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How is heat transferred between bodies?
Compare the mechanisms of heat transfer (conduction, convection and radiation), including the Stefan-Boltzmann law ($P/A = \sigma T^4$) and Wien's displacement law ($\lambda_{\max} T = b$) for thermal radiation
A focused answer to the VCE Physics Unit 1 dot point on heat transfer. Defines conduction, convection and radiation, applies the Stefan-Boltzmann law ($P/A = \sigma T^4$) and Wien's displacement law ($\lambda_{\max} T = b$), and works the VCAA SAC-style problem on Earth-Sun radiation balance.
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What this dot point is asking
VCAA wants you to identify the three modes of heat transfer and apply the radiation laws (Stefan-Boltzmann and Wien) quantitatively. These connect Unit 1's thermal physics to the climate dot points.
Conduction
Heat transfer through a material by direct particle-to-particle interactions. Dominant in solids. Free electrons make metals especially good conductors.
Rate depends on material, area, thickness and temperature gradient.
Convection
Heat transfer through a fluid (gas or liquid) by bulk motion. Hot fluid is less dense, rises; cool fluid sinks. Cannot occur in solid or vacuum.
Natural convection: density-driven (a radiator in a room). Forced convection: fan- or pump-driven (a car radiator).
Radiation
Emission of electromagnetic waves (mostly infrared at terrestrial temperatures). Does not require a medium. Only mode that crosses vacuum (sunlight reaching Earth).
Stefan-Boltzmann law. Power radiated per unit area by a black body:
where W m K. Real bodies emit at where (emissivity, between and ) accounts for the surface.
The fourth-power dependence makes radiation strongly dominant at high temperatures.
Wien's displacement law. Peak emission wavelength is inversely proportional to absolute temperature:
A hot object emits at shorter wavelengths. The Sun's K surface peaks at nm (green light). The Earth's K surface peaks at nm (infrared).
Application: Earth's energy balance
The Sun emits in the visible spectrum; the atmosphere is transparent to visible light. Visible sunlight reaches the surface and warms it. The Earth re-emits as infrared. CO, water vapour and other greenhouse gases are partially opaque to infrared and trap part of the re-emitted radiation. This is the greenhouse mechanism.
The fourth-power dependence in Stefan-Boltzmann is critical to climate dynamics: a small change in surface temperature produces a large change in outgoing radiation, which sets the equilibrium.
VCAA exam style
Year 11 SAC tasks include calculating peak emission wavelengths, comparing thermal power per unit area between two bodies at different temperatures, and explaining how a vacuum flask reduces all three modes of heat transfer.
Common traps
Confusing in metres with nanometres. Wien's law in m K gives metres. Convert by for nm.
Forgetting that Stefan-Boltzmann requires kelvin. Like all thermal radiation formulas; using celsius gives nonsense.
Treating emissivity as when it is not. Real bodies emit less than a perfect black body. The Earth's emissivity is close to in the infrared; many metals have .
Saying radiation needs a medium. Radiation crosses vacuum (sunlight, infrared into space).
In one sentence
Heat transfer occurs by conduction (particle collisions, dominant in solids), convection (bulk fluid motion driven by density differences), and radiation (electromagnetic emission, the only mode that crosses vacuum, with and ).
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
Year 11 SAC4 marksThe Sun's surface is at approximately $5800$ K. (a) Use Wien's law ($b = 2.898 \times 10^{-3}$ m K) to find the wavelength of peak emission. (b) The Earth's surface is at approximately $288$ K. Find the ratio of total power emitted per unit area from the Sun to that from the Earth.Show worked answer →
(a) Wien's law. m nm. This is in the visible range (green-yellow).
(b) Stefan-Boltzmann. . Ratio: .
So the Sun's surface radiates roughly times more power per unit area than the Earth's surface.
Markers reward correct units (m K for Wien's , conversion to nm), Stefan-Boltzmann's fourth-power scaling, and the comparison framed as a ratio.
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