Convection is the movement of heat by the bulk motion of a fluid (liquid or gas). Think of a pot of soup: the hot soup rises, cools, then sinks again, creating a continuous loop that spreads warmth throughout the pot. This process is vital in everyday life, from boiling water to the weather we experience on Earth.
Know that convection is an important method of thermal energy transfer in liquids and gases.
Natural convection happens when a fluid is heated from below and cools from above, creating a buoyancy‑driven flow.
🔹 Example 1: A kettle on a stove – hot water rises, cool water falls.
🔹 Example 2: The atmosphere – warm air rises, cool air descends, forming wind.
Forced convection is when an external force (fan, pump, etc.) moves the fluid.
🔹 Example 1: A computer fan pushing air over a CPU.
🔹 Example 2: A car radiator pump circulating coolant.
| Step | What Happens | Result |
|---|---|---|
| 1 | Heat source warms fluid at the bottom. | Fluid expands → density decreases. |
| 2 | Hot fluid rises. | Cooler fluid moves down to replace it. |
| 3 | Heat is transferred from the source to the surrounding fluid. | Temperature gradient is reduced. |
The rate of heat transfer by convection is given by:
\$ Q = h\,A\,(T{\text{surface}} - T{\text{fluid}}) \$
where:
🔎 Key Points to Remember:
• Convection involves fluid motion.
• Natural vs. forced convection – know the difference.
• Use the convection formula to calculate heat transfer.
• In diagrams, show arrows indicating flow direction.
• For multiple-choice, look for clues like “external force” → forced convection.
• Practice sketching a convection loop: label hot fluid rising, cool fluid sinking, and the heat source.