Chpt 16 Heat transfer

Question 1

1. Which of the following is NOT a method of heat transfer? a. Conduction b. Convection c. Radiation d. Sublimation

Answer 1

Answer: d. Sublimation Explanation: Sublimation is a phase transition directly from solid to gas not a method of heat transfer The three methods of heat transfer are conduction convection and radiation

Question 2

2. Heat transfer by conduction occurs in: a. Solids only b. Liquids only c. Gases only d. All of the above

Answer 2

Answer: d. All of the above Explanation: Conduction is the transfer of heat energy through a substance by the vibration of atoms or molecules This can occur in solids liquids and gases

Question 3

3. Which of the following is the best conductor of heat? a. Wood b. Copper c. Plastic d. Air

Answer 3

Answer: b. Copper Explanation: Metals particularly copper are excellent heat conductors because they have free electrons that can easily transfer thermal energy

Question 4

4. The rate of heat flow through a material depends on: a. The temperature gradient across the material b. The color of the material c. The density of the material d. The magnetic properties of the material

Answer 4

Answer: a. The temperature gradient across the materialExplanation: The greater the difference in temperature between two points in a material the faster the heat flow This difference in temperature over distance is called the temperature gradient

Question 5

5. Thermal conductivity is measured in: a. W m^-1 K^-1 b. J kg^-1 K^-1 c. N m^-2 d. Pa

Answer 5

Answer: a. W m^-1 K^-1 Explanation: The unit of thermal conductivity which represents the rate of heat flow per unit area per unit temperature gradient is watts per meter per Kelvin

Question 6

6. Searle’s method is used to determine the thermal conductivity of: a. Good conductors b. Poor conductors c. Liquids d. Gases

Answer 6

Answer: a. Good conductors Explanation: Searle’s method involves measuring the temperature gradient and heat flow rate through a thick metal rod which is a good conductor of heat

Question 7

7. In Lee’s method the sample must be large to: a. Increase the temperature gradient b. Minimize heat loss from the sides c. Maximize the rate of heat flow d. Facilitate accurate temperature measurement

Answer 7

Answer: b. Minimize heat loss from the sides Explanation: A large sample size in Lee’s method ensures that heat loss from the sides of the sample is negligible allowing for a more accurate determination of the thermal conductivity of the poor conductor

Question 8

8. Convection currents are driven by: a. Density differences due to temperature variations b. Pressure differences c. Viscosity gradients d. Magnetic fields

Answer 8

Answer: a. Density differences due to temperature variations Explanation: In convection warmer less dense fluid rises while cooler denser fluid sinks creating a continuous circulation This circulation is driven by density differences caused by temperature variations in the fluid

Question 9

9. Which of the following is an example of heat transfer by convection? a. Warming your hands near a fire b. Boiling water in a pot c. Feeling the heat from the sun d. Heat transfer through a vacuum flask

Answer 9

Answer: b. Boiling water in a pot Explanation: Boiling water demonstrates convection as the heat source at the bottom of the pot heats the water making it less dense and causing it to rise Cooler water then replaces it creating a circulation pattern

Question 10

10. Heat transfer by radiation: a. Does not require a material medium b. Requires a solid medium c. Requires a liquid medium d. Requires a gaseous medium

Answer 10

Answer: a. Does not require a material medium Explanation: Unlike conduction and convection radiation can transfer heat energy through a vacuum as demonstrated by the heat we receive from the sun

Question 11

11. Radiant energy is transferred by: a. Sound waves b. Water waves c. Electromagnetic waves d. Seismic waves

Answer 11

Answer: c. Electromagnetic waves Explanation: Radiant energy or radiation is the transfer of energy via electromagnetic waves which can travel through a vacuum like space

Question 12

12. Which of the following objects emits thermal radiation? a. An object at 0 K b. A perfect reflector c. The Sun d. A black hole

Answer 12

Answer: c. The SunExplanation: All objects with a temperature above absolute zero 0 K emit thermal radiation The Sun being extremely hot is a significant source of thermal radiation

Question 13

13. A bolometer is used to: a. Measure temperature b. Measure pressure c. Detect thermal radiation d. Generate electricity

