Transfer of heat
A cup of hot tea left on a table slowly becomes cold, and a cold glass of water in the sun becomes warm. In both cases something flows between the object and its surroundings. That something is heat.
Transfer of heat (उष्णतेचे संक्रमण) is the flow of heat from a hot body to a cold body. There are three modes: conduction, convection and radiation.
In the figure : In the figure heat flows from the hot body A (80 °C) to the cold body B (30 °C): A becomes cooler and B becomes warmer.
Rule 1: Heat always flows from hot to cold — The hot body loses heat and becomes cooler; the cold body gains heat and becomes warmer. A halwai wraps cloth around the end of a metal spoon, and we hold a hot steel glass in a handkerchief, to stop the heat reaching the hand. A thermometer measures temperature.
- Easy : A hot tea at 70 °C in a room at 30 °C gives heat to the room and cools.
- Medium : An ice cube in your hand takes heat from the hand, so the hand feels cold.
- Exam-type : Body P at 60 °C touches body Q at 20 °C: heat flows from P to Q, until both reach the same temperature.
Heat flows from higher to lower temperature, never the opposite way by itself.
Thinking cold flows from ice to hand. It is heat that flows from the hand to the ice.
Memory clue: "Heat Hurries Downhill": from Hot to cold, like water flowing down a slope.
Spot the difference
| Mode | Medium needed? | Where? |
|---|---|---|
| Conduction | Yes | Solids |
| Convection | Yes | Liquids and gases |
| Radiation | No | Anywhere, even empty space |
Practice questions
- 1What is transfer of heat?
- 2Name the three modes of heat transfer.
- 3Which instrument measures temperature?
- 4Why does a halwai wrap cloth around his metal spoon?
- 5In which direction does heat flow between 80 °C and 30 °C bodies?
- 6Hot tea at 70 °C is kept in a 30 °C room. What happens to the tea and the room?
- 7Two bodies at 60 °C and 20 °C touch each other. When does the heat flow stop?
- 8Seema says that when she holds ice, cold flows into her hand. Correct her.
Show answers (try them yourself first)
- 1The flow of heat from a hot body to a cold body.
- 2Conduction, convection and radiation.
- 3Thermometer
- 4To stop the heat from reaching his hand.
- 5From the 80 °C body to the 30 °C body.
- 6The tea loses heat and cools; the room gains a little heat.
- 7When both reach the same temperature.
- 8Heat flows from her hand to the ice, so the hand feels cold.
Conduction and convection
A steel spoon left in a hot pan soon becomes too hot to touch, but the hot water at the top of a kettle is not heated by the flame directly. The heat reaches the water by flowing in two different ways.
Conduction is the transfer of heat from the hot part of an object to its cold part, through the object. Convection is the transfer of heat by currents in liquids and gases.
In the figure : In the figure three bars (steel, aluminium and copper) with wax-stuck pins are heated at one end. The pins fall starting from the hot end. The most pins have fallen from the copper bar, so copper conducts heat the fastest.
Rule 1: Conduction — In a solid, heat passes from the hot end to the cold end through the material. It needs a medium, and the speed depends on the substance.
- Easy : A steel spoon in hot tea becomes hot at the handle too.
- Medium : Copper conducts heat faster than steel, so a copper strip heats faster.
- Exam-type : In the experiment, the bar whose pins fall first (copper) is the best conductor of the three.
In the figure : In the figure the water near the bottom, heated by the flame (A), becomes warm and rises (B). Cooler water from above sinks (C) and takes its place. The crystals (D) show these currents.
Rule 2: Convection — When water is heated, the water at the bottom warms, its density decreases and it rises; cold water from above takes its place. These are convection currents, shown by potassium permanganate crystals. Convection needs a medium.
- Easy : Warm air above a heater rises and cooler air comes in to replace it.
- Medium : In a pot of boiling dal, the hot dal from the bottom rises and the cooler dal from the top sinks.
- Exam-type : Heating water from the top is slow to warm the bottom, because the warm water stays on top.
In the figure : In the figure the upper part of the water in a test tube is heated (A). The warm water cannot sink, so the ice at the bottom (B) does not melt quickly.
Conduction and convection
In the app you can move this figure with your finger.
Open in the appConduction: in solids. Convection: liquids and gases, with currents. Both need a medium.
Thinking convection happens in solids. Convection needs something that can flow.
Memory clue: CONDuction = CONtact in solids; CONVection = moving fluids that CONVey heat.
Spot the difference
| Point | Conduction | Convection |
|---|---|---|
| Where | Solids | Liquids and gases |
| How | Through the material | By currents |
| Example | Hot spoon | Boiling water |
Practice questions
- 1What is conduction?
- 2What is convection?
- 3Which of steel, aluminium and copper conducts heat the fastest?
