# IBDP Physics 3.1 – Thermal concepts: IB Style Question Bank -SL Paper 1

### Question

When 40 kJ of energy is transferred to a quantity of a liquid substance, its temperature increases by 20 K. When 600 kJ of energy is transferred to the same quantity of the liquid at its boiling temperature, it vaporizes completely at constant temperature. What is

for this substance?

A. 15

B. 15 K

C. 300

D. 300 K

Ans: D

SPECIFIC HEAT CAPACITY

It is the amount of heat required to raise the temperature of unit mass of substance through 1 degree.
Specific heat capacity,
Latent heat of vaporisation : It is the quantity of heat required to convert unit mass of liquid  into vapour at its boiling point.
$$Q=mL$$
now
$$40 \;kJ=m\times c\times \Delta T$$ where $$\Delta T =20 K$$
$$600 \;kJ = m \times L$$
$$\therefore \frac{m \times L}{m\times c\times \Delta T} =\frac{600 \;kJ}{40 \;kJ}$$
or
$$\frac{L}{c} =\frac{600 \;kJ}{40 \;kJ}\times \Delta T =300 K$$

### Question

Which aspect of thermal physics is best explained by the molecular kinetic model?

A The equation of state of ideal gases

B The difference between Celsius and Kelvin temperature

C The value of the Avogadro constant

D The existence of gaseous isotopes

Ans: A

The kinetic molecular theory can be used to explain each of the experimentally determined gas laws.

Any sample of a gas is made of molecules. A molecule is the smallest unit having all the chemical properties of the sample. The observed behaviour of a
gas results from the detailed behaviour of its large number of molecules. The kinetic theory of gases attempts to develop a model of the molecular behaviour
which should result in the observed behaviour of an ideal gas.

### Question

Molecules leave a boiling liquid to form a vapour. The vapour and the liquid have the same temperature.

What is the change of the average potential energy and the change of the average random kinetic energy of these molecules when they move from the liquid to the vapour?

## Markscheme

B

During a phase change, temperature of the substance remains the same. Average kinetic energy of the molecules of the substance does not change during phase change but average potential energy changes.

Heat supplied at phase change is used in increasing internal energy of molecules. Internal energy of a given body in vapour phase is larger than that in liquid phase

### Question

Which of the following is equivalent to a temperature of –100°C?

A. –373 K

B. –173 K

C. 173 K

D. 373 K

### Markscheme

C

TºK = (tºC + 273) or tºC = (TK – 273)

Hence –100°C in K is –100+273 = 173K

### Question

A sample of solid copper is heated beyond its melting point. The graph shows the variation of temperature with time.

During which stage(s) is/are there an increase in the internal energy of the copper?

A. P, Q and R

B. Q only

C. P and R only

D. Q and R only

### Markscheme

A

When a solid melts, its molecules move apart against the strong molecular attraction. This needs energy which must be supplied from outside. Thus, the  internal energy of a given body is larger in liquid phase than in solid phase. Similarly, the internal energy of a given body in vapour phase is larger than that in liquid phase.

### Question

A pure solid is heated at its melting point. While it is melting the

A. mean kinetic energy of the molecules of the solid increases.
B. mean potential energy of the molecules of the solid increases.
C. temperature of the solid increases.
D. temperature of the solid decreases.

## Markscheme

B

during Phase change (melting) there is no change in Temperature and hence Kinetic Energy remain constant. heat supplied $$Q=mL$$ is used to increase internal potential energy.

### Question

Equal masses of water at 80°C and paraffin at 20°C are mixed in a container of negligible thermal capacity. The specific heat capacity of water is twice that of paraffin. What is the final temperature of the mixture?

A. 30°C

B. 40°C

C. 50°C

D. 60°C

$$ms_w(80-t)=ms_p(t-20)$$
$$(80-t)\times 2s_p=s_p(t-20)$$
$$160-2t=t-20$$
$$t=60^{\circ}C$$