Answer 13

Answer: c. Detect thermal radiation Explanation: A bolometer is a device that measures the intensity of incident thermal radiation by detecting the change in resistance of a blackened platinum strip caused by the absorption of heat energy

Question 14

14. A thermocouple works based on the principle of: a. Resistance change due to temperature b. Thermoelectric effect c. Magnetic induction d. Capacitance change

Answer 14

Answer: b. Thermoelectric effect Explanation: A thermocouple consists of two dissimilar metals joined together When there’s a temperature difference across the junction it generates a voltage a phenomenon known as the thermoelectric effect This voltage is proportional to the temperature difference and can be used to measure temperature

Question 15

15. A black body is: a. A perfect absorber and emitter of radiation b. A perfect reflector of radiation c. A transparent object d. An object that only emits visible light

Answer 15

Answer: a. A perfect absorber and emitter of radiation Explanation: A black body is a theoretical object that absorbs all incident radiation regardless of frequency or angle and emits radiation based solely on its temperature

Question 16

16. The energy distribution in the spectrum of a black body is: a. Not uniform b. Uniform c. Only in the visible region d. Only in the infrared region

Answer 16

Answer: a. Not uniform Explanation: The intensity of radiation emitted by a black body varies with wavelength and temperature This distribution is described by Planck’s law and is not uniform across the electromagnetic spectrum

Question 17

17. Wien’s displacement law states that: a. The wavelength of maximum intensity decreases as temperature increases b. The wavelength of maximum intensity increases as temperature increases c. The intensity of radiation is constant at all temperatures d. The intensity of radiation is inversely proportional to temperature

Answer 17

Answer: a. The wavelength of maximum intensity decreases as temperature increasesExplanation: Wien’s displacement law states that the wavelength at which a black body emits the maximum intensity of radiation is inversely proportional to its absolute temperature This means that hotter objects emit radiation with shorter peak wavelengths

Question 18

18. Which of the following is NOT a characteristic of heat transfer by conduction? a. It requires a material medium b. It can occur in a vacuum c. It involves the transfer of energy through molecular vibrations d. It occurs from a region of higher temperature to a region of lower temperature

Answer 18

Answer: b. It can occur in a vacuum Explanation: Conduction the transfer of heat through direct contact requires a material medium for the transfer of energy via molecular collisions A vacuum by definition lacks a medium

Question 19

19. Which of the following statements about convection is TRUE? a. It involves the movement of fluids b. It occurs primarily in solids c. It is the only method of heat transfer that can occur in a vacuum d. It is independent of temperature differences

Answer 19

Answer: a. It involves the movement of fluids Explanation: Convection is characterized by the movement of fluids liquids or gases driven by density differences caused by temperature variations Warmer less dense fluid rises while cooler denser fluid sinks leading to heat transfer

Question 20

20. What is the primary mechanism of heat transfer from the Sun to the Earth? a. Conduction b. Convection c. Radiation d. None of the above

Answer 20

Answer: c. Radiation Explanation: The vast distance between the Sun and the Earth is a vacuum precluding conduction and convection Therefore radiation which can travel through the vacuum of space is the primary mechanism of heat transfer from the Sun to the Earth

Question 21

21. A metal rod feels colder to the touch than a wooden rod at the same temperature. Why?a. Wood is a better conductor of heatb. Metal is a better conductor of heat drawing heat away from your hand fasterc. Metal has a lower specific heat capacityd. Wood has a higher thermal conductivity

Answer 21

Answer: b. Metal is a better conductor of heat drawing heat away from your hand fasterExplanation: Metal is a better conductor than wood meaning it transfers heat away from your hand more quickly giving a sensation of coldness

Question 22

22. What happens to the rate of heat flow through a rod if its cross-sectional area is doubled keeping all other factors constant?a. The rate of heat flow doublesb. The rate of heat flow halvesc. The rate of heat flow remains the samed. The rate of heat flow increases by a factor of four

Answer 22

Answer: a. The rate of heat flow doublesExplanation: The rate of heat flow is directly proportional to the cross-sectional area Doubling the area doubles the path for heat transfer hence doubling the rate of flow

Question 23

23. Which of the following materials would be the BEST choice for insulating a house?a. Fiberglassb. Copperc. Aluminumd. Steel