- 4What are convection currents?
- 5Which crystals show convection currents in water?
- 6Why does warm water rise?
- 7Why does ice at the bottom of a test tube melt slowly when only the top water is heated?
- 8In a pot of boiling dal, explain how heat reaches the top layer.
Show answers (try them yourself first)
- 1Transfer of heat from the hot part of an object to its cold part.
- 2Transfer of heat by currents in liquids and gases.
- 3Copper
- 4Currents in which warm fluid rises and cooler fluid sinks.
- 5Potassium permanganate crystals.
- 6Its density decreases.
- 7Warm water stays on top and cannot sink; liquids conduct heat poorly.
- 8By convection: hot dal from the bottom rises and cooler dal sinks.
Radiation of heat
Sitting near a bonfire on a winter night, you feel the warmth on your face even though the air between is cold. Standing in the sun, you feel heat from millions of kilometres away.
Radiation (प्रारण) is the transfer of heat without any medium.
In the figure : In the figure radiation needs no medium. There is no air in the part B between the sun (A) and the earth (C), yet the heat of the sun reaches us.
Radiation of heat
In the app you can move this figure with your finger.
Open in the appRule 1: Radiation — The sun's heat reaches us by radiation. We feel a bonfire or a candle mainly by radiation. Many objects (trees, mountains, stones, roads) radiate heat, and an infrared camera uses this to make surroundings visible at night.
- Easy : Heat from a bonfire reaches your hands by radiation.
- Medium : We feel the sun's heat though there is no air in most of the space between the sun and the earth.
- Exam-type : An infrared camera shows a warm road at night because the road radiates heat.
In the figure : In the figure a dull dark surface absorbs more heat than a shiny surface: tin P (dark) shows 48 °C and tin Q (shiny) only 36 °C.
Rule 2: Absorption and reflection — When heat rays fall on an object, part is absorbed and part is reflected. Absorption depends on the colour and nature of the surface. A black object absorbs the most heat.
- Easy : A black umbrella gets hotter in the sun than a white one.
- Medium : The temperature difference between tin P and tin Q is 48 − 36 = 12 °C.
- Exam-type : In summer we prefer white or light clothes because they reflect more heat.
In the figure : In the figure heat rays fall on a surface: part is absorbed and part is reflected. The shiny surface Q reflects more (thick arrow), and the black surface P absorbs more.
Radiation needs no medium. Dark dull surfaces absorb the most; shiny ones reflect.
Thinking the sun's heat travels through air. There is no air for most of the way.
Memory clue: "Black Brings in, White Wards off": black absorbs, white and shiny reflect.
Spot the difference
| Surface | Absorbs | Reflects |
|---|---|---|
| Black, dull | More | Less |
| White, shiny | Less | More |
Practice questions
- 1What is radiation?
- 2How does the sun's heat reach us?
- 3Name a device that uses heat radiation to see at night.
- 4Which colour absorbs the most heat?
- 5Which surface reflects more heat: shiny or dull?
- 6Tin P (dark) shows 48 °C and tin Q (shiny) 36 °C. Find the difference and explain.
- 7Why do we prefer white clothes in summer?
- 8A student says the sun's heat reaches us by convection. Why is that wrong?
Show answers (try them yourself first)
- 1Transfer of heat without a medium.
- 2By radiation.
- 3Infrared camera
- 4Black
- 5Shiny
- 648 − 36 = 12 °C; the dark tin absorbs more heat.
- 7White reflects more heat and absorbs less.
- 8Convection needs a medium, and there is no air between the sun and the earth; it is radiation.
Conductors, expansion and contraction
A steel tumbler of hot tea burns your fingers, but a ceramic cup does not. A tight metal lid on a jar loosens when you dip it in warm water. Both stories are about how materials treat heat.
A conductor (वाहक) passes heat easily; a bad conductor does not. Most substances expand (प्रसरण पावतात) on heating and contract (आकुंचन पावतात) on cooling.
In the figure : In the figure a metal ball that just passes through a ring does not pass after it is heated, because solids expand on heating. They contract again on cooling.
Rule 1: Conductors and bad conductors — Copper and other metals are good conductors. Plastic and wood are bad conductors. We can hold a glass or china cup of hot tea but not a steel or copper cup.
- Easy : A wooden handle on a pan stays cool because wood is a bad conductor.
- Medium : Cooking pots are made of metal, but their handles are often plastic or wood.
- Exam-type : If a steel spoon and a wooden spoon stand in hot soup, the steel one becomes too hot to hold first.
Rule 2: Expansion and contraction — Solids expand on heating and contract on cooling; different solids expand by different amounts. In liquids the particles move farther apart on heating, so the volume increases. A gas also expands on heating.
- Easy : A tight metal lid expands when dipped in warm water, so it loosens.
