IB Physics B.1.3 Mechanisms of Thermal Energy Transfer Notes

Conduction, Convection, and Radiation: How Thermal Energy Moves

1. Thermal energy can move in three distinct ways: conduction, convection, and radiation.
2. Each method relies on different principles and occurs in different contexts.

Conduction: Energy Transfer Through Direct Contact

1. Consider holding one end of a metal rod while the other end is heated by a flame.
2. Over time, the heat travels along the rod, reaching your hand.
3. This process is called conduction.
4. This transfer happens because particles in the hotter region have greater average kinetic energy than particles in the cooler region, so collisions between them gradually pass energy along.

How Conduction Works

1. Particle Interactions:
   1. In solids, particles are closely packed and vibrate around fixed positions.
   2. When one end of the solid is heated, its particles gain kinetic energy and vibrate more vigorously.
2. Energy Transfer:
   1. These energetic particles collide with neighboring particles, transferring some of their kinetic energy.
   2. This chain reaction continues, spreading energy from the hot end to the cooler end.

Quantifying Conduction

The rate of energy transfer by conduction is described by the formula:

$$\frac{\Delta Q}{\Delta t} = k A \frac{\Delta T}{\Delta x}$$

where:

1. $\frac{\Delta Q}{\Delta t}$ is the rate of energy transfer (in watts, W).
2. $k$ is the thermal conductivity of the material (in $\text{W m}^{-1} \, \text{K}^{-1}$).
3. $A$ is the cross-sectional area through which energy is transferred (in $m^2$).
4. $\Delta T$ is the temperature difference between the two ends (in K or °C).
5. $\Delta x$ is the distance between the two ends (in m).

Convection: Energy Transfer Through Fluid Motion

Convection occurs in fluids (liquids and gases) and involves the movement of the fluid itself.

How Convection Works

1. Heating: When a fluid is heated, it expands and becomes less dense.