Answer 23

Answer: a. FiberglassExplanation: Fiberglass is a good insulator because it traps air which is a poor conductor of heat This helps to prevent heat loss from the house in winter and heat gain in summer

Question 24

24. What is the purpose of the lagging used in Searle’s apparatus?a. To increase the temperature gradientb. To reduce the cross-sectional area of the rodc. To minimize heat loss to the surroundingsd. To facilitate the flow of steam

Answer 24

Answer: c. To minimize heat loss to the surroundingsExplanation: Lagging an insulating material is used to minimize heat loss to the surroundings ensuring that the heat flow primarily occurs through the metal rod allowing for accurate measurement of its thermal conductivity

Question 25

25. In Lee’s method why is the brass disc used?a. To provide a heat sourceb. To measure the temperature of the steamc. To measure the rate of heat flow through the sampled. To increase the thermal conductivity of the sample

Answer 25

Answer: c. To measure the rate of heat flow through the sampleExplanation: By measuring the rate of temperature change in the brass disc placed below the sample one can indirectly determine the rate of heat flow through the poor conductor sample

Question 26

26. What happens to the temperature gradient along an insulated rod with a constant heat flow?a. It increases linearlyb. It decreases linearlyc. It remains constantd. It fluctuates randomly

Answer 26

Answer: b. It decreases linearlyExplanation: In an insulated rod with constant heat flow the temperature decreases linearly from the hot end to the cold end creating a uniform temperature gradient

Question 27

27. How does a thermos flask minimize heat transfer by conduction convection and radiation?a. It uses a thick layer of metal to conduct heat awayb. It has a vacuum between its walls a reflective inner surface and an insulated lidc. It uses a circulating fluid to transfer heatd. It relies on the high thermal conductivity of air

Answer 27

Answer: b. It has a vacuum between its walls a reflective inner surface and an insulated lidExplanation: A thermos flask minimizes heat transfer through vacuum which prevents conduction and convection between the walls reflective inner surface which reduces radiation by reflecting heat back into the flask and insulated lid which minimizes heat loss from the opening

Question 28

28. Why do land breezes occur at night?a. Land cools down slower than waterb. Land cools down faster than water creating a convection currentc. Water warms up faster than landd. Air pressure is higher over land at night

Answer 28

Answer: b. Land cools down faster than water creating a convection currentExplanation: Land breezes occur at night because land cools down faster than water The cooler air over land flows towards the warmer air over water creating a convection current known as a land breeze

Question 29

29. Which of the following is NOT an application of convection?a. Heating a room with a radiatorb. Cooking food on a stovetop c. Using a hot air balloon d. Transferring heat through a vacuum

Answer 29

Answer: d. Transferring heat through a vacuumExplanation: Convection requires a fluid medium for heat transfer A vacuum by definition lacks a medium making convection impossible in such an environment

Question 30

30. Which type of radiation has the shortest wavelength?a. Ultraviolet radiationb. Infrared radiationc. Visible lightd. Microwaves

Answer 30

Answer: a. Ultraviolet radiationExplanation: In the electromagnetic spectrum ultraviolet radiation has a shorter wavelength than infrared radiation visible light and microwaves

Question 31

31. What is the approximate value of Wien’s constant? a. 2.93 × 10^-3 m K b. 6.63 × 10^-34 J s c. 1.38 × 10^-23 J K^-1 d. 9.81 m s^-2

Answer 31

Answer: a. 2.93 × 10^-3 m K Explanation: Wien’s constant is a fundamental constant in physics relating the wavelength of peak emission of black-body radiation to its temperature

Question 32

32. Which of the following is a unit of heat flow rate? a. Joule b. Kelvin c. Watt d. Pascal

Answer 32

Answer: c. WattExplanation: The watt (W) is the unit of power which represents the rate of energy transfer per unit time Heat flow rate being a form of power is also measured in watts

Question 33

33. What is the relationship between thermal conductivity and the rate of heat flow? a. Inversely proportional b. Directly proportional c. Exponential d. Logarithmic

Answer 33

Answer: b. Directly proportionalExplanation: The higher the thermal conductivity of a material the faster the rate of heat flow through it assuming all other factors remain constant

