Science notes, chapter 9

Heat

Textbook Science Form 2 (DLP), pages 204–221

What this chapter covers

Compare heat and temperature (9.1)
Explain how heat flows by conduction, convection and radiation, and state when thermal equilibrium is reached (9.2)
Explain sea breeze and land breeze as natural convection currents (9.2)
Give uses of heat conductors and heat insulators in daily life (9.2)
Explain expansion and contraction of solids, liquids and gases in terms of particles, and give their uses (9.3)
Show that dark, dull surfaces absorb and radiate heat better than white, shiny surfaces, and apply this in daily life (9.4)

9.1Relationship between temperature and heat

Heat is a form of energy. It flows from a hotter region to a colder region.
Temperature is a measure of the degree of hotness or coldness of an object. It is measured with a thermometer.

Heat and temperature are related, but they are not the same thing.

	Heat	Temperature
What it is	A form of energy	The degree of hotness or coldness of an object
Unit	Joule (J)	Degree Celsius (°C) or kelvin (K)
Depends on	Type of material, quantity (mass) of material and temperature	The degree of movement of the particles in the matter

Same temperature, different heat

Two beakers, one with 100 mL and one with 200 mL of water, are boiled. Both boil at the same temperature (100 °C), but the 200 mL of water has absorbed more heat and takes longer to boil.

Thinking that a big pot of hot water has a higher temperature than a small cup of the same hot water. Both have the same temperature; the big pot simply holds more heat.

Is the sense of touch a reliable way to tell whether someone has a fever? Why?

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No. Our skin only gives a rough feeling of hot or cold, which depends on the surroundings. A thermometer gives an accurate measurement of temperature.

9.2Heat flow and thermal equilibrium

Heat always flows from a hot object to a cold one. It can travel in three ways: conduction, convection and radiation.

Conduction

Conduction is the transfer of heat through a solid medium, from the hotter region to the colder region.

A copper rod heated at one end by a Bunsen burner, with three thumbtacks K, L and M fixed by wax — Activity 9.1A. The tack nearest the flame (K) falls first, then L, then M. (Redrawn from textbook Figure 9.1, p. 207)

Particles that receive heat vibrate more and collide with their neighbours, passing the heat along the whole solid.
The particles do not travel along the rod; only the energy does.

Convection

Convection is the transfer of heat by the movement of a fluid (liquid or gas) from hotter regions to colder regions.

The part of the fluid that receives heat expands, becomes less dense and rises.
The colder, denser fluid moves downwards to take its place.
This continuous circulation is a convection current.

A beaker of water heated at one bottom corner, with arrows showing water rising above the heat, moving across the top, sinking on the far side and returning along the bottom — A convection current in water. The potassium permanganate crystal shows the path of the current. (Redrawn from textbook Figure 9.2, p. 207)

In Activity 9.1B(ii), a glowing incense stick is held at the mouth of the beaker so that the smoke shows how the air moves. Air goes down on the side of the incense, passes under the cardboard and rises over the candle.

Radiation

Radiation is the transfer of heat without a medium. It can pass through an empty space (a vacuum).

Radiation is the only way heat reaches the Earth from the Sun, because space is almost a vacuum.
Radiation is the fastest way for heat to travel, because it needs no medium.
The type of surface, the temperature and the total surface area of an object affect the rate of heat flow.

Method	Medium needed	Happens in	Example
Conduction	Yes, a solid	Solids (best in metals)	Metal spoon in hot soup warms up
Convection	Yes, a fluid	Liquids and gases	Water heated in a kettle
Radiation	No	Everywhere, including a vacuum	Warmth of the Sun

Why is the heating coil of an electric kettle at the bottom? The water that is heated rises and the cold water sinks to the coil, so all the water is heated by convection.

Which method of heat transfer can pass through a vacuum? How do you know?

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Radiation. Heat from the Sun reaches the Earth through space, and in Activity 9.1C the bell jar is warm even though the air has been pumped out.

Heat flow in natural phenomena

Heat from the Sun reaches the Earth by radiation. Land and sea absorb it, and the warming of the Earth causes climate and natural phenomena such as sea breeze and land breeze.

Two coastal diagrams. By day, air rises over warm land and cool air moves in from the sea. At night, air rises over the warmer sea and cool air moves from the land to the sea — Sea breeze and land breeze are natural convection currents. (Redrawn from textbook Figures 9.6 and 9.7, p. 210)

	Sea breeze	Land breeze
When	Daytime	Night
Warmer surface	Land (heats up faster than the sea)	Sea (cools slower than the land)
What happens	Warm air above the land rises; cool, denser air from the sea moves in	Warm air above the sea rises; cool, denser air from the land moves out to sea
Wind blows	From sea to land	From land to sea

Mixing up the direction. A breeze is named after the place it comes from: a sea breeze blows from the sea.

Thermal equilibrium

Two objects in thermal contact exchange heat until there is no net transfer of heat between them. They are then in thermal equilibrium and have the same temperature.

