Q238 · Practice questionComplex familiar5 marks
QUESTION 238 (5 marks)
A stationary box receives of energy from an external source. During the transfer, escapes as heat. The box is then sealed and retains the remaining energy. Compare its rest mass before the transfer with its rest mass after it is sealed. Use .
a)[3 marks]
Calculate the increase in the box's rest mass.
b)[2 marks]
Inside the sealed box, stored electrical energy is later converted to light that remains inside. Does this conversion change the total rest mass of the box and its contents? Explain.
WORKED SOLUTION
5 marksPractice marking scheme
ANSWER
(a) . (b) No. Total contained energy remains the same.
Worked solution
(a) The net retained energy is . Its rest-mass contribution is .
(b) The energy changes form, but no energy enters or leaves the system. The rest mass of the complete stationary box-plus-contents system depends on its total contained energy, including radiation, so it is unchanged by this internal conversion.
Equivalent physically justified methods and consistent equivalent units accepted.
Displayed decimals are model answers; accept appropriate significant figures and consistent rounding from stated constants.
Part a: Determine the net retained energy.
Part a: Convert megajoules and apply mass-energy equivalence.
Part a: Calculate a positive mass increase.
Part b: Identify that no energy crosses the system boundary.
Part b: Relate unchanged total system energy to unchanged total rest mass.
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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