Question 34

34. Which of the following statements is TRUE about thermal insulators? a. They have high thermal conductivity b. They resist the flow of heat c. They are typically made of metals d. They readily transfer heat by convection

Answer 34

Answer: b. They resist the flow of heatExplanation: Thermal insulators are materials with low thermal conductivity meaning they impede the flow of heat making them suitable for applications like insulation

Question 35

35. Why are metals generally good conductors of electricity? a. They have a high density of free protons b. They have a strong magnetic field c. They have a high density of free electrons d. They have a high specific heat capacity

Answer 35

Answer: c. They have a high density of free electrons Explanation: The free electrons in metals can readily move throughout the material carrying both electric charge and thermal energy making metals good conductors of both electricity and heat

Question 36

36. What is the temperature gradient across a 1-meter long rod with one end at 100°C and the other end at 0°C? a. 100°C/m b. 50°C/m c. 1°C/m d. 0.01°C/m

Answer 36

Answer: a. 100°C/m Explanation: The temperature gradient is the change in temperature per unit length In this case the temperature difference is 100°C over a distance of 1 meter resulting in a gradient of 100°C/m

Question 37

37. What happens to the rate of heat flow through a wall if its thickness is doubled keeping all other factors constant? a. The rate of heat flow halves b. The rate of heat flow doubles c. The rate of heat flow remains the same d. The rate of heat flow increases by a factor of four

Answer 37

Answer: a. The rate of heat flow halvesExplanation: The rate of heat flow is inversely proportional to the thickness of the material Increasing the thickness increases the resistance to heat flow reducing the rate

Question 38

38. Why is a black object considered a good emitter of thermal radiation? a. It reflects all incident radiation b. It transmits all incident radiation c. It absorbs all incident radiation and re-emits it efficiently d. It only emits radiation in the visible spectrum

Answer 38

Answer: c. It absorbs all incident radiation and re-emits it efficientlyExplanation: A black object being an ideal absorber absorbs all incident radiation According to Kirchhoff’s law of thermal radiation good absorbers are also good emitters meaning a black object re-emits the absorbed radiation efficiently

Question 39

39. What is the relationship between the temperature of a black body and the wavelength of peak emission? a. Directly proportional b. Inversely proportional c. Exponential d. Logarithmic

Answer 39

Answer: b. Inversely proportional Explanation: Wien’s displacement law establishes that the wavelength of peak emission is inversely proportional to the absolute temperature of the black body As temperature increases the peak emission shifts towards shorter wavelengths

Question 40

40. Why does hot air rise? a. It is less dense than cold air b. It is more dense than cold air c. It has higher viscosity than cold air d. It has lower pressure than cold air

Answer 40

Answer: a. It is less dense than cold airExplanation: When air is heated its molecules move faster and spread out resulting in lower density Being less dense than the surrounding cooler air hot air rises due to buoyancy

Question 41

41. Which of the following is NOT a factor affecting the rate of heat transfer by convection? a. The temperature difference between the fluid and the object b. The surface area of the object in contact with the fluid c. The thermal conductivity of the object d. The viscosity of the fluid

Answer 41

Answer: c. The thermal conductivity of the object Explanation: While the thermal conductivity of the object matters in conduction convection primarily relies on the fluid’s properties and the temperature difference for heat transfer

Question 42

42. Why do birds soar effortlessly in thermals? a. Thermals are regions of low air pressure b. Thermals are rising columns of warm air c. Thermals are areas with strong horizontal winds d. Thermals have high humidity providing lift

Answer 42

Answer: b. Thermals are rising columns of warm air Explanation: Thermals are columns of rising warm air created by uneven heating of the Earth’s surface Birds exploit these thermals for lift soaring effortlessly by gliding within the rising air currents

Question 43

31. What is the approximate value of Wien’s constant? a. 2.93 × 10^-3 m K b. 6.63 × 10^-34 J s c. 1.38 × 10^-23 J K^-1 d. 9.81 m s^-2

Answer 43

Answer: a. 2.93 × 10^-3 m K Explanation: Wien’s constant is a fundamental constant in physics relating the wavelength of peak emission of black-body radiation to its temperature

