B.1.13—Convection
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
Explain Convection in Fluids
Density difference drives convection
When part of a liquid or gas is heated, it generally expands and becomes less dense. The warmer region experiences greater buoyancy and rises while cooler, denser fluid sinks.
A convection current
The rising warm fluid and sinking cool fluid form a circulation. The fluid’s bulk motion carries internal energy from the warmer region to other parts of the fluid.
What the syllabus asks
This objective is qualitative: identify the density change, the direction of motion and how that motion transfers energy. It does not require a detailed fluid-dynamics calculation.
Common trap
Convection occurs in fluids, not in a rigid solid. A solid can conduct energy even though it does not circulate as a bulk fluid.
B.1.13 Exam Analysis
Assessment in practice
2 marksHow it is assessed
The evidence asks why convection regions form in a star and awards the hot-core/cool-surface temperature contrast plus the corresponding density-driven motion.
Command terms
Outline / Explain
What earns marks
For a qualitative convection explanation, identify the temperature difference, the resulting density difference and the direction of bulk fluid motion. Hotter fluid becomes less dense and rises; cooler denser fluid sinks, producing a circulation that transfers energy.
Watch for
Saying that hot fluid sinks, or describing conduction without fluid motion.
Representative question
Question 1
[Maximum number: 2]
Outline why regions of convection form in Star A.
The core of star A is at a much higher temperature than the surface
Gas/plasma in the hot core becomes less dense and moves to the surface of the star
OR
Gas/plasma at the cool surface becomes more dense and moves toward the core