- Heat is a form of energy that flows from a hotter body to a colder body. Temperature tells us how hot or cold a body is.
- Temperature is measured with a thermometer; the clinical thermometer measures body temperature (normal ≈ 37 °C) and the laboratory thermometer measures other temperatures.
- Heat travels by three methods: conduction (in solids), convection (in liquids and gases) and radiation (no medium needed).
- Conductors (metals) allow heat to pass easily; insulators (wood, plastic, air) do not.
- Convection currents explain sea breeze, land breeze and the working of room heaters and cooling.
- Radiation brings heat from the Sun to the Earth through empty space; dark, dull surfaces absorb and emit heat better than light, shiny ones.
- These ideas explain why we wear dark clothes in winter and light ones in summer, and why coastal places have moderate climate.
- Weightage: ~6 marks/year — 1-mark definitions, 2-mark conductor/insulator and thermometer questions, 3-mark conduction/convection/radiation comparison questions.
1. Heat and Temperature
When we touch a cup of hot tea we feel warm, and when we touch an ice cube we feel cold. Our sense of touch tells us roughly how hot or cold something is, but it is not reliable. Heat is a form of energy, and temperature is a measure of the degree of hotness or coldness of a body.
Take three bowls — one with cold water, one with hot (not too hot) water, and one with lukewarm water. Put your left hand in cold water and your right hand in hot water for a minute, then dip both hands into the lukewarm water. The same lukewarm water feels warm to the left hand and cool to the right hand. This shows that our sense of touch cannot reliably measure temperature, so we use a thermometer.
2. Measuring Temperature — Thermometers
A thermometer is an instrument used to measure temperature. Common thermometers contain a thin tube with a liquid (such as coloured alcohol or, earlier, mercury) that expands on heating and rises up the tube.
2.1 Clinical thermometer
A clinical (doctor's) thermometer is used to measure the temperature of the human body. Its scale usually runs from 35 °C to 42 °C, because the body temperature always lies in this narrow range. It has a sharp kink (bend) near the bulb that stops the liquid from flowing back, so the reading stays put until you shake it down.
2.2 Laboratory thermometer
A laboratory thermometer measures temperatures other than body temperature, such as that of water or air. Its range is much wider, usually from about −10 °C to 110 °C. It has no kink, so the reading must be taken while the bulb is still in contact with the object.
| Feature | Clinical thermometer | Laboratory thermometer |
|---|---|---|
| Use | Human body temperature | Temperature of water, air, etc. |
| Range | 35 °C to 42 °C | −10 °C to 110 °C |
| Kink | Present | Absent |
| Reading taken | After removing from body | While bulb is still in the object |
The SI unit of temperature is the kelvin (K), but in everyday life we use the degree Celsius (°C). Today, safe digital thermometers are often used in place of mercury ones.
3. Three Methods of Heat Transfer
Heat always flows from a hotter region to a colder region. It does so by three different methods: conduction, convection and radiation.
4. Conduction
Conduction is the transfer of heat through a material without the material itself moving. It is the main way heat travels through solids. When one end of a metal rod is heated, the particles there vibrate faster and pass on the energy to neighbouring particles, so heat moves along the rod to the cooler end.
Fix small wax balls at intervals along a metal rod and clamp it horizontally. Heat one end with a candle. The wax balls fall off one by one, starting from the end nearest the flame. This shows that heat travels along the rod from the hot end to the cold end by conduction.
4.1 Conductors and insulators
- Conductors: materials that allow heat to pass through them easily. Most metals — iron, copper, aluminium, silver — are good conductors.
- Insulators: materials that do not allow heat to pass through easily. Wood, plastic, rubber, glass, cloth, paper and air are good insulators (poor conductors).
This is why cooking pans are made of metal (good conductor) but their handles are made of wood or plastic (insulator) so we do not burn our hands. Woollen clothes keep us warm in winter because wool traps air, and trapped air is a poor conductor that reduces the loss of body heat.
5. Convection
Convection is the transfer of heat in liquids and gases by the actual movement of the heated particles. When a fluid is heated, the warmer part expands, becomes lighter (less dense) and rises; the cooler, denser fluid sinks to take its place. This sets up a circulation called a convection current.
