Physics unit guide
The particulate nature of matter
Thermal concepts, gas laws, the kinetic model, internal energy and phase change.
Unit 2 is conceptually gentle and numerically unforgiving. Almost every lost mark comes from one of two things: leaving a temperature in Celsius where the physics needs kelvin, or forgetting that a substance changing phase absorbs energy without changing temperature. Both are avoidable by habit rather than understanding, which makes this one of the cheapest units to secure.
Topics in this unit
What gets examined
- Converting between Celsius and kelvin, and knowing which equations require absolute temperature.
- Specific heat capacity and specific latent heat calculations, including multi-stage heating problems.
- The ideal gas law, and the assumptions of the kinetic model that make it an approximation.
- Relating average kinetic energy of particles to absolute temperature.
- Interpreting a heating curve, including the flat sections where phase change occurs.
- Explaining pressure in terms of particle collisions with container walls.
How to revise it
Convert to kelvin before anything else
Write the conversion as the first line of every thermal question. Gas laws and kinetic energy relationships are wrong in Celsius, and the error is invisible in the working.
Break heating problems into stages
Heat to the melting point, melt, heat to the boiling point, boil. Each stage is its own calculation with its own equation. Trying to do it in one step is the classic way to lose the latent heat term entirely.
Learn the kinetic model assumptions as a list
Negligible volume, no intermolecular forces, elastic collisions, random motion. These are recalled and stated, and questions ask for them directly.
Say why temperature is constant during a phase change
Energy goes into breaking intermolecular bonds rather than increasing kinetic energy. That sentence is the answer to a question that appears almost every session.
Where marks get lost
- Using Celsius in pV = nRT or in kinetic energy relationships.
- Omitting the latent heat stage in a problem that crosses a phase change.
- Saying particles in an ideal gas have no volume rather than negligible volume.
- Confusing heat with temperature in an explanation.
- Using the specific heat capacity of the wrong substance in a mixture problem.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.