How do I tell the three heat transfer mechanisms apart?

Moving matter means convection, particle collisions inside a solid mean conduction, and needing no medium means radiation.

Why is thick glass worse than a vacuum?

Thick glass only slows conduction, while a vacuum stops both conduction and convection.

S4 Physics heat transfer paper 4: ten common slips

2026-11-10

Tutor Prep Guide · S4 Physics Heat Transfer

All ten items are rewritten from the most common slips; redo each one before checking.

Direct answers　Items 01 to 03 retry the conduction formula and units, 04 to 06 ratios and double glazing, 07 to 09 efficiency and insulation, and 10 the thickness multiplied instead of divided.

01　Paper details (check answers afterwards)

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

02　Questions (10 items)

FoundationA copper plate has k = 400 W/(m K), area 0.5 m2, thickness 20 mm and a 50 K difference across it. Find the conduction rate.
FoundationA glass window of k = 1.0 W/(m K), area 2 m2 and thickness 5 mm has a 10 K difference. How much energy passes each second?
FoundationAn insulation board 0.05 m thick and 0.1 m2 in area passes 10 W across a 20 K difference. Find k.
IntermediateFor the same glass, the thickness doubles while the area triples. What is the new rate as a multiple of the old?
IntermediateDouble glazing: two 4 mm panes (k = 1.0) and a 12 mm air gap (k = 0.026), area 2 m2, 20 K difference. Find the total rate.
IntermediateA 250 W heater runs for 45 minutes. How much energy does it release?
ChallengeA panel receives 800 W per square metre over 1.5 m2 at 18 per cent efficiency. Find the electrical output.
ChallengeA 2 m2 wall insulated with k = 0.04 faces a 25 K difference. Find the rate at 0.1 m, and the minimum thickness for at most 15 W.
ChallengeA metal rod of k = 200 W/(m K) has a 2 cm2 cross-section, is 0.5 m long, and spans 180 K. Find the conduction rate.
Error redoA student writes kA(delta T) x L and gets 0.32 W. What is the correct rate and what went wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	500 000.0 W	Convert L to 0.02 m: 400 x 0.5 x 50 / 0.02 = 500 000.0 W; leaving the thickness in millimetres is a hundredfold error.
2	4 000.0 J	5 mm is 0.005 m, so 1.0 x 2 x 10 / 0.005 = 4 000.0 W, and a watt is a joule per second.
3	0.25 W/(m K)	Rearrange: k = (Q/t) x L / (A delta T) = 100 x 0.05 / (0.1 x 20) = 2.5, in W/(m K).
4	1.5 times	The rate goes as A and as 1/L, so 3 / 2 = 1.5; doubling the thickness alone would halve it.
5	about 85.2 W	Thermal resistances add: R = 2 x 0.004 / 1.0 + 0.012 / 0.026 = 0.470, so 2 x 20 / R = 85.2 W against 10 000.0 W for single glazing.
6	675 000 J	45 minutes is 2700 s, so Q = 250 x 2700 = 675 000 J; using 45 directly undercounts sixtyfold.
7	216.0 W	Incident power is 800 x 1.5 = 1200 W, times 0.18 gives 216.0 W, with 984.0 W becoming heat.
8	20.0 W; at least 0.133 m (13 cm)	First 0.04 x 2 x 25 / 0.1 = 20.0 W, then L = 0.04 x 2 x 25 / 15 = 0.1333 m, rounded up to 13 cm.
9	14.4 W	2 cm2 is 2 x 10^-4 m2, so 200 x 2e-4 x 180 / 0.5 = 14.4 W; treating cm2 as m2 inflates it ten thousandfold.
10	800.0 W	The formula divides by thickness: 0.8 x 0.5 x 40 / 0.02 = 800.0 W. Multiplying treats thickness as helping heat through, which reverses the physics.

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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