12.4Chapter 12: Energy Transfer

Energy transfer: convection — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 7 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision

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Lesson summary

Physics 12.4 Energy transfer: convection — Fluid Density Changes and Convection Currents — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how energy is transferred by convection and how convection currents form in everyday situations.Convection = the transf…

Fluids only
Conduction first
Cooling at the top
Fire in a house
Freezer compartment
Rising air

Part 1 of 5What convection is

1Convection

Convection the transfer of energy by the movement of a heated liquid or gas.

2Fluids only

It occurs only in liquids and gases that are free to move, where particles can move from place to place.

3Saucepan

Heating the bottom warms all the water, not just the bottom part.

4Conduction first

Energy passes through the solid bottom of the pan to the water by conduction.

Part 2 of 5How a convection current forms

The water in contact with the pan is warmed.

Its molecules move faster and further apart.

The warmer water becomes less dense.

The warmer water rises (floats up).

Cooler, denser water moves down to take its place.

Eventually all the water circulates up and down, forming a convection current.

Cooling at the top
- Fluid that reaches the top cools, becomes denser and sinks.

Part 3 of 5Everyday examples

Everyday examples

Fire in a house

Hot smoky air expands and rises, so firefighters crawl along the floor where the air is cleanest.

Radiators

They are placed low in rooms so that warm air rises and cooler air flows down to be heated.

Freezer compartment

It is at the top of a fridge so that cold, dense air sinks and cools the food.

Thermals

Convection currents in the air caused by the Sun heating the ground are used by birds to climb high.

Part 4 of 5Sea breezes

Daytime

The land heats faster than the sea, so the air above the ground gets hot.
The hot air expands and rises like hot water in a pan.
Cooler air from above the sea replaces it, so the breeze comes off the sea.

Night

The ground cools more quickly than the sea.
Air above the sea is now warmer and rises.
Cooler air from above the land replaces it, so the breeze comes off the land.

Part 5 of 5Convection in the atmosphere

Equator

The Sun heats the Earth more at the equator than near the poles.

Rising air

Air near the equator rises and is replaced by cooler air.

Weather

This moves the air in the atmosphere and affects weather patterns across the world.

Key idea

When a gas or liquid is heated it expands and becomes less dense, and the less dense fluid rises.

Key terms

Term	Meaning
Convection	Energy transfer by the movement of a heated liquid or gas.
Convection current	The circulation of a fluid as warm fluid rises and cool fluid sinks.
Density	Mass per unit volume.
Thermal	A convection current in the air.
Fluid	A liquid or a gas.