Temperature and thermal energy
IB Physics SL· Unit 2: Thermal Physics, Topic 2.1· 15 min read
1. Core Definitions: Key Distinctions★☆☆☆☆⏱ 4 min
The most common mistake in IB thermal physics is confusing temperature, thermal energy and internal energy: each describes a different property of a system of particles.
Internal energy
The total sum of the random kinetic and potential energies of all particles in a system
Example:
A 1 kg block of iron at 50°C has more internal energy than a 0.1 kg block at the same temperature
Temperature
A measure of the average random kinetic energy of particles in a system, defines the direction of thermal energy transfer
Example:
Two objects at the same temperature have particles with the same average kinetic energy, regardless of mass
Thermal energy
Non-mechanical energy transferred between a system and surroundings due to a temperature difference
Example:
Thermal energy flows from a hot coffee cup to cool surrounding air
A student claims a bucket of boiling water has the same temperature and same thermal energy as a cup of boiling water. Evaluate this claim.
- 1
First compare temperature: both are boiling at standard pressure, so they are at 100°C, so their temperature is equal.
- 2
Next compare thermal energy: the bucket holds far more water molecules than the cup. Total energy of all molecules is much larger for the bucket.
- 3
Conclusion: the claim is incorrect. Temperature is the same, but thermal energy of the bucket is much greater.
Exam tip:
Examiners frequently test the distinction between these three quantities in both Paper 1 and Paper 2.
2. Temperature Scales: Celsius and Kelvin★★☆☆☆⏱ 4 min
IB Physics requires conversion between the Celsius (°C) and absolute thermodynamic Kelvin (K) temperature scales. Absolute zero, the lowest theoretical temperature where particles have minimum kinetic energy, is defined as 0 K = -273.15°C.
For almost all IB exam calculations, rounding to 273 instead of 273.15 is fully acceptable.
Convert normal body temperature (37°C) to Kelvin, and convert the boiling point of liquid nitrogen (77 K) to degrees Celsius.
- 1
Step 1: Convert 37°C to Kelvin:
- 2
- 3
Step 2: Rearrange to convert 77 K to °C:
- 4
Check your understanding:
What is 0 K in degrees Celsius?
0°C
-273°C
273°C
100°C
Reveal answer
1 —Correct! Absolute zero is defined as 0 K, equal to -273°C.
3. Thermal Energy Transfer and Equilibrium★★☆☆☆⏱ 4 min
Net thermal energy is only transferred when there is a temperature difference between two systems in thermal contact. Transfer always flows from higher temperature to lower temperature, until thermal equilibrium is reached.
Thermal equilibrium
A state where two systems in thermal contact have no net transfer of thermal energy, meaning they are at the same temperature
An 80°C iron block is placed in contact with a 20°C copper block. Describe energy transfer and the final state.
- 1
Step 1: Identify the temperature difference: iron has a higher temperature than copper.
- 2
Step 2: State the direction of net transfer: net thermal energy flows from iron to copper.
- 3
Step 3: Temperature changes: iron cools down, copper warms up.
- 4
Step 4: Final state: when both blocks reach the same temperature, net transfer stops, they are in thermal equilibrium.
4. Particle Model Explanation★★☆☆☆⏱ 3 min
We can explain all thermal processes using the particulate nature of matter, the core foundation of thermal physics:
Faster moving (higher kinetic energy) particles collide with slower moving particles.
Collisions transfer kinetic energy from faster to slower particles.
Over time, average kinetic energy equalizes across the combined system, resulting in equal temperature and thermal equilibrium.
5. Common Pitfalls
Wrong move:
Confusing temperature with thermal or internal energy
Why:
Temperature measures average kinetic energy, so mass does not change it; total energy depends on mass
Correct move:
Always check which quantity the question asks for: same temperature does not mean same total energy for different masses.
Wrong move:
Using Celsius instead of Kelvin in thermal formulas
Why:
Most thermal physics formulas (e.g. ideal gas law) require absolute temperature, so using Celsius gives wrong results
Correct move:
Always convert Celsius to Kelvin before substituting into formulas, except for temperature changes (ΔT is the same in both scales).
Wrong move:
Claiming no energy transfer at all at thermal equilibrium
Why:
Examiners mark this wrong because small random energy exchanges still occur
Correct move:
State that there is no net thermal energy transfer between systems at equilibrium.
Wrong move:
Overcomplicating temperature conversion with 273.15
Why:
Unnecessary precision leads to arithmetic errors in exams
Correct move:
Use T = θ + 273 for all standard IB calculations, unless explicitly asked for higher precision.
6. Quick Reference Cheatsheet
Quantity | Symbol | Definition | Unit |
|---|---|---|---|
Temperature | T (K), θ (°C) | Average random KE of particles | K / °C |
Thermal energy | Q | Energy transferred due to ΔT | Joule (J) |
Internal energy | U | Total KE + PE of all particles | Joule (J) |
Temperature conversion | n/a | n/a | |
Thermal equilibrium | n/a | No net transfer, equal | n/a |
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 · Paper 1
Temperature scale conversion
- 2024 · Paper 2
Distinguish thermal vs internal energy
- 2023 · Paper 1
Thermal equilibrium concept
What's Next
This sub-topic provides the foundational definitions for all thermal physics in IB SL Physics. Mastering the distinction between temperature, thermal energy and internal energy will help you avoid common costly exam mistakes and correctly solve problems involving heat transfer, phase changes and ideal gases. These core concepts are also revisited in higher level topics if you continue studying physics beyond IB SL.