Line graph: hot object P cools from 80 to 45 degrees Celsius and cold object Q warms from 20 to 45 degrees Celsius; both lines meet and stay level — The hot object loses heat and the cold object gains it until both are at 45 °C (after 12 minutes).

Heat conductors and heat insulators

	Heat conductor	Heat insulator
Meaning	Allows heat to flow through it easily	Prevents (slows down) heat flow
Examples of material	Metals such as copper, iron, aluminium; mercury	Wood, fibreglass, polystyrene, cotton, felt, air
Uses	Base of an iron and bottom of a pan; mercury in a thermometer	Oven gloves; wooden handles; ice box wall; blanket

Experiment 9.1: which is the best heat insulator?

Four flasks of boiling water: K uncovered, L covered with cotton, M with felt, N with aluminium foil.
Manipulated: type of insulator. Responding: final temperature after 10 minutes. Constant: volume of water.
Flask K is the control. Flask N loses heat quickly because aluminium is a conductor. Cotton and felt keep the water hot longest, so they are the good insulators.

Why is a polystyrene container used to store ice cubes?

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Polystyrene is a heat insulator. It stops heat from the surroundings flowing in, so the ice melts slowly.

9.3Expansion and contraction of matter

Expansion: the volume of matter increases when it is heated.
Contraction: the volume of matter decreases when it is cooled.

Solids, liquids and gases all expand when heated and contract when cooled. Gases expand the most and solids the least.

When a solid is heated its particles vibrate faster and move further apart, so the solid expands.

	When heated	When cooled
Solid	Particles vibrate faster about fixed positions and move further apart. The solid expands.	Particles vibrate slower and move closer. The solid contracts.
Liquid and gas	Particles move faster and randomly; the distance between them increases. Volume increases.	Particles move slower and closer together. Volume decreases.

The particles themselves do not get bigger. Only the spaces between them increase.

Uses of expansion and contraction

Situation	Explanation
Mercury in a thermometer	Mercury expands when hot and rises up the tube, and contracts when cold. It is a good heat conductor, so it responds quickly.
Gaps between railway tracks	Gaps let the rails expand in hot weather without buckling.
Steel bridge with rollers and a gap at one end	Allows the bridge to expand in hot weather.
Bimetallic strip	Two different metals expand at different rates, so the strip bends when heated.
Tight bottle lid	Put the lid in hot water. It expands and loosens.
Dented table tennis ball	Put it in hot water. The air inside expands and pushes the dent out.
Loose electric cables	Hung loosely so they can contract in cold weather without snapping.

A bimetallic strip of copper and iron straight at room temperature, curving when heated and the opposite way when cooled, and a circuit with a fire alarm that completes when the strip bends to the contact point — A fire alarm uses a bimetallic strip. Heat from a fire makes the copper side expand faster than the iron side, so the strip bends and touches the contact point. The circuit is completed and the alarm rings. (Textbook Figure 9.14, p. 215)

Textbook figure: Science Form 2 (DLP), Figure 9.14, p. 215.

Forgetting that the metal which expands more ends up on the outside of the curve. When heated, copper expands faster, so it is on the outside and the strip bends towards the iron.

A glass bottle cracks when boiling water is poured into it. Explain.

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The inner surface of the glass is heated and expands quickly, but the outer surface is still cold and has not expanded. The uneven expansion produces a stress that cracks the glass.

9.4Types of surface, and heat absorption and emission

An object absorbs heat: its temperature rises. An object radiates (emits) heat: its temperature falls.
Dark and dull surfaces are better heat absorbers and better heat radiators than white and shiny surfaces.

	Dark, dull surface	White, shiny surface
Absorbs heat	Well	Poorly
Radiates heat	Well	Poorly

Bar chart: white can J rises 12 degrees Celsius, black can K rises 31 degrees Celsius — Experiment 9.2A (example results): both cans start at 30 °C. After 10 minutes the black can has risen 31 °C (61 − 30), the white can only 12 °C.

Experiment 9.2: dark and white cans

Manipulated: colour of surface. Responding: increase (or decrease) in temperature. Constant: distance from heat source (or volume of hot water).
Absorbing: the black can warms more. Radiating: the hot water in the black can cools more. The hypothesis is accepted.

Uses in daily life

Fuel tanks and lorries are painted white or silver: little heat is absorbed, so less fuel evaporates.
Bright clothes are worn in hot weather: they absorb less heat, so we feel cooler.
Thermos flask: shiny walls with a vacuum between them reduce heat radiation and conduction, so the drink stays hot.
Cooking pots have a black base to absorb heat well; the shiny outside reduces heat lost.
Green building concept: solar or renewable energy, good water flow, air circulation and lighting, and recycled materials.

Writing that a shiny object "attracts" or "traps" heat. Say it reflects radiant heat, so it absorbs less.

Why does a house with a white roof stay cooler than one with a black roof?

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A white (shiny) surface is a poor absorber of heat radiation. Less heat from the Sun enters the house, so it stays cooler.