D. Energy and Society · Thermal and Nuclear Energy

Heat Transfer: Conduction, Convection and Radiation

D3.8 D2.1 A1.5 A1.10About 65 minutes

Learning goals

I can explain conduction, convection and radiation using the kinetic molecular theory and electromagnetic waves.
I can explain why some materials are good thermal conductors and others are insulators.
I can identify which methods of heat transfer are happening in everyday and technological situations.
I can explain how a device or building design reduces or increases heat transfer.

Words to know

conduction
Heat transfer through a material, or between materials in contact, by particles colliding with their neighbours.

thermal conductor
A material that transfers heat easily by conduction, such as most metals.

thermal insulator
A material that transfers heat poorly, such as air, wool, foam, wood or fibreglass.

convection
Heat transfer by the movement of a fluid (liquid or gas): warmer, less dense fluid rises and cooler, denser fluid sinks.

convection current
A circular flow of fluid set up by uneven heating.

radiation (thermal)
Heat transfer by electromagnetic waves, mostly infrared. It needs no particles and can cross a vacuum.

absorber / emitter
A surface that takes in (absorbs) or gives off (emits) radiation. Dark, dull surfaces are good at both; shiny surfaces are poor at both.

The cold railing mystery

On a −15 °C morning, grab a metal railing with a bare hand, then touch a wooden bench beside it. The metal feels painfully cold and the wood feels only cool. But they have both been outside all night, so they are at the same temperature. Why do they feel so different? (Touch the metal only for a moment, and never with wet skin or your tongue: very cold metal can freeze to damp skin.)

Your skin does not sense temperature directly. It senses how fast heat leaves your hand. Metal pulls heat out of your skin much faster than wood does. To understand why, we need to look at the three ways heat moves: conduction, convection and radiation.

Remember from Grade 7

In Grade 7 you met conduction, convection and radiation and looked at how insulation and different surfaces affect heat transfer. In Grade 11 you will explain each one with the kinetic molecular theory and with electromagnetic waves, and use them to analyse technologies.

Conduction: passing it along

Put one end of a metal spoon in hot soup. Soon the handle is warm, even though it never touched the soup. The fast-vibrating particles at the hot end bump into their slower neighbours and give them kinetic energy. Those particles bump the next ones, and so on. Energy travels along the spoon, but the particles themselves stay in place. This is conduction.

Conduction happens best in solids, where particles are close together.

Metals are excellent conductors. Besides vibrating atoms, metals have free electrons that move easily through the whole metal and carry energy quickly from hot to cold regions.
Non-metals such as wood, plastic, glass, wool and foam are insulators. They have no free electrons, so energy passes slowly.
Gases are very poor conductors because their particles are far apart. Trapped, still air is an excellent insulator. That is why a puffy winter coat, fibreglass insulation and a double-pane window all work: they trap air in small pockets so it cannot move.

Now the railing mystery is solved: the metal conducts heat away from your hand much faster than wood, so it feels colder.

Convection: the fluid carries the energy

In a fluid (a liquid or a gas), the particles can move from place to place. When part of a fluid is heated, its particles move faster and spread out, so that part becomes less dense. The warm, less dense fluid rises, and cooler, denser fluid sinks to take its place. The sinking fluid is heated in turn, and a circular flow called a convection current forms.

In convection, the particles themselves travel, carrying their energy with them. That is the key difference from conduction.

Natural convection: driven only by density differences, like air rising above a baseboard heater or warm air collecting near the ceiling.
Forced convection: a fan or pump moves the fluid, like a furnace blower, a car's radiator pump or a convection oven's fan.

Convection cannot happen in a solid, and it cannot happen in a vacuum.

Radiation: no particles required

The Sun is about 150 million kilometres away across the near-vacuum of space, yet it warms your face. Energy that crosses empty space cannot be using conduction or convection. It travels as electromagnetic waves, the same family as visible light. Warm objects give off mostly infrared waves, which we feel as warmth but cannot see. This is radiation.

Every object above absolute zero emits radiation. Hotter objects emit much more (and at shorter wavelengths: a stove element glows red, then orange).
Radiation travels at the speed of light and needs no medium.
When radiation hits a surface, it can be absorbed (the surface warms), reflected or transmitted.
Dark, dull (matte) surfaces are good absorbers and good emitters. Light, shiny surfaces are poor absorbers and poor emitters; they reflect most radiation.

That is why emergency blankets are shiny, why black asphalt gets hot in July, and why a light-coloured roof keeps a house cooler.

Three ways to move heat. In conduction energy passes from particle to particle while the particles stay put. In convection warm fluid travels and carries its energy. Radiation is electromagnetic waves and needs no particles at all.

Method	Where it happens	How energy moves	Everyday example
Conduction	mainly solids; any materials in contact	collisions between neighbouring particles (and free electrons in metals)	frying-pan handle gets hot
Convection	liquids and gases only	the warm fluid itself moves (currents)	warm air rising from a heating vent
Radiation	anywhere, even a vacuum	electromagnetic waves (mostly infrared)	feeling a campfire's warmth on your face

Comparing the three methods of heat transfer.

Worked example

Example 1: A thermos bottle

A vacuum flask (thermos) keeps soup hot for hours. It has a double wall with a vacuum between the walls, the inside surfaces of the walls are silvered, and it has a tight stopper. Explain how each feature reduces heat loss.

