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Thermal Physics - Transfer of thermal energy (Conduction, convection and radiation)

Grade 11IGCSEPhysics

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

🔑Concepts

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Conduction: The process by which thermal energy is transmitted through a medium from a region of higher temperature to a region of lower temperature without the movement of the medium as a whole. In solids, this occurs via lattice vibrations and, in metals, primarily through the movement of free electrons.

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Convection: The transfer of thermal energy in fluids (liquidsliquids and gasesgases) through the bulk movement of the fluid itself. When a fluid is heated, it expands, its density ρ\rho decreases (ρ=mV\rho = \frac{m}{V}), causing it to rise and be replaced by cooler, denser fluid, creating a convection current.

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Radiation: The transfer of energy by infrared (IR) electromagnetic waves. It does not require a medium and can occur in a vacuum. All objects above 0 K0\ K emit radiation; the rate depends on surface temperature TT and surface color/texture.

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Emission and Absorption: Dull, black surfaces are the best emitters and absorbers of infrared radiation. Shiny, silver, or white surfaces are the best reflectors and the poorest emitters and absorbers.

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Thermal Insulation: Reducing the rate of heat transfer using materials with low thermal conductivity (insulators), trapping air to prevent convection, or using reflective surfaces to reduce radiation (e.g., in a vacuum flask).

📐Formulae

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

ρ=mV\rho = \frac{m}{V}

ΔQ=mcΔθ\Delta Q = mc\Delta \theta

P∝A(Tsurface−Tsurroundings)P \propto A(T_{surface} - T_{surroundings})

💡Examples

Problem 1:

A solar water heater has a copper pipe painted matte black. Explain why the pipe is made of copper and why it is painted black when the solar intensity is 1000 W/m21000\ W/m^2.

Solution:

Copper is used because it has a high thermal conductivity, allowing efficient conduction of heat to the water. The matte black paint ensures the maximum absorption of incident infrared radiation from the Sun.

Explanation:

Black surfaces have an absorption coefficient close to 11, whereas copper ensures the energy reaches the fluid inside via molecular collisions and free electron transfer.

Problem 2:

A vacuum flask (Thermos) has double silvered glass walls with a vacuum in between. How does this design minimize heat loss for a liquid at 95∘C95^\circ C?

Solution:

The vacuum eliminates conduction and convection as these require a medium. The silvered surfaces reflect infrared radiation back into the flask.

Explanation:

Since a vacuum contains no particles, lattice vibrationslattice\ vibrations and fluid flowfluid\ flow cannot occur. The silver coating acts as a poor emitter and a high-quality reflector for radiation waves.

Problem 3:

Calculate the thermal energy QQ required to heat 0.5 kg0.5\ kg of water in a kettle from 20∘C20^\circ C to 100∘C100^\circ C. (Specific heat capacity of water c=4200 J/kg∘Cc = 4200\ J/kg^\circ C).

Solution:

Δθ=100∘C−20∘C=80∘C\Delta \theta = 100^\circ C - 20^\circ C = 80^\circ C ΔQ=mcΔθ\Delta Q = mc\Delta \theta ΔQ=0.5 kg×4200 J/kg∘C×80∘C=168,000 J=168 kJ\Delta Q = 0.5\ kg \times 4200\ J/kg^\circ C \times 80^\circ C = 168,000\ J = 168\ kJ

Explanation:

This represents the total energy transferred into the system, primarily via convection currents within the water.

Transfer of thermal energy (Conduction, convection and radiation) Revision - Grade 11 Physics IGCSE