o NAMIBIA UNIVERSITY OF SCIENCE AND TECHNOLOGY

FACULTY OF HEALTH AND APPLIED SCIENCES

DEPARTMENT OF NATURAL AND APPLIED SCIENCES

QUALIFICATION: BACHELOR OF SCIENCE

QUALIFICATION CODE: 07BOSC LEVEL: 6

COURSE CODE: TPH601S COURSE NAME: THERMAL PHYSICS

SESSION: JUNE 2016 PAPER: THEORY DURATION: 3 HOURS MARKS: 100

FIRST OPPORTUNITY EXAMINATION QUESTION PAPER

EXAMINER(S) | Prof. Idugba M. Echi

MODERATOR: | Dr. Al-Mas Sendegeya

INSTRUCTIONS

1. Answer ALL the questions.

2. Write clearly and neatly.

3. Number the answers clearly.

PERMISSIBLE MATERIALS

Non-programmable Calculators

ATTACHMENT

None

THIS QUESTION PAPER CONSISTS OF 6 PAGES (Including this front page)

In question 1, write the letter corresponding to your choice in your answer booklet.

Each question in this section carries 1 mark

QUESTION 1 [20]

1.1 Heat is a form of energy which can be transferred from one body to another if there is a nooo difference between the bodies.

A temperature B pressure C volume D mass (1)

1.2. The normal body temperature is about 98.6°F. If a nurse measures the temperature of a patient to be 311K, what is the deviation from the normal value?

A 1.0°F B 1.6°F C 1.8°F D 212.4°F (1)

1.3. The thermometric property of a thermocouple thermometer is ------

A the variation of resistance with temperature

B the variation of expansion coefficient with temperature

B the variation emf with pressure

C the variation of emf with temperature (1)

1.4 The mode of heat transfer which can take place in solids is/are:

| radiation Il conduction Ill convection

A I, Il B I, Ill C il, ll D llonly (1)

1.5 The unit of thermal conductivity is:

A Jm*K* =B Wm?K* = C WmK? D Wm'k (1)

1.6 Examples of intensive thermodynamic variables are:

| temperature Il volume Ill pressure IV mass

A |, Il B I. tll C 1,1IV D Il, Ill (1)

1.7 ‘The internal energy of a gas increased by 105J at constant volume. This means that:

A the mechanical work done on the gas is 105J

B the mechanical work done by the gas is 105J

C The heat gained by the gas is 105J

D The heat lost by the gas is 105J (1)

1.8 In Joule —Thompson effect,

the gas expands at the expense of its internal energy

the temperature of gas remains constant

the volume of gas is directly proportional to its temperature

the internal energy of gas doubles. (1) 0ANQa >

1.9

1.10

1.11

1.12

1.13

1.14

isotherm

“

VY

"I 2

‘ yr > V

the isothermal work done by the system on the surrounding

the adiabatic work done on the system

isobaric work done by the system

isothermal work done on the system (1) OO WD

A beaker of water at initial temperature T; is heated for 8 minutes and it was found that

there was no change in temperature. Which of the following statements is true about the

beaker of water?

A The specific heat capacity of water is very high.

B Tj is the boiling point of water.

C Water is a poor conductor of heat.

D The anomalous effect of water prevents the rise in temperature. (1)

The measure of the disorderliness of a system is called

A internal energy B heat capacity C entropy D enthalpy (1)

The efficiency of a Carnot engine operating between the temperatures 90°C and 30°C is

A 70% B 60% C 17% D 25% (1)

In the kitchen refrigerator the two heat reservoirs are located in------

A the freezer and outside the fridge |B the compressor and condenser

C compressor and outside the fridge Dthe condenser and outside the fridge (1)

A Brayton heat engine operates with the following data: heat input= 1860J at Ty = 98°C, heat

out put 1525) at Tc = 27°C. The thermal efficiency of the engine is:

A 19% B 17% C 21% D 72% (1)

1.15 The P-V diagram below is the operating cycle of a ------ engine

1.16

1.17

1.18

1.19

1.20

Pp

- isotherms

| l 1 4 Vv

A Carnot B Sterling C Brayton C Rankine (1)

The temperature at which the pressure and volume of a gas becomes zero is called the--

