Heat Transfer, Grade 7: Conduction, Convection and Radiation

This topic contains a page that states the rules of fair testing correctly, and five activities that break them.

The skills page on variables is genuinely well written. Independent, dependent and control variables are properly defined, and it says plainly that only one thing may change.

And the conductivity test sixty pages earlier used three separate burners. This topic runs the same investigation on one hotplate, which is the correct design. So the same material carries both the broken version and its own remedy, and nobody joined them up.

The three ways heat moves

Needs How it works Example
Conduction Solids, mostly Particles vibrate against their neighbours and pass energy along without moving themselves A metal spoon in hot soup
Convection A fluid: liquid or gas The fluid itself moves, carrying its energy with it A kettle, a room heater, sea breezes
Radiation Nothing at all Energy travels as waves and crosses empty space The Sun. A braai on your face

The quickest way to tell convection from conduction: ask whether the stuff moved. In conduction the energy travels and the material stays. In convection the material carries the energy with it.

And radiation is the one that works in a vacuum, which is how the Sun's energy reaches us across 150 million km of nothing.

Where this fits in the curriculum

Subject Natural Sciences
Grade 7
Term 3
Strand Energy and Change
Topic Heat transfer
Status Class activities. Nothing here is formally assessed
Marks The topic revision is 20. Our worksheet is 40

Heat is not something an object has

You will see heat defined as "energy that an object has because of the movement of its particles".

That is thermal energy, also called internal energy. It is not heat.

Heat is energy in transit. It only exists while it is moving from somewhere hotter to somewhere cooler. An object does not contain heat any more than a wire contains current.

And a page later the same topic gets it exactly right: heat is energy transferred from a place of higher temperature to a place of lower temperature. That second version is the one to teach.

The test for a learner: can you say how much heat is in this cup? No. You can say how much thermal energy is in it, and how much heat flowed out of it in the last minute. Heat is a verb pretending to be a noun.

Two over-reaches that are worth catching

What you will read What actually happens
"Eventually the pot will become as hot as the stove" It will not, while the stove is switched on. The plate keeps supplying energy and the pot keeps losing it to the air and the food. It settles at a steady temperature well below the plate
"Your hands become as cold as the water" Not under a running tap. Your blood keeps delivering heat and the water keeps carrying it away

The rule is right and the examples are wrong. Heat does flow until the temperature difference is gone, but only in a closed situation where nothing is adding or removing energy. A pot on a lit plate and a hand under a running tap are the two cases where that does not apply, and they are the two examples chosen.

The conduction test, done the way this topic does it

This is the correct design and it is worth using.

  1. Four rods of different metals, the same length and thickness
  2. A blob of candle wax at the SAME distance along each one
  3. All four resting on ONE hotplate, so every rod gets the same heat
  4. Time how long each blob takes to melt

One heat source is the whole point. Three separate burners cannot be matched by eye, and flame size is the variable that matters most, so the version with a burner per rod is measuring the flames rather than the metals. Our Grade 7 properties of materials page covers that earlier test and the same correction.

Expect copper first, then aluminium, then iron, then steel.

The six-cup conduction activity needs three fixes

You will find a cup comparison with three problems in one method.

Problem Fix
The apparatus list says "warm water" and the method says "boiling water" Decide before the lesson. Warm is safer and the comparison still works
A stopwatch is listed and never used. There is no timing step at all Time it. Readings at fixed intervals are the whole experiment
The measurement is "touch the cups with your hand" Use a thermometer. Touch is subjective, and with boiling water and a glass cup it is a burn and a breakage waiting to happen

A thermometer turns this from a hand test into an experiment, and it is one of four activities in this topic that needs one.

Convection: it is density, not weight

You will read that the warm part of a fluid becomes "lighter" and rises, and the cooler part is "heavier" and sinks.

It is density, not weight. Heating does not remove anything from the water, so its weight has not changed at all.

And the same sentence gets the correct half: it says the warm fluid "takes up more space". That is exactly right, and it is the reason. Same mass, bigger volume, so lower density, so it floats on the colder water underneath.