Take water in a beaker and drop a small crystal of potassium permanganate near one side at the bottom. Heat that spot gently. The coloured water rises from the heated region, moves across the top, comes down the other side and back — tracing a clear circular path. This visible loop is a convection current showing how heat spreads through the liquid.
5.1 Sea breeze and land breeze
Convection currents in air explain the breezes near coasts.
- Sea breeze (daytime): During the day the land heats up faster than the sea. Warm air over the land rises, and cooler air from the sea moves in to take its place. This cool wind blowing from the sea to the land is the sea breeze.
- Land breeze (night-time): At night the land cools faster than the sea. The air over the warmer sea rises, and cooler air from the land moves towards the sea. This wind blowing from the land to the sea is the land breeze.
Convection currents also explain why a room heater warms the whole room, why the cooling part of a refrigerator is placed at the top, and why ventilators are kept high on walls to let hot air escape.
6. Radiation
Radiation is the transfer of heat without any medium. Heat from the radiation travels in the form of invisible heat rays (a kind of energy wave) and can pass through empty space (vacuum).
We feel the warmth of a fire, a heater or the Sun by radiation. All hot bodies give out heat by radiation. When these heat rays fall on an object, part is absorbed (warming it), part is reflected, and part may pass through.
6.1 Absorbers and reflectors of heat
- Dark, dull (black) surfaces are good absorbers and good emitters of heat.
- Light, shiny (white or polished) surfaces are poor absorbers but good reflectors of heat.
Take two similar metal cans, paint one black and leave the other shiny silver. Fill both with the same amount of water at the same temperature and place them in the Sun. After some time, the water in the black can is warmer. This shows that black (dark, dull) surfaces absorb more heat radiation than shiny surfaces.
This is why we prefer dark-coloured clothes in winter (to absorb more heat and stay warm) and light-coloured clothes in summer (to reflect heat and stay cool). It also explains why tea stays hot longer in a shiny steel flask.
7. Heat Transfer in Nature and Everyday Life
The three methods of heat transfer act together all around us.
- The Sun warms the Earth by radiation through the vacuum of space.
- Winds and ocean currents form because of convection in the atmosphere and seas, helping spread heat around the planet.
- Coastal areas have a moderate climate because the sea heats up and cools down slowly and the sea and land breezes keep temperatures even.
- Cooking uses conduction (heat through the pan), convection (heat through the boiling water) and radiation (heat from the flame) at the same time.
- A thermos flask reduces all three: a vacuum between its walls stops conduction and convection, and shiny silvered walls reduce radiation, keeping drinks hot or cold for hours.
7.1 Conduction vs Convection vs Radiation
| Feature | Conduction | Convection | Radiation |
|---|---|---|---|
| Occurs in | Solids | Liquids & gases | All; needs no medium |
| Particles move? | No (only vibrate) | Yes (actual movement) | No particles needed |
| Medium needed? | Yes | Yes | No |
| Example | Hot metal spoon in tea | Boiling water, sea breeze | Heat of the Sun |
- Lower temperature to higher temperature
- Higher temperature to lower temperature
- One of equal temperature to another
- Larger size to smaller size
- 0 °C
- 37 °C
- 42 °C
- 100 °C
- Make it look attractive
- Stop the liquid from flowing back so the reading stays
- Increase its range
- Measure very high temperatures
- Convection
- Radiation
- Conduction
- Evaporation
- Wood
- Plastic
- Copper
- Air
- Good conductors
- Good insulators
- Cheap to buy
- Shiny in appearance
- Conduction
- Convection
- Radiation
- Insulation
- Land breeze
- Sea breeze
- Monsoon wind
- Trade wind
- Conduction
- Convection
- Radiation
- All three together
- Shiny white
- Polished silver
- Dark dull (black)
- Transparent glass
- Absorb more heat
- Reflect most of the heat and keep us cool
- Are heavier
- Conduct heat into the body
- 35 °C to 42 °C
- 0 °C to 50 °C
- −10 °C to 110 °C
- 100 °C to 500 °C
- Convection
- Radiation
- Conduction
- Evaporation
- Increases conduction
- Reduces all three types of heat transfer
- Adds heat to the drink
- Allows free convection
- Wool produces heat
- Wool traps air, which is a poor conductor, reducing loss of body heat
- Wool is a good conductor of heat
- Wool reflects sunlight
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