IB Physics HL B.1 Thermal energy transfers questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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Question 1

Paper 2Easy7 marks

A student models the apparent brightness $b$ of a star at different distances $d$ assuming the star’s luminosity $L$ remains constant. The relationship is

b=\frac{L}{4\pi d^2}

where $b$ is measured in $\mathrm{W\,m^{-2}}$ $L$ in $\mathrm{W}$ and $d$ in $\mathrm{m}$

Figure 1 shows the modelled apparent brightness as the distance from the star is changed.

(a)

Use Figure 1 to determine the apparent brightness of the star when $d=1.0\times10^{16}\,\mathrm{m}$

[1]

(b)

Using your answer to (a), calculate the luminosity of the star.

[3]

(c)

Use Figure 1 to determine the apparent brightness when $d=2.0\times10^{16}\,\mathrm{m}$

[1]

(d)

State the factor by which the apparent brightness changes when the distance increases from $1.0\times10^{16}\,\mathrm{m}$ to $2.0\times10^{16}\,\mathrm{m}$ and explain this change using the relationship given.

[2]

Question 2

Multiple choiceMedium1 mark

During a 10.0 min interval, a sample is melting at constant temperature. A 400 W heater is on for a total of 5.0 min; assume all energy supplied while it is on enters the sample. Which row gives the energy supplied by the heater and the particle behaviour during melting?

Show answer and explanation

Answer: B. The energy supplied is $E=Pt$ using the time for which the heater is on: $E=(400\ \text{W})(5.0\times60\ \text{s})=1.2\times10^5\ \text{J}=120\ \text{kJ}$ During melting, the energy changes particle behaviour and arrangement while the average kinetic energy, and therefore the temperature, remains constant.
A: treats 5.0 min as 5.0 s, giving 2.0 kJ.
C: calculates the energy correctly but applies the temperature-rise model instead of recognizing a phase change.
D: uses the full 10.0 min interval as the heater-on time, giving 240 kJ.
Remember: energy can be transferred during a phase change without increasing the temperature.

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