molecules of the liquid are gravitational forces.
R: The force between two molecules of a liquid separated by 10^–10 m is of order of 10 N.

10. A: The work done in blowing a bubble of volume V is W, then work done in blowing a soap bubble to volume 2V is (2)^3/2 W.
R: Work done = Surface tension × Increase in surface area.

11. A: Gas laws are not obeyed at low temperature and high pressure.
R: At low temperature and high pressure, the molecular attraction between the molecules become appreciable.

12. A: Root mean square velocity of gas molecules is greater than the most probable velocity of gas molecules.
R: vrms : vmp = M: √3RT/M : √2RT/M

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