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Heat Transfer - Convection

Grade 8IB

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

🔑Concepts

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Convection is the process of heat transfer in fluids (liquids and gases) through the actual movement of the matter itself.

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When a fluid is heated, its particles gain kinetic energy, move faster, and spread apart. This causes the fluid to expand and its density (ρ=mV\rho = \frac{m}{V}) to decrease.

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The less dense, warmer fluid rises, while the cooler, denser fluid sinks to take its place. This continuous cycle is known as a convection current.

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Convection cannot occur in solids because the particles are held in fixed positions and are not free to flow.

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Real-world phenomena such as sea breezes, land breezes, and the circulation of magma in the Earth's mantle are driven by convection.

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The rate of heat transfer is proportional to the temperature difference (ΔT\Delta T) between the surface and the fluid.

📐Formulae

ρ=mV\rho = \frac{m}{V}

Q=h×A×(Ts−Tf)Q = h \times A \times (T_s - T_f)

💡Examples

Problem 1:

Explain why a heater is placed on the floor of a room rather than near the ceiling.

Solution:

A heater warms the air near the floor. As the air heats up, it expands, its density decreases (ρwarm<ρcold\rho_{warm} < \rho_{cold}), and it rises toward the ceiling. Cooler, denser air from the top sinks to the floor to be heated, creating a convection current that warms the entire room efficiently.

Explanation:

This demonstrates the formation of convection currents where gravity causes denser fluids to sink and buoyancy causes less dense fluids to rise.

Problem 2:

During the day, a 'sea breeze' blows from the ocean toward the land. Calculate the density of a parcel of air if its mass is 1.225 kg1.225 \text{ kg} and its volume is 1 m31 \text{ m}^3.

Solution:

ρ=mV\rho = \frac{m}{V} ρ=1.2251\rho = \frac{1.225}{1} ρ=1.225 kg/m3\rho = 1.225 \text{ kg/m}^3

Explanation:

During the day, land heats up faster than water. The air above the land becomes hot and rises, creating a low-pressure area. The cooler, denser air over the sea (ρ≈1.225 kg/m3\rho \approx 1.225 \text{ kg/m}^3) moves in to fill the gap, creating the breeze.

Problem 3:

If the heat transfer coefficient hh is 10 W/m2K10 \text{ W/m}^2\text{K}, the surface area AA is 2 m22 \text{ m}^2, and the temperature difference ΔT\Delta T is 20 K20 \text{ K}, calculate the rate of heat transfer QQ.

Solution:

Q=h×A×ΔTQ = h \times A \times \Delta T Q=10×2×20Q = 10 \times 2 \times 20 Q=400 WattsQ = 400 \text{ Watts}

Explanation:

This uses Newton's Law of Cooling, which describes the rate of heat loss from a surface via convection.