Ch 7 · 2.3.2 Convection · O Level Physics 5054
Topic 7 of 9 in Thermal Physics — O Level Physics 5054 — 40 lessons.
Introduction
A heater under a window warms the whole room, and the water at the top of a pan gets hot even though the flame is underneath. In liquids and gases, thermal energy is carried by the fluid itself moving. This is convection. (Syllabus section 2.3.2.)
Key ideas — What convection is
Convection is the transfer of thermal energy in a fluid (a liquid or a gas) by the movement of the fluid itself. It cannot happen in solids, because their particles cannot move from place to place.
🧑🏫 Teacher's explanation
Imagine a crowded swimming pool where a group of people starts splashing vigorously at the bottom. The water near the bottom becomes warmer and lighter, so it rises, while the cooler, heavier water sinks to take its place. This continuous circular movement is convection. Unlike conduction, where heat 'crawls' through particles, convection is the actual physical movement of the substance itself.
📖 Definitions
🔍 Worked example — Easy
A beaker of water is heated from the bottom. Explain why the water at the top becomes warmer.
🔍 Worked example — Standard
In an experiment to show convection in a liquid, a crystal of potassium permanganate is placed at the bottom of a glass tube filled with water. The bottom is heated. Describe the observation.
🔍 Worked example — Exam-style
A gas in a closed container is heated uniformly. If the mass m remains constant, how does the density rho change as the volume V increases due to heating?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — How it works: density changes
When the warm fluid moves away from the heater, it cools, becomes denser and sinks again, completing the loop.
🧑🏫 Teacher's explanation
Imagine a crowded elevator. If everyone in the elevator suddenly started waving their arms wildly and taking up more space, the elevator would feel much more 'full' even though the number of people hasn't changed. In a fluid, heating does something similar: it makes the particles move faster and push further apart, increasing the volume occupied by that mass of fluid.
📖 Definitions
🔍 Worked example — Easy
A volume of 0.5 m^3 of air has a mass of 0.6 kg. If the air is heated so that its volume increases to 0.75 m^3 while the mass remains constant, calculate the new density.
🔍 Worked example — Standard
In a convection experiment, a heated beaker of water shows a rising current. If the density of the cold water is 1000 kg/m^3 and the density of the heated water is 980 kg/m^3, calculate the change in density Δrho.
🔍 Worked example — Exam-style
A gas occupies 2.0 m^3 and has a density of 1.2 kg/m^3. When heated, the gas expands to a volume of 2.5 m^3. Calculate the new density of the gas.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Everyday convection
🧑🏫 Teacher's explanation
Imagine a crowded elevator. If the people inside start getting very excited and spread out, they take up more space (lower density). If they calm down and huddle together, they take up less space (higher density). Convection works exactly like this, but with particles in a fluid.
📖 Definitions
🔍 Worked example — Easy
A heater is placed at the bottom of a container of water. Explain why the water at the top of the container eventually reaches the same temperature as the water at the bottom.
🔍 Worked example — Standard
In a sea breeze during the day, the land heats up faster than the sea. If the air temperature directly above the land is 30 circC and the air above the sea is 25 circC, describe the direction of the air movement.
🔍 Worked example — Exam-style
A student performs an experiment to show convection. They place a candle at the bottom of a glass jar and cover it with a beaker of water. As the water enters the jar, the candle goes out. Explain this in terms of convection.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe, in terms of density, why the water moves in a circular pattern. [3] (b) State one way the student could modify the experiment to show convection in a gas. [1] [4 marks]
Practical notes
Safety: handle potassium manganate(VII) with tweezers (it stains and is harmful), and take care with flames.
Worked example
Air next to the freezing compartment is cooled, so it contracts and becomes denser. The dense cold air sinks, cooling the food below. Warmer air from below is less dense and rises to the top, where it is cooled in turn. A convection current keeps the whole fridge cold.
🔍 Worked example — Easy
A beaker of water is heated from the bottom. Describe the movement of the water particles.
🔍 Worked example — Standard
In a convection current, if the density of the heated fluid is rho_hot and the density of the cool fluid is rho_cold, and rho_cold > rho_hot, explain why the fluid moves.
🔍 Worked example — Exam-style
A student performs an experiment to show convection using potassium permanganate crystals in a beaker of water. The crystal is placed at the bottom. Describe the observation and explain it using density.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) The transfer of thermal energy in a liquid or gas by the movement of the fluid itself. (2) Water at the bottom is heated, expands and becomes less dense, so it rises; cooler, denser water sinks to replace it and is heated in turn; the circulation is a convection current. (3) Hot water rises from the element and cooler water sinks towards it, so convection heats all the water. (4) Drop a crystal of potassium manganate(VII) to the bottom of a beaker of cold water at one side, heat gently beneath it, and watch the purple colour rise, move across the top and sink on the other side.