Question 1

Which of the following best describes the process of convection as illustrated in the first image (boiling water in a pot)?

Accepted Answer

Heat is transferred by the movement of fluid, where hot, less dense water rises and cooler, denser water sinks, creating a convection current.

Question 2

A glider takes advantage of rising air currents above a sun-warmed field, as shown in the second image. What is the primary mechanism of heat transfer responsible for these updrafts?

Accepted Answer

Convection

Question 3

The granules seen on the Sun's surface (fourth image) are evidence of which heat transfer process occurring within the Sun?

Accepted Answer

Convection

Question 4

Which of the following materials would be the best thermal insulator based on the thermal conductivity values provided in the table (eighth image)?

Accepted Answer

Styrofoam

Question 5

A metal rod is heated at one end, as shown in the sixth image. Which statement best explains how heat is transferred along the rod?

Accepted Answer

Heat is transferred by the movement of free electrons and the vibration of atoms.

Question 6

Given the equation for heat transfer by conduction $$Q = \frac{kA \Delta T t}{L}$$, which change would most effectively reduce the heat loss through a house wall?

Accepted Answer

Increase the wall thickness $$L$$

Question 7

A blacksmith heats a metal rod and places one end on an anvil (fifth image). Which property of the metal is most responsible for the rapid transfer of heat to the anvil?

Accepted Answer

High thermal conductivity

Question 8

Which of the following best explains why a silver-coated block heats up more slowly than a lampblack-coated block under sunlight (eleventh image)?

Accepted Answer

Silver reflects more radiation due to its high albedo, absorbing less energy.

Question 9

Which equation would you use to calculate the net radiant power emitted by an object hotter than its environment?

Accepted Answer

$$P_{net} = e \sigma A (T_{obj}^4 - T_{env}^4)$$

Question 10

A wall is made of two layers: insulation and plywood (ninth image). If the thermal conductivity of insulation is lower than that of plywood, which layer is more effective at reducing heat transfer?

Accepted Answer

Insulation, because it has lower thermal conductivity.

Question 11

Which of the following statements about blackbodies is correct?

Accepted Answer

A blackbody is a perfect absorber and a perfect emitter of radiation.

Question 12

The Sun emits energy that reaches Earth through which heat transfer process?

Accepted Answer

Radiation

Question 13

If the temperature of an object doubles (in Kelvin), by what factor does the power it radiates change, according to the Stefan-Boltzmann law?

Accepted Answer

It increases by a factor of 16

Question 14

A house has walls made of two materials in series: insulation and plywood. If the inside temperature is $$25^\circ C$$ and the outside is $$4^\circ C$$, and the interface temperature is $$5.8^\circ C$$, what does this interface temperature represent?

Accepted Answer

The temperature at the boundary between insulation and plywood.

Question 15

Which of the following is NOT a method of heat transfer discussed in the lecture?

Accepted Answer

Diffusion

Question 16

Why does Styrofoam make a good coffee cup for keeping drinks hot?

Accepted Answer

It has a low thermal conductivity, reducing heat loss by conduction.

Question 17

Which of the following best describes why the air near the ground is warmer on a sunny day?

Accepted Answer

The ground absorbs sunlight, warms up, and then heats the air above by convection.

Question 18

If you touch a piece of metal and a piece of wood that are both at room temperature, the metal feels colder. Why?

Accepted Answer

Metal conducts heat away from your hand faster than wood.

Question 19

Which of the following would increase the rate of heat transfer by conduction through a wall?

Accepted Answer

Increasing the temperature difference $$\Delta T$$

Question 20

A star has twice the radius and half the temperature of the Sun. What is its luminosity relative to the Sun, assuming both are blackbodies?

Accepted Answer

$$L = 2^2$ 	imes (0.5)^4$ = 4 	imes 0.0625 = 0.25$$ times the Sun's luminosity

Heat Transfer: Convection, Conduction, and Radiation- lecture 28

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