GRADE 7
Name:
Conduction is the transfer of heat between two substances that are in direct contact
with each other. This can occur between solids (stove plate and pot), a solid and
liquid (pot and water inside the pot) and a solid or liquid and a gas (heater heating
the air around it).
Convection is the transfer of heat energy within a liquid or gas in a closed system
and relates to the movement of the particles of the substance.
Do you remember the properties of
all particles?
They have spaces between them; they are always
moving around and they move around more
quickly when heated.
Can you remember how particles
move around in liquids and gases?
In liquids, they flow over and around each other.
In gases, they bounce around in the container.
When a liquid or gas is warmed up, the particles
move around more quickly. The warmer particles that are moving around more
quickly, also take up more space and expand. This means they are lighter (or less
dense) than the cooler particles. The lighter, warm particles rise and the heavier,
cooler particles sink. This continues to happen while the substance is
being heated.
See convection in action
Have you ever studied a lava lamp? The lava lamp uses convection.
This is how the “lava” rises and sinks.
The heat comes from a light bulb at the base of the lamp. There are
two liquid substances in the lamp that are insoluble in each other
(this means they will always be separate, like oil and water).
When the light bulb is turned on, the heat from the bulb is
transferred to the liquids (by conduction). As the “lava” is heated, it
expands and becomes lighter. This means it floats to the top.
When it reaches the top, it cools down and becomes heavier, so it
sinks to the bottom again where the light bulb heats it again and
the process repeats itself.
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Term 3 Natural Science: Energy and Change: Heat Transfer: Convection
Heat transfer: Convection
GRADE 7
cool
molecules
heat
The circular motion shown in this picture (tea pot of water on a gas stove) is called
the convection current.
It is the flow of the warm and cold particles in a closed system (closed tea pot on a gas
stove), showing how heat energy is transferred within the liquid or gas of that system.
Activity 1:
Answer the following in your class workbook:
1) Write a definition for convection.
2) Give a synonym for ‘current’.
3) When a house or building is on fire, why do firefighters always stay close to the
ground and sometimes crawl through the building?
4) Look at this picture of a firefighter crawling into a room where there is a couch on fire. Draw the convection current on the picture. Use a RED pen or pencil to show the hot air and a BLUE pen or pencil to show the cold air.
Convection
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Term 3 Natural Science: Energy and Change: Heat Transfer: Convection
warm
molecules
GRADE 7
Activity 1:
1) Write a definition for convection
Convection is the transfer of heat energy within a liquid or gas in a closed system.
2) Give a synonym for current.
A synonym for current is flow.
3) When a house or building is on fire, why do firefighters always stay close to
the ground and sometimes crawl through the building?
The hot air with the smoke rises to the top of the room, and the cool air is found
at the bottom of the room, closest to the floor.
It is safer for firefighters to crawl on the ground because they can see better and
do not risk more damage to themselves from the heat and smoke-inhalation.
4) Look at this picture of a firefighter crawling into a room where there is a
couch on fire. Draw the convection current on the picture. Use a RED pen or
pencil to show the hot air and a BLUE pen or pencil to show the cold air.
Red
Convection
Blue
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Term 3 Natural Science: Energy and Change: Heat Transfer: Convection
Answer sheet