1. Thermal Energy and Temperature
Understanding the distinction between these two is fundamental to thermodynamics.
Temperature: A measure of the average kinetic energy of the particles in a substance. It determines the direction of thermal energy flow (from high to low temperature).
Thermal Energy: The total internal energy of an object due to the motion of its particles.
Note: A giant iceberg has more thermal energy than a cup of boiling water because it has vastly more particles, even though its temperature is lower.
Example: If you have two beakers of water at 50°C, one containing 100ml and the other 200ml, they have the same temperature, but the 200ml beaker has double the thermal energy.
2. Internal Energy
Internal energy is the "energy store" kept within a substance.
Definition: The sum of the random distribution of kinetic and potential energies of all the molecules in a system.
Kinetic Energy: Due to the motion of particles (vibration, rotation, or translation).
Potential Energy: Due to the intermolecular forces (bonds) between particles.
Changes in State: When a substance changes state (e.g., melting), the temperature remains constant because the energy is used to increase the potential energy (breaking bonds) rather than kinetic energy.
3. Heat Transfer Mechanisms
Thermal energy always moves from a region of higher temperature to a region of lower temperature.
A. Conduction
The transfer of thermal energy through a medium without the movement of the medium itself.
Mechanism: Energetic particles vibrate and collide with neighbors, transferring energy. In metals, delocalized electrons also carry energy rapidly through the lattice.
Example: A metal spoon heating up in a hot bowl of soup.
B. Convection
The transfer of thermal energy in fluids (liquids and gases) by the movement of the fluid itself.
Mechanism: When heated, fluid expands, becomes less dense, and rises. Cooler, denser fluid sinks to take its place, creating a convection current.
Example: The heating of water in a kettle or sea breezes.
C. Radiation
The transfer of energy by infrared (IR) waves.
Mechanism: Unlike conduction and convection, radiation does not require a medium; it can travel through a vacuum.
Surfaces: Dull, black surfaces are the best emitters and absorbers. Shiny, silver surfaces are the best reflectors.
Example: Thermal energy from the Sun reaching the Earth.
4. Thermal Expansion and Contraction
As the internal energy of a substance changes, its physical dimensions change.
Expansion: When heated, particles gain kinetic energy and move/vibrate more vigorously. This push against neighboring particles increases the average distance between them, causing the volume to increase.
Contraction: When cooled, particles lose kinetic energy and move closer together, decreasing the volume.
States of Matter: Gases expand the most, followed by liquids, then solids.
Applications: Railway tracks have small gaps to allow for expansion in summer.
Consequences: Bridges are often built on rollers at one end to prevent buckling.
5. Temperature Scales and Absolute Zero
IGCSE requires knowledge of both the Celsius and Kelvin scales.
The Kelvin Scale (Thermodynamic Scale)
The Kelvin scale starts at Absolute Zero, the lowest possible theoretical temperature.
Absolute Zero (0 K): The temperature at which particles have the minimum possible kinetic energy (molecular motion effectively ceases). This occurs at $-273^\circ\text{C}$.
Conversion Formulas
To convert between the two scales, use the following:
Solve these problems and compare your answers with my video explanation.
