B.1 Temperature and thermal energy
IB Physics HL· Theme B: The particulate nature of matter, B.1· 15 min read
1. 1. Key Definitions: Temperature, Thermal Energy and Internal Energy★☆☆☆☆⏱ 5 min
Temperature
(kelvin), (Celsius)
A measure of the average random kinetic energy of the particles in a substance. It determines the direction of thermal energy transfer between systems.
Example:
Hot coffee has higher average particle kinetic energy than cold juice, so higher temperature.
Temperature is an intensive property, meaning it does not depend on the amount of substance present. This is a key distinction from thermal energy, which is an extensive property that depends on the total amount of energy in the system.
Explain why a 10 kg block of iron at 50°C has more thermal energy than a 1 kg block of iron at 50°C, despite having the same temperature.
- 1
Temperature depends on average kinetic energy per particle. Both blocks have the same average kinetic energy per iron atom, so they have equal temperature.
- 2
Thermal energy (total energy of all particles) scales with the number of particles. The 10 kg block has 10 times more iron atoms than the 1 kg block.
- 3
Therefore, the total thermal energy of the 10 kg block is 10 times higher than the 1 kg block, even at the same temperature.
Exam tip:
Always check if questions ask for temperature or thermal energy — these are the most commonly mixed up terms in thermal physics exams.
2. 2. Temperature Scales and Conversions★★☆☆☆⏱ 4 min
IB Physics uses two main temperature scales: the Celsius scale (°C) and the Kelvin (absolute thermodynamic) scale (K). The Kelvin scale is aligned with the definition of absolute zero, the lowest possible temperature, making it the standard for thermodynamics calculations.
Absolute Zero
The lowest possible temperature, where particles have their minimum possible average kinetic energy. No further energy can be removed from the system at this temperature.
Convert 22 °C (room temperature) to Kelvin, and 180 K to degrees Celsius.
- 1
Recall the conversion relationship between Kelvin and Celsius:
- 2
- 3
Convert 22 °C: K, rounded to 295 K for most IB questions.
- 4
Convert 180 K: °C, rounded to -93 °C.
3. 3. Thermal Equilibrium★★☆☆☆⏱ 4 min
When two systems at different temperatures are placed in thermal contact (able to exchange energy), net thermal energy will always transfer from the higher temperature system to the lower temperature system. This continues until thermal equilibrium is reached.
Thermal Equilibrium
A stable state between two systems in thermal contact where there is no net transfer of thermal energy. At equilibrium, both systems have equal temperature.
A hot copper block at 120 °C is placed in contact with an aluminum block at 10 °C inside an insulated container. Describe what happens when the system reaches equilibrium.
- 1
Particles in the hot copper block have higher average kinetic energy, so they transfer energy to the cold aluminum particles via collisions.
- 2
Energy transfer continues until the average kinetic energy of particles in both blocks is equal, meaning both blocks reach the same final temperature.
- 3
At equilibrium, random energy transfers still occur between the blocks, but there is no net transfer of energy in either direction, so the temperature of both blocks remains constant.
4. 4. Internal Energy★★★☆☆⏱ 5 min
Internal energy is the total energy stored by the particles of a system, combining two components: random kinetic energy (from particle motion: translational, rotational, vibrational) and intermolecular potential energy (from the forces between particles).
Internal Energy
The sum of the total random kinetic energy and total intermolecular potential energy of all particles in a system. Temperature is only related to the kinetic energy component.
Ice melts into liquid water at a constant temperature of 0 °C. Explain why the internal energy of liquid water is higher than the internal energy of the original ice.
- 1
Temperature is constant during melting, so the average kinetic energy of the water particles does not change. This means the kinetic component of internal energy stays the same.
- 2
To break the strong intermolecular bonds in solid ice and change state, energy (latent heat) must be added to the system from the surroundings.
- 3
This added energy increases the intermolecular potential energy of the water particles.
- 4
Since internal energy is the sum of kinetic and potential energy, the total internal energy of the liquid water is higher than the original solid ice, even at the same temperature.
5. Common Pitfalls
Wrong move:
Claiming temperature measures total kinetic energy of particles.
Why:
Temperature is average kinetic energy per particle, an intensive property, not total energy.
Correct move:
Recognize that total thermal energy depends on both average kinetic energy and number of particles, so larger objects can have lower temperature but higher total thermal energy.
Wrong move:
Assuming constant temperature always means constant internal energy.
Why:
State changes occur at constant temperature, but intermolecular potential energy changes, so total internal energy changes.
Correct move:
Always account for both kinetic and potential components of internal energy when analyzing temperature or state changes.
Wrong move:
Claiming all particle motion stops at 0 K.
Why:
Quantum mechanics requires particles to have minimum zero-point energy at absolute zero, so motion does not stop completely.
Correct move:
State that 0 K is the temperature where particles have minimum possible average kinetic energy, with no more energy able to be removed from the system.
Wrong move:
Claiming thermal energy flows from higher internal energy to lower internal energy.
Why:
Direction of thermal energy transfer depends on temperature difference, not total internal energy difference.
Correct move:
Net thermal energy always flows from higher temperature to lower temperature, regardless of the total internal energy of each system.
Wrong move:
Confusing the conversion offset between Celsius and Kelvin, using +100 instead of +273.
Why:
The zero point of the Kelvin scale is 273 degrees below the zero point of the Celsius scale.
Correct move:
Memorize the conversion: for all IB calculations.
6. Quick Reference Cheatsheet
Concept | Key Definition | Core Property |
|---|---|---|
Temperature | Average random kinetic energy of particles | Intensive, determines energy flow direction |
Thermal energy | Net energy transferred due to temperature difference | Extensive, always flows high → low temperature |
Internal energy | Total kinetic + potential energy of all particles | Extensive, changes even at constant T for state changes |
Celsius → Kelvin | Conversion formula | |
Absolute zero | 0 K | -273.15 °C, minimum average kinetic energy |
Thermal equilibrium | No net energy transfer | Equal temperature for systems in contact |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 · 1
Convert scales, distinguish core terms
- 2023 · 2
Explain thermal equilibrium concept
Going deeper
What's Next
The core concepts covered in this sub-topic are the foundation for all subsequent thermal physics topics in IB Physics HL. Distinguishing between temperature, thermal energy, and internal energy is critical for analyzing heat transfer, specific heat capacity, and changes of state, which build directly on these definitions. The relationship between temperature and average particle kinetic energy is also extended when you study ideal gases, where it is used to derive the ideal gas law and explain gas macroscopic behavior. Mastery of these basic definitions will help you avoid common exam pitfalls in more complex thermodynamics problems later in the course.