Our Grade 8 density page is where this gets measured properly, and it is worth linking a class to it rather than re-explaining density here.

The hot and cold bottle demonstration only works if you do both halves

Two bottles, one of hot coloured water and one of cold, held mouth to mouth with a card between them.

Arrangement What happens Usually shown?
Cold on top of hot They mix immediately. The cold water is denser, so it sinks through the warm Yes
Hot on top of cold They do not mix. The warm water is less dense and stays floating on top, sometimes for many minutes No, and this is the half that teaches

Only the second arrangement is usually left out, and the contrast between the two is the entire point. As printed, a class gets a jar of mixed colour and no conclusion. Set them up side by side and the result explains itself.

Radiation, and one picture that is the wrong mechanism

Radiation needs no medium, which is what separates it from the other two. Hold your hand beside a braai, not above it, and what you feel is radiation.

One photograph offered as an example of radiation is an oil-filled column radiator.

Those warm a room mainly by convection. The oil heats the metal, the metal heats the air touching it, and that air rises and circulates. It is called a radiator and it is mostly a convector, which in the one topic that separates the three mechanisms is a misleading choice.

A better example is already in the topic: the Sun. Or an electric bar heater, where you feel it instantly on the side facing you and not at all behind it.

Absorption and reflection are handled well, including why a white car stays cooler than a black one. One wording to sharpen: absorbed light is sometimes described as "disappearing". It does not disappear, it becomes thermal energy, which is exactly why the black car gets hot.

Two more things worth a minute

The Sun's energy is nuclear, and some multiple-choice questions do not offer that option. You may meet a question asking whether the Sun has gravitational, chemical or elastic potential energy. None of the three is right. The Sun's energy comes from nuclear fusion, and if that choice is not on the list the honest answer is that the question has no correct option. Our Grade 8 solar system page covers it.

And "prove" should be "test". You will see instructions to design an experiment to prove an explanation. You test a hypothesis; you do not set out to prove yourself right. It is a small word with a large effect on how a class treats a result that disagrees with them.

If it does not work

Problem Cause Fix
All the metal rods seem the same Blobs at different distances, or separate heat sources One hotplate, wax at equal distances
The hot and cold bottles always mix Only the cold-on-top arrangement was done Do hot on top of cold as well. That one does not mix, and the contrast is the lesson
The cup results are inconsistent Touch was used as the measurement A thermometer in each cup, read at fixed times
Nothing circulates in the convection beaker Heated in the middle, so it rises evenly Heat one side only. That is what makes the current visible
The black and silver bottles read the same One was in shade, or the thermometers went in after filling Same sun, same start temperature, and almost fill so they do not overflow
A learner says the pot gets as hot as the plate The printed example Only in a closed situation. The plate keeps supplying energy

How the 40 marks are made up

Part Marks
The three mechanisms, and telling them apart 10
Heat against thermal energy 8
Conduction, tested fairly 8
Convection and density 8
Radiation, absorption and reflection 6

Nothing in this topic is formally assessed. The topic revision is 20 marks and it adds up correctly. Our worksheet is 40 and the split above is ours.

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

Related practicals

What you need to run it

Item Price What it is for
Thermometer, red spirit, −10 to 110 °C R35 Four separate activities in this topic need one. Buy a class set
Beaker, borosilicate R7 The convection current, heated on one side
Beaker, borosilicate, low form R19 The heavier-walled option for a school cupboard
Safety goggles R30 Hot water and a hotplate

The thermometer is the purchase, and this is the topic that proves it. It is needed for the cup comparison, the convection work, the black-against-silver fair test and again in the insulation topic that follows. With the earlier properties topic and the assessed Term 3 project, a thermometer is used in eight separate activities across the Grade 7 year. A set of ten is R350.

And the honest gap: we do not stock metal rod sets or hotplates. The conduction comparison needs four rods of different metals, equal length and thickness, and a single heat source. Offcuts of copper, aluminium and steel rod from a hardware shop work, and a kitchen hotplate or a single shared burner does the heating. We are not going to pretend otherwise.

Thermometers are in thermometers and the glassware is in laboratory glassware.