Question 44

32. Which of the following is a unit of heat flow rate? a. Joule b. Kelvin c. Watt d. Pascal

Answer 44

Answer: c. WattExplanation: The watt (W) is the unit of power which represents the rate of energy transfer per unit time Heat flow rate being a form of power is also measured in watts

Question 45

33. What is the relationship between thermal conductivity and the rate of heat flow? a. Inversely proportional b. Directly proportional c. Exponential d. Logarithmic

Answer 45

Answer: b. Directly proportionalExplanation: The higher the thermal conductivity of a material the faster the rate of heat flow through it assuming all other factors remain constant

Question 46

34. Which of the following statements is TRUE about thermal insulators? a. They have high thermal conductivity b. They resist the flow of heat c. They are typically made of metals d. They readily transfer heat by convection

Answer 46

Answer: b. They resist the flow of heatExplanation: Thermal insulators are materials with low thermal conductivity meaning they impede the flow of heat making them suitable for applications like insulation

Question 47

35. Why are metals generally good conductors of electricity? a. They have a high density of free protons b. They have a strong magnetic field c. They have a high density of free electrons d. They have a high specific heat capacity

Answer 47

Answer: c. They have a high density of free electrons Explanation: The free electrons in metals can readily move throughout the material carrying both electric charge and thermal energy making metals good conductors of both electricity and heat

Question 48

36. What is the temperature gradient across a 1-meter long rod with one end at 100°C and the other end at 0°C? a. 100°C/m b. 50°C/m c. 1°C/m d. 0.01°C/m

Answer 48

Answer: a. 100°C/m Explanation: The temperature gradient is the change in temperature per unit length In this case the temperature difference is 100°C over a distance of 1 meter resulting in a gradient of 100°C/m

Question 49

37. What happens to the rate of heat flow through a wall if its thickness is doubled keeping all other factors constant? a. The rate of heat flow halves b. The rate of heat flow doubles c. The rate of heat flow remains the same d. The rate of heat flow increases by a factor of four

Answer 49

Answer: a. The rate of heat flow halvesExplanation: The rate of heat flow is inversely proportional to the thickness of the material Increasing the thickness increases the resistance to heat flow reducing the rate

Question 50

38. Why is a black object considered a good emitter of thermal radiation? a. It reflects all incident radiation b. It transmits all incident radiation c. It absorbs all incident radiation and re-emits it efficiently d. It only emits radiation in the visible spectrum

Answer 50

Answer: c. It absorbs all incident radiation and re-emits it efficientlyExplanation: A black object being an ideal absorber absorbs all incident radiation According to Kirchhoff’s law of thermal radiation good absorbers are also good emitters meaning a black object re-emits the absorbed radiation efficiently

Question 51

39. What is the relationship between the temperature of a black body and the wavelength of peak emission? a. Directly proportional b. Inversely proportional c. Exponential d. Logarithmic

Answer 51

Answer: b. Inversely proportional Explanation: Wien’s displacement law establishes that the wavelength of peak emission is inversely proportional to the absolute temperature of the black body As temperature increases the peak emission shifts towards shorter wavelengths

Question 52

40. Why does hot air rise? a. It is less dense than cold air b. It is more dense than cold air c. It has higher viscosity than cold air d. It has lower pressure than cold air

Answer 52

Answer: a. It is less dense than cold airExplanation: When air is heated its molecules move faster and spread out resulting in lower density Being less dense than the surrounding cooler air hot air rises due to buoyancy

Question 53

41. Which of the following is NOT a factor affecting the rate of heat transfer by convection? a. The temperature difference between the fluid and the object b. The surface area of the object in contact with the fluid c. The thermal conductivity of the object d. The viscosity of the fluid

Answer 53

Answer: c. The thermal conductivity of the object Explanation: While the thermal conductivity of the object matters in conduction convection primarily relies on the fluid’s properties and the temperature difference for heat transfer

Question 54

42. Why do birds soar effortlessly in thermals? a. Thermals are regions of low air pressure b. Thermals are rising columns of warm air c. Thermals are areas with strong horizontal winds d. Thermals have high humidity providing lift

Answer 54

Answer: b. Thermals are rising columns of warm air Explanation: Thermals are columns of rising warm air created by uneven heating of the Earth’s surface Birds exploit these thermals for lift soaring effortlessly by gliding within the rising air currents