krit.club logo

Heat Transfer - Modes of Heat Transfer

Grade 8IB

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

🔑Concepts

•

Heat energy always flows from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached, where T1=T2T_1 = T_2.

•

Conduction: The transfer of thermal energy through direct contact between particles. In solids, kinetic energy is passed via vibrations and collisions. Metals are excellent conductors due to the presence of free electrons that move rapidly through the lattice structure.

•

Convection: The transfer of heat in fluids (liquids and gases) through the bulk movement of the fluid itself. When a fluid is heated, it expands, its density (ρ=mV\rho = \frac{m}{V}) decreases, and it rises, creating a convection current.

•

Radiation: The transfer of energy by infrared electromagnetic waves. This mode of heat transfer does not require a material medium and can occur in a vacuum (e.g., heat from the Sun reaching Earth).

•

Surface Properties: Dark, dull, or matt surfaces are better absorbers and emitters of thermal radiation compared to light-colored, shiny, or polished surfaces, which reflect most radiation.

•

Specific Heat Capacity (cc): The amount of heat energy required to raise the temperature of 1 kg1\text{ kg} of a substance by 1∘C1^{\circ}\text{C} (or 1 K1\text{ K}).

📐Formulae

Q=mcΔTQ = mc\Delta T

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

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

ρ=mV\rho = \frac{m}{V}

💡Examples

Problem 1:

Calculate the amount of heat energy required to raise the temperature of 2 kg2\text{ kg} of water from 25∘C25^{\circ}\text{C} to 75∘C75^{\circ}\text{C}. (Specific heat capacity of water, c=4200 J/kg∘Cc = 4200\text{ J/kg}^{\circ}\text{C})

Solution:

Given: m=2 kgm = 2\text{ kg} c=4200 J/kg∘Cc = 4200\text{ J/kg}^{\circ}\text{C} ΔT=75∘C−25∘C=50∘C\Delta T = 75^{\circ}\text{C} - 25^{\circ}\text{C} = 50^{\circ}\text{C}

Using the formula: Q=mcΔTQ = mc\Delta T Q=2×4200×50Q = 2 \times 4200 \times 50 Q=420,000 JQ = 420,000\text{ J} Q=420 kJQ = 420\text{ kJ}

Explanation:

The total heat energy needed is found by multiplying the mass of the water, the specific heat capacity, and the change in temperature.

Problem 2:

A metal spoon is placed in a hot cup of coffee. After a few minutes, the handle of the spoon becomes hot. Identify the mode of heat transfer and explain the process.

Solution:

The mode of heat transfer is conduction.

Explanation:

Heat energy is transferred from the hot coffee to the submerged part of the spoon. The particles in the metal vibrate more vigorously and collide with neighboring particles, passing the kinetic energy along the handle. Additionally, free electrons in the metal move from the hot end to the cold end, accelerating the transfer.

Problem 3:

Why are houses in hot countries often painted white?

Solution:

Houses are painted white to reduce heat gain through radiation.

Explanation:

White and light-colored surfaces are poor absorbers and good reflectors of thermal radiation. By reflecting the infrared radiation from the Sun, the temperature inside the house remains lower (Tinside<ToutsideT_{inside} < T_{outside}).