Identify all three methods and ask which feature blocks each one.
Vacuum gap: a vacuum has (almost) no particles. Conduction needs particles to collide, and convection needs a fluid to move, so the vacuum stops both conduction and convection between the inner and outer walls.
Silvered walls: radiation can cross a vacuum. Shiny surfaces are poor emitters and good reflectors, so very little infrared leaves the inner wall, and what does is reflected back. This reduces radiation.
Stopper: a plastic stopper is a poor conductor, and closing the top stops warm air and vapour from rising out of the flask, so it reduces convection (and evaporation).
Heat still leaks slowly, mostly by conduction through the neck and stopper, which is why soup is only warm, not hot, after a day.

The vacuum blocks conduction and convection, the silvering reduces radiation, and the stopper blocks convection and evaporation at the top.

How a thermos fights all three methods of heat transfer. The only solid path between inside and outside is through the neck and stopper.

Worked example

Example 2: Sea breezes at a cottage

On a sunny summer afternoon at a lake in Muskoka, a cool breeze blows from the lake toward the beach. Explain this using heat transfer.

The Sun warms both land and water by radiation. On a sunny day the land surface warms up much faster than the lake (you will learn why in the next lesson, Specific Heat Capacity).
The warm ground heats the air touching it by conduction. That air becomes less dense and rises (convection).
Cooler, denser air over the lake moves in along the surface to replace the rising air. That is the breeze you feel.
Higher up, the air flows back out over the lake, cools and sinks, completing a convection current.

The breeze is the lower part of a convection current: air rises over the warm land and cooler air from over the lake flows in to replace it.

A daytime lake breeze is a convection current: warm air rises over the sun-heated land, and cooler air from over the lake flows in along the surface (not to scale).

Just for fun

For the record, the toque is great, but the old saying that you lose "most of your body heat through your head" is a myth. An uncovered head loses heat at about the same rate as any other uncovered patch of skin of the same size. So please also wear mittens.

Hands-on activity

Three heat-transfer mini-investigations

Grown-up note: a grown-up should pour the hot water (from a kettle or very hot tap; it can scald). Do not use a stove burner.

A. Conduction race. You need: a mug of very hot water, a metal spoon, a wooden spoon, a plastic spoon (same length if possible) and a pat of butter. Put a small dab of butter near the top of each handle, the same distance from the bowl of the spoon. Stand all three in the hot water at once and time how long each dab takes to start sliding. Question: which material is the best conductor? Write a hypothesis first and name your independent, dependent and controlled variables.

B. Convection you can see. You need: a clear glass of warm water and an ice cube made with a few drops of food colouring. Gently place the ice cube in the water and watch from the side. Sketch the pattern. Look for: coloured cold water sinking from the cube to the bottom, because cold water is denser.

C. Radiation and colour. You need: two identical glasses of water at the same temperature, black paper, white paper, tape and a thermometer. Wrap one glass in black paper and one in white. Set both in direct sunlight for 30 minutes, then measure each temperature.

What to look for: usually the metal spoon wins A by a large margin; in C the black-wrapped glass ends up warmer because dark surfaces absorb more radiation. Write a conclusion for each, and one source of error (for example, butter dabs of different sizes).

Your turn

Name the method of heat transfer: the bottom of a pot gets hot on an electric stove element.

Name the method: warm air from a floor vent spreads to the far side of the room.

Name the method: you feel warmth from a fireplace while sitting beside it, not above it.

Why are metals much better conductors than wood?

Why can't convection happen in a solid?

Why is a puffy down jacket warm?

Explain why black asphalt gets hotter in the sun than a light concrete sidewalk.

More practice: warm-up

Which method of heat transfer can cross a vacuum?

In which states of matter can convection occur?

Give one example of a good thermal insulator.

What kind of electromagnetic waves do warm objects mostly give off?

Is a shiny surface a good or poor absorber of radiation?

More practice: on your own

Why are baseboard heaters placed low on a wall, under windows, rather than near the ceiling?

Explain, using conduction, why a tile floor feels colder on bare feet than a carpet in the same room.

Why is the inside of a car on a sunny day much hotter than the air outside?

Triple-pane windows are common in new Canadian homes. How do the gaps between the panes reduce heat loss?

A student says, "Radiation only comes from very hot objects like the Sun." Is this correct?

Challenge

Use all three methods of heat transfer to describe how heat from a furnace in the basement reaches a person sitting in an upstairs bedroom.

An igloo can keep the inside many degrees warmer than the outside air, even though it is made of snow. Explain, referring to two methods of heat transfer.

A hot drink cools from 80 °C to 60 °C in 10 minutes in a ceramic mug, but takes about 40 minutes in an insulated travel mug with a lid. Design a fair test to find out how much of the difference is due to the lid. What would you change, measure and control?

Check your understanding

Quiz

Which method of heat transfer can occur through a vacuum?

Why do metals conduct heat well?

Warm air rising above a radiator is an example of:

Which surface would absorb the most sunlight?

A wool sweater keeps you warm mainly because:

In conduction, energy moves through a solid because:

For parents and teachers

Big idea: expectation D3.8 asks students to distinguish conduction, convection and radiation and give examples. At Grade 11 the explanation should use particles (KMT), free electrons in metals, density differences, and electromagnetic (infrared) waves.

How to help: Walk through your home and ask your teen to point out heat transfer: the furnace and vents, windows, the fridge, the stove, a winter coat. For each, ask: which method, and how is it being encouraged or blocked?

Common difficulties:

Saying "heat rises". It is warm fluid that rises, because it is less dense. Heat itself travels in every direction by conduction and radiation.
Thinking cold objects give off no radiation.
Mixing up conduction and convection: in conduction the particles stay put; in convection they travel.
Forgetting that a vacuum blocks conduction and convection but not radiation.

Safety: a grown-up should pour the hot water in the activity. Do not use the stove for the conduction race.