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Thermal Physics - Transfer of thermal energy

Grade 12IGCSEPhysics

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Conduction: The process by which thermal energy is transferred through a substance by the vibration of atoms and the movement of free electrons. In metals, 'delocalized' electrons gain kinetic energy and move rapidly, colliding with distant metal ions to transfer energy, which is why metals are better conductors than insulators.

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Convection: The transfer of heat in fluids (liquids and gases) caused by the movement of the fluid itself. When a fluid is heated, it expands, its density ρ\rho decreases, and it rises. Cooler, denser fluid sinks to take its place, creating a convection current.

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Radiation: The transfer of thermal energy via infrared electromagnetic waves. Unlike conduction and convection, radiation does not require a physical medium and can travel through a vacuum at the speed of light c≈3×108 m/sc \approx 3 \times 10^8 \text{ m/s}.

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Emission and Absorption: Dull, black surfaces are the best absorbers and emitters of infrared radiation. Conversely, light-colored, shiny surfaces are poor emitters and absorbers but excellent reflectors.

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Factors affecting Radiation: The rate of emission depends on the surface temperature TT (measured in Kelvin), the surface area AA, and the nature of the surface (emissivity). The power emitted is proportional to T4T^4.

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Thermal Equilibrium: Two objects are in thermal equilibrium when they are at the same temperature and there is no net transfer of thermal energy between them.

📐Formulae

ΔQ=mcΔT\Delta Q = mc\Delta T

P=QtP = \frac{Q}{t}

P=σAeT4P = \sigma A e T^4

ΔQΔt=kAΔTd\frac{\Delta Q}{\Delta t} = \frac{kA\Delta T}{d}

💡Examples

Problem 1:

A glass window has an area of 2.0 m22.0 \text{ m}^2 and a thickness of 0.005 m0.005 \text{ m}. The temperature inside the room is 20∘C20^{\circ}\text{C} and the temperature outside is 5∘C5^{\circ}\text{C}. If the thermal conductivity of glass is k=0.8 W m−1K−1k = 0.8 \text{ W m}^{-1}\text{K}^{-1}, calculate the rate of thermal energy loss through the window.

Solution:

Using the conduction formula: P=kAΔTdP = \frac{kA\Delta T}{d}. Given: k=0.8k = 0.8, A=2.0A = 2.0, ΔT=20−5=15 K\Delta T = 20 - 5 = 15\text{ K}, and d=0.005 md = 0.005\text{ m}. P=0.8×2.0×150.005=240.005=4800 WP = \frac{0.8 \times 2.0 \times 15}{0.005} = \frac{24}{0.005} = 4800 \text{ W}

Explanation:

The rate of heat transfer (Power) is directly proportional to the area and temperature gradient, and inversely proportional to the thickness of the material.

Problem 2:

Explain why a vacuum flask (Thermos) has silvered internal walls and a vacuum between the two layers.

Solution:

The vacuum prevents heat transfer by conduction and convection because both processes require a medium (particles). The silvered walls reflect infrared radiation back into the flask (or prevent external radiation from entering), as shiny surfaces are poor emitters and good reflectors.

Explanation:

To minimize thermal energy transfer, all three mechanisms (conduction, convection, and radiation) must be addressed. The vacuum eliminates the first two, and the silvering minimizes the third.

Transfer of thermal energy - Revision Notes & Key Formulas | IGCSE Grade 12 Physics