Convection | Physics for GCSE/IGCSE - Year 11 PDF Download

Introduction

• Heat primarily propagates through liquids and gases via convection.
  • Convection exclusively takes place within fluids.
  • Solids do not facilitate convection.

Density & Convection

Descriptions of convection currents always involve changes in temperature leading to changes in density.

• Both temperature decreases and increases can initiate convection currents.
• When a liquid or gas is heated, the molecules repel each other, causing the substance to expand.
  • This expansion results in the heated liquid or gas being less dense than its surroundings.
  • The less dense, heated substance rises, while the cooler surrounding substance moves in to fill the vacated space.
  • Eventually, as the hot substance cools, it contracts and descends back down.
  • The resultant movement constitutes a convection current.

• When a liquid or gas undergoes cooling, the molecules draw closer together, causing the substance to contract.
  • This contraction leads to the cold liquid or gas being denser than its surroundings.
  • Consequently, the denser, colder substance descends, allowing warmer liquid or gas to occupy the vacated space.
  • The warmer substance, in turn, undergoes cooling, contracts, and descends.
  • This motion constitutes a convection current.

Demonstrating Convection Currents

• A straightforward illustration of convection in liquids entails taking a beaker of water and introducing several crystals of potassium permanganate to one side, as depicted in the diagram.
• As heat is applied to that side, causing the water to warm up, the potassium permanganate will dissolve in the heated water and ascend along with it, thus demonstrating the convection current.

Diagram showing an experiment with potassium permanganate to demonstrate convection