Quantity of Heat and Heat Transfer
Senior 3 · Physics
Quantity of Heat and Heat Transfer
Lesson 4 of 5
25 min
Stand beside a charcoal stove with a pan of water on it. Hold your hand above the stove, and the rising air warms it. Hold your hand to the side, well out of the rising air, and your palm still feels the heat. Touch the pan's metal handle after a few minutes, and it burns, although it never touched the fire. Heat reaches you in three different ways, and all three are happening at the same stove.
Heat a metal rod at one end, and the far end soon gets hot although the rod does not move. In a solid, the particles at the hot end vibrate more strongly and pass energy to their neighbours through their bonds. In a metal there are also free electrons, which move through the whole metal and carry energy quickly from the hot end to the cold end. This is why metals are the best conductors.
A simple test: fix drawing pins with wax at equal spacing along rods of copper, aluminium, iron and glass of the same size, and heat one end of each equally. The pins fall off the copper rod first and the glass rod last.
| Good conductors of heat | Poor conductors (insulators) |
|---|---|
| Copper, aluminium, iron, steel and other metals | Wood, plastic, rubber, glass, cloth, paper |
| (Metals conduct well because of their free electrons) | Liquids such as water, and above all gases such as air |
Still air is a very poor conductor. Materials that trap air in small pockets, such as wool, feathers, foam and fibreglass, are therefore good insulators. Water is also a poor conductor: in a test tube of water weighted with ice at the bottom, the water at the top can be boiled while the ice below stays frozen.
Definition
Conduction: the transfer of heat through a material from particle to particle, without the material itself moving. It is the main mode of heat transfer in solids.
Thermal conductor: a material that lets heat pass through it easily (e.g. copper). Thermal insulator: a material that conducts heat poorly (e.g. wood, wool, air).
Worked example 1: the cold floor
In the morning, a tiled floor feels much colder to bare feet than a woollen mat lying on it, though both are at the same room temperature. Explain why.
Solution. Your feet are warmer than both, so heat flows from your feet into each. The tiles are a much better conductor than the wool, which traps air, so they carry heat away from your feet much faster. Your skin senses the rate at which it loses heat, not the temperature of the floor, so the tiles feel colder.
Liquids and gases (fluids) conduct heat poorly, but they can carry it by moving. When part of a fluid is heated it expands, becomes less dense and rises; cooler, denser fluid sinks to take its place. The result is a circulating convection current. Drop a crystal of potassium permanganate into one bottom corner of a beaker of water and warm that corner gently: a purple stream rises above the flame, crosses the top, and sinks down the far side.
Convection explains why the heating element of a kettle is at the bottom, why the freezer compartment of a refrigerator is at the top (the air it cools sinks through the cabinet), and why smoke goes up a chimney. It also drives winds. By day the land beside a large lake warms faster than the water, because the land has a much smaller specific heat capacity (lesson 1), and a breeze blows from the lake onto the land:
At night the land cools faster than the lake, and the circulation reverses: a land breeze blows out over the water.
Definition
Convection: the transfer of heat by the movement of the heated fluid (liquid or gas) itself, as warmer, less dense fluid rises and cooler, denser fluid sinks. Convection cannot happen in a solid or in a vacuum.
Between the Sun and the Earth there is almost empty space, so neither conduction nor convection can bring us the Sun's heat. It arrives as radiation, mainly infrared waves, which travel at the speed of light and need no material at all. Every object gives out infrared radiation; the hotter it is, the more it gives out.
How much radiation a surface absorbs or gives out depends on its colour and texture:
| Surface | As an absorber | As an emitter | Reflects radiation? |
|---|---|---|---|
| Dull (matt) black | Best | Best | Very little |
| White | Poor (of sunlight) | Good (of infrared) | Much of the Sun's radiation |
| Shiny silver | Worst | Worst | Most |
The rule to remember is that a good absorber is also a good emitter: dull black is best at both, and shiny silver is worst at both. White only seems to break the rule. It reflects most visible sunlight, so it absorbs little of it, but it still gives out infrared well. In exam questions, compare dull black with shiny silver.
Definition
Radiation (thermal radiation): the transfer of heat by electromagnetic waves, mainly infrared. It needs no medium and can cross a vacuum. Example: the warmth of sunlight on your skin.
Worked example 2: black can, shiny can
Two identical cans, one painted matt black and one polished silver, are each filled with 300 g of water at 80.0 °C and left side by side. After 10 min the black can reads 64.0 °C and the shiny can 71.0 °C (illustrative results). Find each can's average rate of cooling and explain the difference.
The black can cools almost twice as fast because a matt black surface is a much better emitter of infrared radiation. The test is fair only if the cans are the same size and material, hold the same mass of water at the same starting temperature, have lids, and stand in the same place.
A vacuum flask keeps tea hot (or water cold) for hours by blocking all three modes of heat transfer:
| Device or practice | Heat transfer used or blocked |
|---|---|
| Pan with a wooden or plastic handle | Metal body conducts heat to the food; the insulating handle stays cool |
| Solar water heater | Dark tubes or plates absorb radiation; hot water rises by convection into an insulated tank above |
| Ceiling boards under an iron roof | The trapped air layer reduces conduction and convection from the hot roof into the room |
| Light-coloured clothes in the sun | Reflect more of the Sun's radiation than dark clothes |
| Woollen jumper, blanket | Trapped air is a poor conductor and cannot circulate |
| Refrigerator | Cooling unit at the top: cold air sinks and sets up convection through the cabinet |
Worked example 3: how the flask works
Explain how each part of a vacuum flask reduces heat loss from hot tea.
A little heat still escapes, mostly by conduction through the stopper and the glass at the neck, so the tea cools slowly over many hours.
Warning
"Heat rises" is wrong. Heat itself spreads in every direction by conduction and radiation. It is hot fluid that rises, because it is less dense. Also: a vacuum stops conduction and convection, but not radiation.
Tip
Solids conduct, fluids convect, radiation goes anywhere. For any device, ask three questions: can heat pass through a solid part? can a fluid move? what colour and texture are the surfaces?
Conduction, convection and radiation (Cognito). Watch for why conduction is best in solids, how a convection current forms, and why radiation can cross a vacuum.
Practice
Key points
Sources and credits
Written by Ganzaa: the lesson text, the three diagrams, the tables, worked examples and practice exercises.