A triple point B absolute zero C boiling point D freezing point. (1)

The practical use the Maxwell relation is

A to display the beauty of partial derivatives of thermodynamic potentials

B to substitute control experiments with equivalent ones

C to reduce the entropy changes of the universe

D reduce the Helmholtz energy in reversible processes. (1)

From the microscopic point of view, the pressure of a gas is due to ----

A the potential energy of the bonds holding molecules together

B the random collision of molecules with the walls of the containing vessel

C the cohesive forces between the molecules

D the repulsive forces between the molecules (1)

If f(v) is a normalised molecular speed distribution function, the integral [ v?t)dv

gives the---

A mean molecular speed

B mean square molecular speed

C root mean square molecular speed

B mean square free path (1)

The mean free path of a nitrogen gas in a container is 3.697x10’ and the rms speed of the

molecules is 460 ms”. Evaluate the mean time between collisions of molecules.

A 8.04x107°s_ =B 7.50x107°s_ = C:~8.00x10"°ms* D 7.50x10"°ms™ (1)

QUESTION 2 [20]

ded Explain the meaning of the statement that the specific heat capacity of water is

4200Jkg*k* (2)

2.2 Define the specific latent heat of condensation of water. (2)

aud If you are required to determine the specific latent heat of condensation of water:

2.3.1 List the apparatus and materials you will use. (3)

2.3.2 List the measurements you will take. (3)

Loe Write the theory and show how the specific latent heat will be obtained.

2.4 Ice block at initial temperature of -10°C is dropped into a copper calorimeter of mass 800g

and containing hot water of mass 700g at temperature of 70°C. If the steady temperature of

the mixture is 25°C, evaluate the mass of ice [Specific heat capacity of ice = 2100Jkgk™,

latent heat of fusion of ice = 3.4x10°Jkg™, specific heat capacity of

water = 4200JkgK™, specific heat capacity of copper 400Jkg“K“] (7)

QUESTION 3 [20]

3.1 Write the first law of thermodynamics for adiabatic, isochoric and isobaric processes. (3)

3.2. Air is trapped in the conical flask as shown in the figure below.

water

air

water

3.2.1 Write the law of thermodynamics for the trapped air as it is heated. (3)

3.2.2 As the gas expands, explain why the pressure of the trapped air remains constant.

(4)

3.3 A compressed ideal gas with a volume of 20m? at temperature of 120°C and pressure of

2x10°Nm™ suddenly expands to a volume of 50m° and the temperature drops to 80°C.

3.3.1 Explain why the temperature drops. (4)

3.3.2 Evaluate the work done by the gas. (6)

QUESTION 4 [20]

4.1 State the second law of thermodynamics from the entropy point of view. (3)

4.2 Sketch the P-V diagram of a Rankine. (3)

4.3 Evaluate the entropy change of 900g of ice at 0°C as it is heated and completely evaporates

at 100°C [latent heat of fusion of ice = 3.4x10°Jkg™, specific heat capacity of

water = 4200JkgK’” specific latent heat of evaporation of water =2.26x10°kg™] (6)

4.4 Explain the functions of compressor and evaporator in refrigerators. (4)

4.5 The hot reservoir of a Carnot engine is 130°C. If the thermal efficiency of the engine is 32%,

determine the temperature of the cold reservoir. (4)

QUESTION 5 [20]

5.1 Explain how the third law of thermodynamics establishes the absolute zero temperature.

(4)

5.2 The thermodynamic potential U may be expressed as a function of the extensive variables

entropy S and volume V.

5.2.1 Write the intensive variables associated with the differential of U. (3)

5.2.2 Hence obtain the Maxwell relation and explain the practical importance of the

relation. (4)

53 The Maxwell-Boltzmann molecular speed distribution is given by 3/2 2

f(v)=42 —— vexp| — ay 2uk, T 2k T

Name the symbols m, v, T. List two things you can use this distribution for. (4)

5.4 Evaluate the most probable speed of CO, gas at a temperature of 300 K

(kg = 1.3x10°JK“, Na = 6.022 x10°°, molar mass of CO = 4.4x10* Kgmol”) (5)

2K 2 Ae oe oe ok OK OK oo OK ok oe ok ok KK GOODLUCK KR KK KK KK KK