Q: A 0.5 kg block of ice at -10°C is heated until it becomes water at 10°C. Given $$c_{ice} = 2100 J/kg$$·K, $$c_{water} = 4186 J/kg$$·K, and $$L_f = 334 \times 10^3$ J/kg, how much total heat Q$ is required?

$$Q = 0.5 \times 2100 \times 10 + 0.5 \times 334 \times 10^3$ + 0.5 \times 4186 \times 10$$

Q: A 240 W heater is used to melt ice at 0°C. If it takes 700 seconds to melt all the ice, and $$L_f = 334 \times 10^3$ J/kg, what was the initial mass of ice?

$$m = \frac{240 \times 700}{334 \times 10^3$}$$

Question 1

A 0.5 kg block of ice at -10°C is heated until it becomes water at 10°C. Given $$c_{ice} = 2100 J/kg$$·K, $$c_{water} = 4186 J/kg$$·K, and $$L_f = 334 	imes 10^3$ J/kg, how much total heat Q$ is required?

Accepted Answer

$$Q = 0.5 	imes 2100 	imes 10 + 0.5 	imes 334 	imes 10^3$ + 0.5 	imes 4186 	imes 10$$

Question 2

During the melting of ice at 0°C, which of the following statements is correct?

Accepted Answer

The temperature remains constant while heat is added.

Question 3

A 240 W heater is used to melt ice at 0°C. If it takes 700 seconds to melt all the ice, and $$L_f = 334 	imes 10^3$ J/kg, what was the initial mass of ice?

Accepted Answer

$$m = \frac{240 	imes 700}{334 	imes 10^3$}$$

Question 4

Why does a metal seat at 0°C feel colder than a plastic seat at 0°C when you sit on it?

Accepted Answer

Metal has a higher thermal conductivity, so it transfers heat away from your body faster.

Question 5

Which equation correctly describes the heat current $$H$$ through a material of cross-sectional area $$A$$, length $$L$$, and thermal conductivity $$k$$, with a temperature difference $$T_H - T_C$$?

Accepted Answer

$$H = k A \frac{T_H - T_C}{L}$$

Question 6

If you double the cross-sectional area $$A of a $$conducting strip between two reservoirs at different temperatures, what happens to the heat current $$H$$?

Accepted Answer

The heat current doubles.

Question 7

If you double the length $$L of a $$conducting strip between two reservoirs at different temperatures, what happens to the heat current $$H$$?

Accepted Answer

The heat current is halved.

Question 8

Objects A and B have the same mass and size. $$c_A  k_B. $$Both are dropped into 80°C water. Which statement is correct?

Accepted Answer

Object A will reach equilibrium faster, but both will end up at the same temperature.

Question 9

A mass $$M of $$ice at $$T_1  0$. Which expression gives the total heat added?

Accepted Answer

$$Q = M c_{ice} (-T_1$) + M L_f + M c_{water} (T_2$)$$

Question 10

In a steady-state heat conduction scenario, three surfaces are marked along the direction of heat flow through a slab between 100°C and 0°C. What can you say about the heat current $$H$$ through these surfaces?

Accepted Answer

$$H_1$$ = $$H_2$$ = $$H_3$$

Question 11

Which of the following materials is the best thermal insulator based on its thermal conductivity?

Accepted Answer

Styrofoam ($$k = 0.027 W/m$$·K)

Question 12

Which process is responsible for the transfer of heat from a hot brick to the air in a fireplace?

Accepted Answer

Convection

Question 13

Which equation would you use to calculate the heat required to raise the temperature of $$n$$ moles of a substance by $$\Delta T$$?

Accepted Answer

$$Q = n C \Delta T$$

Question 14

Why is the temperature vs. time graph for heating ice water initially flat during the melting phase?

Accepted Answer

All the heat is used for the phase change, not for increasing temperature.

Question 15

Which of the following best explains why the temperature vs. time graph curves upward after all the ice has melted?

Accepted Answer

The specific heat of water is lower than that of ice, so temperature rises faster.

Heat, Phase Changes, and Thermal Conductivity: PHYS157 Study Notes

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