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Physics: Thermal Physics - Conduction, Convection, and Radiation

Grade 8IB

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

🔑Concepts

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Thermal energy always flows from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached (Thot=TcoldT_{hot} = T_{cold}).

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Conduction: The transfer of thermal energy through a substance without the substance itself moving. It occurs mainly in solids through molecular vibrations and, in metals, via free electrons.

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Convection: The transfer of thermal energy in fluids (liquids and gases) by the movement of the fluid itself. As a fluid is heated, it expands, its density ρ\rho decreases, and it rises, creating a convection current.

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Radiation: The transfer of energy by infrared electromagnetic waves. Unlike conduction and convection, it does not require a 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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Surface Effects: Dull, black surfaces are the best absorbers and emitters of infrared radiation, while shiny, silver (white) surfaces are the best reflectors and worst emitters.

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Insulation: Materials with low thermal conductivity (like air, wood, or plastic) are used to reduce the rate of heat transfer by conduction.

📐Formulae

ΔT=Tfinal−Tinitial\Delta T = T_{final} - T_{initial}

ρ=mV\rho = \frac{m}{V}

Q=mcΔTQ = mc\Delta T

Rate of Heat Transfer∝ΔT\text{Rate of Heat Transfer} \propto \Delta T

💡Examples

Problem 1:

A metal rod has one end placed in a flame. After some time, the other end becomes hot. If the initial temperature of the cool end was 25∘C25^{\circ}C and it rose to 75∘C75^{\circ}C, calculate the change in temperature ΔT\Delta T.

Solution:

ΔT=75∘C−25∘C=50∘C\Delta T = 75^{\circ}C - 25^{\circ}C = 50^{\circ}C

Explanation:

This is an example of conduction. Thermal energy is transferred through the metal rod via the vibration of atoms and the movement of free electrons from the hot end to the cold end.

Problem 2:

During the day, the air above the land heats up faster than the air above the sea. Explain the movement of air using the concept of density ρ\rho.

Solution:

Land air heats up →\rightarrow Volume VV increases →\rightarrow Density ρ=mV\rho = \frac{m}{V} decreases →\rightarrow Air rises.

Explanation:

As the warm air over the land rises due to lower density, cooler air from the sea moves in to take its place. This creates a 'sea breeze,' which is a classic example of a convection current.

Problem 3:

Two identical cans, one painted dull black and one painted shiny silver, are filled with boiling water at 100∘C100^{\circ}C. Which can will cool down to 60∘C60^{\circ}C faster?

Solution:

The dull black can.

Explanation:

Dull black surfaces are better emitters of infrared radiation than shiny silver surfaces. Therefore, the black can loses thermal energy to the surroundings more rapidly via radiation, leading to a faster decrease in temperature.

Conduction, Convection, and Radiation Grade 8 Notes & Examples