CAIE A-Level Physics 14.1 Thermal Equilibrium

Practise explaining thermal-energy transfer from higher to lower temperature and identifying thermal equilibrium as equal temperature with no net transfer.

Exam points

explain thermal energy transfer from higher to lower temperature
identify thermal equilibrium as equal temperature with no net thermal transfer

Covered syllabus points

14.1.1—(thermal) energy is transferred from a region of higher temperature to a 14.1.2—Regions of equal temperature are in thermal equilibrium

[Maximum number: 4]

Use the information in (i) to draw, on Fig. 1.1, a line to represent the temperature of the block, assuming no energy losses to the surroundings.

(a)

State

[ 1 ]

(i)

what may be deduced from the difference in the temperatures of two objects,

[ 1 ]

(b)

A block of aluminium of mass 670 g is heated at a constant rate of 95 W for 6.0 minutes. The specific heat capacity of aluminium is $910 \mathrm{Jkg}^{-1} \mathrm{~K}^{-1}$ . The initial temperature of the block is $24^{\circ} \mathrm{C}$ .

[ 3 ]

(i)

In practice, there are energy losses to the surroundings. The actual variation with time t of the temperature $\theta$ of the block is shown in Fig. 1.1.

Fig. 1.1

[ 3 ]

1. sketch: straight line from (0,24) to (6,80)

2. temperature drop due to energy loss $=(80-64)=16^{\circ} \mathrm{C}$

energy loss $=0.670 \times 910 \times(80-64)=9800\,\mathrm{J}$

or

energy to raise temperature to $64^{\circ} \mathrm{C}=0.670 \times 910 \times(64-24)$ $=24400\,\mathrm{J}$ $\text{loss}=(95 \times 6 \times 60)-24400=9800\,\mathrm{J}$

[Maximum number: 3]

Fig. 2.1 shows a laboratory thermometer that is calibrated to measure temperature in degrees Celsius.

Fig. 2.1

The thermometer makes use of the fact that the density of mercury varies with temperature.

The thermometer is initially at $23.0^{\circ} \mathrm{C}$ , as shown in Fig. 2.1. It is used to measure the temperature of an insulated beaker of water that is at $37.4^{\circ} \mathrm{C}$ . The bulb of the thermometer is inserted into the water, and the water is stirred until the reading on the thermometer becomes steady.

The mass of water in the beaker is 18.7 g .
The mass of mercury in the thermometer is 6.94 g .
The specific heat capacity of water is $4.18 \mathrm{~J} \mathrm{~g}^{-1} \mathrm{~K}^{-1}$ .
The specific heat capacity of mercury is $0.140 \mathrm{~J} \mathrm{~g}^{-1} \mathrm{~K}^{-1}$ .
The glass of the thermometer and the beaker containing the water can be considered to have negligible heat capacity.

Calculate, to three significant figures, the final steady temperature indicated by the thermometer in the water. ${ }^{\circ} \mathrm{C}$

$Q=m c \Delta T$

C1

evidence of realisation that Q lost by water =Q gained by mercury

C1

$18.7 \times 4.18 \times(37.4-T)=6.94 \times 0.140 \times(T-23.0)$

C1

$T=37.2^{\circ} \mathrm{C}$

A1

All question bank results loaded

Covered syllabus points

14.1 Thermal equilibrium question 1

Question (a)

Question (i)

Question (b)

Question (i)

14.1 Thermal equilibrium question 2