Home / IBDP Physics B.1 Thermal energy transfers IB Style Question Bank : HL Paper 2

# IBDP Physics B.1 Thermal energy transfers IB Style Question Bank : HL Paper 2

IBDP Physics HL 2025 – B.1 Thermal energy transfers HL  Paper 2 Exam Style Questions

## B.1 Thermal energy transfers HL  Paper 2

Kinetic Theory, Temperature, Internal Energy, Specific Heat Capacity, Latent Heat, Heat Transfer, Stefan-Boltzmann Law

### Question- B.1 Thermal energy transfers  HL  Paper 2

A solid piece of chocolate of mass $$82 \mathrm{~g}$$ is placed in a pan over fire. Thermal energy is transferred to the chocolate at a constant rate. The graph shows the variation with time $$t$$, of the temperature $$T$$ of the chocolate. At 6.0 minutes all the chocolate has melted.

The specific heat capacity of solid chocolate is $$1.6 \times 10^3 \mathrm{~J} \mathrm{~kg}^{-1} \mathrm{~K}^{-1}$$.

(a) Show that the average rate at which thermal energy is transferred into the chocolate is about $$15 \mathrm{~W}$$.[3]

(b) Estimate the specific latent heat of fusion of chocolate.[2]

(c) Compare the internal energy of the chocolate at t = 2 minutes with that at t = 6 minutes. [2]

Ans:

Reads change in temperature to be $$45-31$$ OR 14 ‘ $$\mathrm{C} V$$
$Q=0.082 \times 1.6 \times 10^3 \times 14=1.84 \times 10^3 \alpha \mathrm{J} n$
$P=\frac{1.84 \times 10^3}{2.0 \times 60}=15.3 \approx 15 \ll W_n$

b.\begin{aligned} & Q=15.3 \times 4.0 \times 60=3.67 \times 10^3 \ll \mathrm{J} » \\ & L=\frac{3.67 \times 10^3}{0.082}=4.5 \times 10^4 \alpha \mathrm{J} \mathrm{kg}^{-1} »\end{aligned}

c.Internal energy is greater at $$t=6 \mathrm{~min} O R$$ internal energy is lower at $$t=2 \mathrm{~min} O R$$ internal energy increases «as energy is added to the system»
Because kinetic energy «of the molecules» is the same AND potential energy w of the molecules was increased / OWTTE

### Question

A sample of vegetable oil, initially in the liquid state, is placed in a freezer that transfers thermal energy from the sample at a constant rate. The graph shows how temperature of the sample varies with time t.

The following data are available.

Mass of the sample = 0.32 kg

Specific latent heat of fusion of the oil = 130 kJ kg–1

Rate of thermal energy transfer = 15 W

(a ) (i) Calculate the thermal energy transferred from the sample during the first 30 minutes.     [1]

(ii) Estimate the specific heat capacity of the oil in its liquid phase. State an appropriate unit for your answer.     [2]

(b) The sample begins to freeze during the thermal energy transfer. Explain, in terms of the molecular model of matter, why the temperature of the sample remains constant during freezing. [3]

(c) Calculate the mass of the oil that remains unfrozen after 60 minutes.                   [2]

Ans:

a i « » 15 × 30 × 60 » = 27000 «J»

a ii  27 × 10 3 = 0.32 × c × (290 250) OR 2100  J kg-1 K-1 OR J kg-1 0C-1

b

«intermolecular» bonds are formed during freezing

bond-forming process releases energy

OR

«intermolecular» PE decreases «and the difference is transferred as heat»

«average random» KE of the molecules does not decrease/change

temperature is related to «average» KE of the molecules «hence unchanged»

c

mass of frozen oil « = $$\frac{27\times 10^3}{130\times 10^3}$$ » 0.21 «kg»

unfrozen mass «= 0.32 – 0.21»= 0.11 «kg »

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