Why is a vacuum used in a flask?

A vacuum layer has no matter, so conduction and convection stop and only radiation remains.

Why is real heating always slower than the calculation?

Some energy leaks to the air and the container, so the water absorbs only part of what is supplied.

S4 Physics: Heat Capacity and Heat Transfer Paper 2 (power, efficiency and the three transfer routes)

2026-11-03

S · 4

Paper 2 turns energy into time: power and efficiency first, then conduction, convection and radiation kept apart.

Quick answer　70 s; 560 W; conduction, convection, radiation; conduction; 32 C; 630 W; 235 s; 32,000 J lost (16%); a vacuum stops conduction and convection; convection does not occur inside solids.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Physics: Heat Capacity and Heat Transfer
Level	S4

02　Questions (10 items)

FoundationA 2 kW heater delivers 90 per cent of its energy to 1 kg of water. How long does it take to raise the water from 20 C to 50 C?
FoundationOne kilogram of water rises from 20 C to 60 C in 5 minutes. Find the average power of the heater.
FoundationWhat are the three ways in which heat is transferred?
FoundationThe handle of a metal spoon in hot soup soon becomes hot. Which transfer route dominates?
IntermediateA 600 W heater runs for 140 s in 500 g of water and 80 per cent of the energy is absorbed. Find the temperature rise.
IntermediateA heating curve for 1 kg of water has a slope of 0.15 degrees per second. Find the heating power.
IntermediateA 2.2 kW kettle brings 1.5 kg of water from 18 C to boiling at 100 C. Ignoring losses, how many seconds, to the nearest whole number?
ChallengeA 1 kW heater runs 200 s in 2 kg of water and the temperature rises only 20 C. Find the energy lost and its percentage of the supply.
ChallengeExplain why the vacuum layer of a flask slows both conduction and convection.
Error redoA student writes that convection is the transfer of heat inside solids. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	70 s	The water needs 126,000 J, so the heater must supply 140,000 J, which at 2,000 W takes 70 s.
2	560 W	The energy is 168,000 J over 300 s, giving 560 W.
3	Conduction, convection and radiation	They work by different means: particle vibration, bulk fluid flow and electromagnetic waves.
4	Conduction	Free electrons and lattice vibrations carry energy along the metal without bulk movement.
5	32 C	The absorbed energy is 67,200 J, and dividing by 2,100 gives 32 degrees.
6	630 W	Power is m times c times the slope, so 4,200 times 0.15 gives 630 W.
7	About 235 s	The energy is 516,600 J, and dividing by 2,200 W gives 234.8 s, rounded to 235 s.
8	32,000 J lost, which is 16 per cent	The supply is 200,000 J, the water absorbs 168,000 J, and the 32,000 J difference is 16 per cent.
9	A vacuum has no particles to vibrate and no fluid to flow, so both routes are cut off.	Both conduction and convection need matter; a vacuum removes matter and leaves only radiation.
10	Convection needs bulk flow of a fluid, so it happens only in liquids and gases; solids rely on conduction.	Particles in a solid are locked in a lattice and cannot flow, so convection cannot occur.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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