- Medium : The liquid in a thermometer rises when heated because it expands.
- Exam-type : A balloon tied on a bottle swells when the bottle is put in hot water.
In the figure : In the figure a liquid expands on heating and contracts on cooling. The level in the tube rises from mark 3 at 30 °C to mark 7 at 60 °C.
In the figure : In the figure gases also expand on heating: the balloon on the bottle swells when the bottle is put in hot water.
Metals conduct; wood and plastic do not. Solids, liquids and gases expand on heating.
Thinking the particles themselves become bigger. The particles move farther apart.
Memory clue: "Hot = Higher volume, Cold = Contracts"; metals are the Mighty conductors.
Spot the difference
| State | On heating | On cooling |
|---|---|---|
| Solid | Expands a little | Contracts |
| Liquid | Expands more | Contracts |
| Gas | Expands the most | Contracts |
Practice questions
- 1Name a good conductor of heat.
- 2Name a bad conductor of heat.
- 3What happens to a solid on heating?
- 4What happens to a liquid on cooling?
- 5Why is the handle of a pan made of wood or plastic?
- 6Why does a tight metal lid loosen in warm water?
- 7In a liquid tube the level rises from mark 3 to mark 7 on heating. By how many marks did it rise?
- 8A steel spoon and a wooden spoon stand in hot soup. Which becomes too hot to hold first, and why?
Show answers (try them yourself first)
- 1Copper
- 2Wood (or plastic)
- 3It expands.
- 4It contracts.
- 5They are bad conductors, so the handle stays cool.
- 6The metal expands on heating.
- 77 − 3 = 4 marks
- 8The steel spoon, because steel is a good conductor and wood is a bad conductor.
The thermos flask
Tea poured into a thermos in the morning is still hot in the evening, while the same tea in an ordinary cup is cold in an hour. The thermos stops all three ways heat can move.
A thermos flask (थर्मास) is a double-walled flask with a vacuum between the walls, which keeps hot things hot and cold things cold. It is also called the Dewar flask.
In the figure : In the figure of the thermos flask A is the cap, B the outer jar, C the gap between the two glass walls, from which the air is removed (vacuum) and whose surfaces are shiny, and D the sponge or rubber pieces that protect the flask.
The thermos flask
In the app you can move this figure with your finger.
Open in the appRule 1: Structure — Sir James Dewar, a Scottish scientist, made the first flask in 1892. It is two glass tubes, one inside the other, sealed, with a shiny silver coating on the surfaces and the air between removed. A metal or plastic jar protects it, with sponge or rubber pieces between.
- Easy : The vacuum has no air, so conduction and convection stop.
- Medium : The shiny coating reflects heat, so radiation stops.
- Exam-type : Heat is still lost slowly from the lid and through the glass at the mouth, so after 2 or 3 hours the tea cools a little.
Rule 2: How it works — The shiny surface reflects heat, so there is no radiation; the vacuum stops conduction and convection. Some heat is still lost from the lid and by conduction through the glass, so the contents cool slightly after two or three hours.
- Easy : Hot tea stays hot because heat cannot easily leave.
- Medium : A thermos also keeps cold water cold, because outside heat cannot easily get in.
- Exam-type : Mode stopped: radiation by the shiny coating; conduction and convection by the vacuum.
Thermos: shiny walls + vacuum + protective jar. First made by Dewar in 1892.
Thinking a thermos only keeps things hot. It keeps cold things cold as well.
Memory clue: "Shiny stops Shine-heat (radiation); Vacuum stops the rest".
Spot the difference
| Part | What it stops |
|---|---|
| Shiny silver coating | Radiation |
| Vacuum | Conduction and convection |
| Sponge or rubber pieces | Protect the glass |
Practice questions
- 1Who made the first thermos flask, and when?
- 2Why is it called the Dewar flask?
- 3What is between the two glass walls of a thermos?
- 4What does the shiny coating do?
- 5Which two modes of heat transfer does the vacuum stop?
- 6Why does a thermos keep cold water cold?
- 7Why does tea in a thermos cool slightly after 2 or 3 hours?
- 8Meena says a thermos works because it has thick walls. Correct her using the structure.
Show answers (try them yourself first)
- 1Sir James Dewar, 1892.
- 2It was invented by Dewar.
- 3A vacuum (no air).
- 4It reflects heat and so stops radiation.
- 5Conduction and convection.
- 6Outside heat cannot easily get in.
- 7Some heat is lost from the lid and through the glass.
- 8It works because of the vacuum between the walls and the shiny coating, not thickness.
Quick revision
- Heat flows from hot to cold by conduction (solids), convection (liquids and gases) and radiation (no medium).
- Solids, liquids and gases expand on heating and contract on cooling.
- A thermos flask has a vacuum and shiny walls, so it keeps things hot or cold.