Chapter 20

Heat and the First Law of Thermodynamics

Quick Quiz 20.1

Imagine you have 1 kg each of iron, glass, and water, and that all three samples are at 10°C. Rank the samples from lowest to highest temperature after 100 J of energy is added to each sample.

(a) iron, glass, water

(b) iron, water, glass

(c) glass, iron, water

(d) glass, water, iron

(e) water, glass, iron

(f) water, iron, glass

Answer: (e). Because water has the highest specific heat (4 186 J/kg · °C), it has the smallest change in temperature. Glass is next (837 J/kg · °C), and iron is last (448 J/kg · °C).

Quick Quiz 20.1

Quick Quiz 20.2

Imagine you have 1 kg each of iron, glass, and water, and that all three samples are at 10°C. Rank them from least to greatest amount of energy transferred by heat if each sample increases in temperature by 20°C.

(a) iron, glass, water

(b) iron, water, glass

(c) glass, iron, water

(d) glass, water, iron

(e) water, glass, iron

(f) water, iron, glass

Answer: (a). Iron, glass, water. For a given temperature increase, the energy transfer by heat is proportional to the specific heat.

Quick Quiz 20.2

Quick Quiz 20.3

Suppose energy is added to an ice cube until it eventually vaporizes into steam, and we graph the internal energy of the system as a function of energy input. What would this graph look like?

Answer: The figure below shows a graphical representation of the internal energy of the ice in parts A to E as a function of energy added. Notice that this graph looks quite different from Figure 20.2 – it doesn’t have the flat portions during the phase changes. Regardless of how the temperature is varying in Figure 20.2, the internal energy of the system simply increases linearly with energy input.

Quick Quiz 20.3

Quick Quiz 20.4

Calculate the slopes for the A, C, and E portions of the figure below. Rank the slopes from least to greatest.

(a) A, C, E

(b) A, E, C

(c) C, E, A

(d) C, A, E

(e) E, A, C

(f) E, C, A

Answer: (d). C, A, E. The slope is the ratio of the temperature change to the amount of energy input. Thus, the slope is proportional to the reciprocal of the specific heat. Water, which has the highest specific heat, has the smallest slope.

Quick Quiz 20.4

Quick Quiz 20.5

In the last three columns of the following table, fill in the boxes with -, +, or 0. For each situation, the system to be considered is identified.

Situation System Q W ΔEint

(a) Rapidly pumping up a bicycle tire

Air in the pump

     

(b) Pan of room-temperature water sitting on a hot stove

Water in the pan

     

(c) Air quickly leaking out of a balloon

Air originally in the balloon

     

Answer:

Quick Quiz 20.5

Situation System Q W ΔEint

(a) Rapidly pumping up a bicycle tire

Air in the pump

0 + +

(b) Pan of room-temperature water sitting on a hot stove

Water in the pan

+ 0 +

(c) Air quickly leaking out of a balloon

Air originally in the balloon

0 - -

Quick Quiz 20.6a

Characterize path A in the figure below:

(a) isobaric

(b) isovolumetric

(c) isothermal

(d) adiabatic

Answer: (b).

Quick Quiz 20.6a

Quick Quiz 20.6b

Characterize path B in the figure below. Note that Q = 0.

(a) isobaric

(b) isovolumetric

(c) isothermal

(d) adiabatic

Answer: (d).

Quick Quiz 20.6b

Quick Quiz 20.6c

Characterize path C in the figure below:

(a) isobaric

(b) isovolumetric

(c) isothermal

(d) adiabatic

Answer: (c).

Quick Quiz 20.6c

Quick Quiz 20.6d

Characterize path D in the figure below:

(a) isobaric

(b) isovolumetric

(c) isothermal

(d) adiabatic

Answer: (a).

Quick Quiz 20.6d

Quick Quiz 20.7

An ice cube wrapped in a wool blanket will remain frozen for a _____ length of time than an identical ice cube exposed to air at room temperature.

(a) shorter

(b) same

(c) longer

Answer: (c). The blanket acts as a thermal insulator, slowing the transfer of energy by heat from the air into the cube.

Quick Quiz 20.7

Quick Quiz 20.8

You have two rods of the same length and diameter but they are formed from different materials. The rods will be used to connect two regions of different temperature such that energy will transfer through the rods by heat. They can be connected in series, as in part (a) of the figure below, or in parallel, as in part (b). In which case is the rate of energy transfer by heat larger?

(a) when the rods are in series

(b) when the rods are in parallel

(c) The rate is the same in both cases.

Answer: (b). In parallel, the rods present a larger area through which energy can transfer and a smaller length.

Quick Quiz 20.8