CIE IGCSE Physics (0625) Convection Study Notes - New Syllabus
CIE IGCSE Physics (0625) Convection Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Convection
Key Concepts:
- Convection in Liquids and Gases
- Convection in Liquids and Gases – Particle and Density Explanation
Convection in Liquids and Gases
Convection in Liquids and Gases
Convection is the transfer of thermal energy in a fluid (liquid or gas) through the bulk movement of particles.It occurs when particles gain heat energy, move apart, and cause density changes within the fluid.
How Convection Works:
- When a fluid is heated, the particles at the heated region gain kinetic energy and move faster.
- This makes the fluid expand and become less dense.
- The less dense (warmer) fluid rises and the denser (cooler) fluid sinks.
- This creates a convection current, circulating heat through the fluid.
Key Conditions:
- Convection only occurs in liquids and gases (fluids), because the particles can move freely.
- It does not occur in solids, as particles in solids are fixed in place and cannot flow.
Example:
A beaker of water is placed on a tripod and heated from below using a Bunsen burner. After some time, the water at the top becomes warm, even though only the bottom is directly heated.
Explain how the water becomes evenly heated using the concept of convection.
▶️ Answer/Explanation
- The water at the bottom of the beaker gains heat energy and the particles move faster.
- This warm water expands and becomes less dense than the cooler water above.
- As a result, the warmer, less dense water rises.
- The cooler, denser water from above sinks to take its place.
- This creates a convection current which continues to circulate the water.
- Eventually, the thermal energy is distributed throughout the beaker, making the whole volume warm.
Convection transfers heat from the bottom to the top of the beaker by circulating water through density-driven currents.
Convection in Liquids and Gases - Particle and Density Explanation
Convection in Liquids and Gases – Particle and Density Explanation
Why convection occurs:
- When part of a fluid (liquid or gas) is heated, the particles in that region gain kinetic energy and move faster.
- This causes the fluid to expand, making it less dense than the surrounding cooler fluid.
- The warmer, less dense fluid rises, and the cooler, denser fluid sinks to take its place.
- This movement sets up a convection current – a continuous flow that transfers heat throughout the fluid.
Experiment 1: Convection in a Liquid (Water + Dye)
Aim: To observe convection currents in water.
Apparatus:
- Glass beaker or large glass container
- Tripod stand and Bunsen burner (or heater)
- Potassium permanganate crystals or food dye
- Cold water
Method:
- Fill the beaker with cold water and let it settle.
- Gently place a few crystals of potassium permanganate or a drop of dye at the bottom corner of the beaker (use a straw or glass rod).
- Gently heat the bottom of the beaker directly under the dye using the Bunsen burner.
Observation:
- You will see purple or colored streaks slowly rising from the bottom and spreading through the water.
- This is the path of the rising warm, less dense water and sinking cooler, denser water.
Conclusion:
- The movement of the dye shows the convection current formed by density differences.
Experiment 2: Convection in a Gas (Smoke Box)
Aim: To observe convection currents in air.
Apparatus:
- Convection box (or a metal box with two chimneys — can be made using a shoebox)
- Smoke source (incense stick or smoldering paper)
- Candle or small spirit lamp
Method:
- Place the candle inside the box under one chimney.
- Light the incense stick and hold it near the other chimney opening.
Observation:
- Smoke enters through one chimney, flows inside the box, and exits through the chimney above the candle.
Conclusion:
- The candle heats the air above it, making it less dense, so it rises and escapes.
- Cooler, denser air (with smoke) is drawn in through the other chimney to replace it — forming a convection current in air.
Example:
In winter, a person places a small electric heater on the floor of a cold room. After some time, the upper part of the room becomes noticeably warmer than the lower part.
Explain this observation using the concept of convection in air.
▶️ Answer/Explanation
- The air near the heater gains thermal energy and the particles move faster.
- This heated air expands, becomes less dense, and rises upwards.
- Cooler, denser air near the ceiling sinks down to take its place.
- This sets up a convection current that slowly warms the room.
- As a result, the top part of the room becomes warmer first, while the bottom remains cooler until convection fully circulates the heat.
Warm air from the heater rises due to its lower density, and cooler air sinks, creating a convection current that warms the upper part of the room first.