⚛️ Physics · Topic 5 of 10
Start with the study notes on Heat & Temperature (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Temperature is a measure of how hot or cold something is. Heat is a form of energy that flows from a hotter object to a colder one.
Heat flows on its own from hot → cold, until both are at the same temperature.
Examples:
Remember: Touch is not reliable. A steel spoon and a wooden spoon in the same room feel different, even though they are at the same temperature.
A thermometer measures temperature. Its liquid expands and rises up a thin tube when it is heated.
Temperature is usually measured in degrees Celsius (°C). The SI unit is the kelvin (K).
Measures body temperature, with a range of about 35 °C to 42 °C. A kink in its tube stops the liquid from falling back, so the reading stays until you shake it down.
Measures other temperatures. It has no kink and a wider range, about –10 °C to 110 °C, so it must be read while it is still in the substance.
Remember: Digital thermometers are now preferred, because they contain no poisonous mercury.
Conduction is the transfer of heat through a material, mainly a solid, from the hotter part to the colder part. The material itself does not move.
Example: A steel spoon left in hot milk becomes warm all the way up to the handle.
Example: Cooking pans are made of metal, so heat reaches the food quickly, but their handles are plastic or wood, so we can hold them.
Convection is the transfer of heat in liquids and gases by the movement of the heated fluid itself. The warmed part becomes lighter and rises, and cooler fluid sinks to take its place. This loop is called a convection current.
Examples:
Radiation is the transfer of heat without any material medium. It can even cross empty space.
Examples:
Examples:
Most solids, liquids and gases expand (get bigger) when they are heated and contract (get smaller) when they are cooled.
Examples:
Trapped air is a poor conductor of heat, so it slows down heat loss.
Examples:
| Misconception | Correct understanding |
|---|---|
| Heat and temperature mean the same thing. | Temperature measures how hot something is; heat is energy that flows. |
| Cold flows into a drink to cool it. | Heat flows out of the warmer thing; there is no flow of “cold”. |
| A sweater produces heat. | It only slows down the heat leaving your body. |
| Metal objects are colder than wooden ones in the same room. | Both are at room temperature; metal just conducts heat away from your hand faster. |
| Radiation needs air to travel. | Radiation needs no medium at all. That is how the Sun's heat crosses space. |
What is normal human body temperature?
About 37 °C.
Why does a clinical thermometer have a kink?
To stop the liquid falling back, so the reading can be taken after the thermometer is removed.
Why are pan handles made of plastic or wood?
They are insulators, so they stay cool enough to hold.
How does the Sun's heat reach the Earth?
By radiation, which needs no medium.
Why are ventilators placed near the ceiling?
Warm air rises by convection and escapes through them.
Why are gaps left between railway rails?
So the rails have room to expand on hot days.
Why do we wear light-coloured clothes in summer?
Light colours reflect most of the Sun's heat.
Why are two thin sweaters warmer than one thick one?
The air trapped between them is an extra insulating layer.
Key takeaway: Heat always flows from hot to cold: by conduction in solids, by convection in liquids and gases, and by radiation through empty space.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
A reliable measure of how hot or cold an object is, is its:
Answer: A — Temperature
Temperature is the measure of the degree of hotness or coldness of an object. Our sense of touch is unreliable, so we use a thermometer to measure temperature accurately.
Q2. easy
The device used to measure temperature is the:
Answer: A — Thermometer
A thermometer measures temperature. Common types are the clinical thermometer (for body temperature) and the laboratory thermometer (for other measurements).
Q57. easy
Which instrument is used to measure body temperature?
Answer: A — A clinical thermometer
A clinical thermometer measures human body temperature, with a range of about 35°C to 42°C. A ruler measures length and a weighing scale measures mass.
Q3. medium
The thermometer used to measure the temperature of the human body is called a:
Answer: A — Clinical thermometer
A clinical thermometer is designed to measure human body temperature. Its range is small (usually 35°C to 42°C) because the body temperature stays within this range.
Q4. medium
The normal temperature of the healthy human body is about:
Answer: A — 37°C (98.6°F)
The normal human body temperature is about 37 degrees Celsius, which equals 98.6 degrees Fahrenheit. A higher temperature usually indicates fever.
Q5. medium
The liquid traditionally used inside a clinical thermometer is:
Answer: A — Mercury (a shiny metal that is liquid at room temperature)
Traditional thermometers use mercury, a silvery liquid metal that expands evenly on heating. (Because mercury is toxic, digital thermometers are now widely used.)
Q8. medium
The range of a laboratory thermometer is usually:
Answer: A — About -10°C to 110°C
A laboratory thermometer measures a wide range of temperatures (typically -10°C to 110°C), unlike the clinical thermometer's narrow body-temperature range. It has no kink.
Q10. medium
Heat always flows from a body at:
Answer: A — Higher temperature to a body at lower temperature
Heat flows naturally from a hotter object (higher temperature) to a colder object (lower temperature) until both reach the same temperature. It never flows on its own from cold to hot.
Q12. medium
The three ways in which heat can travel (transfer) are:
Answer: A — Conduction, convection and radiation
Heat transfers by three methods: conduction (through solids, by contact), convection (in liquids and gases, by movement of the fluid) and radiation (through empty space, like sunlight).
Q13. medium
The transfer of heat through a SOLID (like a metal spoon in hot tea getting warm) is called:
Answer: A — Conduction
Conduction is the transfer of heat through a material (especially solids) from the hotter part to the cooler part, without the material itself moving. Metals are good conductors of heat.
Q16. medium
Why are cooking utensils often made of metal but their handles of plastic or wood?
Answer: A — Metal conducts heat to cook food; the handle insulates so we can hold it safely
Metal is a good conductor, so it quickly carries heat to cook the food. The plastic or wooden handle is an insulator, so it stays cool enough to hold without burning our hands.
Q17. medium
The transfer of heat in LIQUIDS and GASES, by the actual movement of the heated fluid, is called:
Answer: A — Convection
In convection, the heated part of a liquid or gas becomes lighter and rises, while cooler fluid sinks to take its place, setting up convection currents that carry heat. This is how water boils and rooms warm up.
Q19. medium
How does heat from the Sun reach the Earth through empty space (vacuum)?
Answer: A — By radiation
Radiation is the transfer of heat without any medium (it can travel through empty space/vacuum). The Sun's heat reaches Earth by radiation, since there is no air in space for conduction or convection.
Q21. medium
We feel warm sitting near a fire mainly because of heat transfer by:
Answer: A — Radiation
The warmth we feel from a fire (or the sun) travelling directly to us through the air is mainly radiation. (Some warm air also rises by convection.)
Q23. medium
Most substances EXPAND (increase in size) when they are:
Answer: A — Heated
Most solids, liquids and gases expand on heating and contract (shrink) on cooling. This thermal expansion is why a thermometer's liquid rises when heated.
Q25. medium
In summer, why do electrical wires between poles sag (hang loosely)?
Answer: A — The wires expand in the heat and become slightly longer
Metal wires expand in hot summer weather, becoming longer and sagging. In winter they contract and become tight. Wires are strung with some slack to allow for this.
Q27. medium
The common units of temperature are:
Answer: A — Degree Celsius (°C) and degree Fahrenheit (°F)
Temperature is commonly measured in degrees Celsius (°C) and degrees Fahrenheit (°F). The SI unit is the kelvin (K). On the Celsius scale, water freezes at 0°C and boils at 100°C.
Q29. medium
Why do we use a thermometer instead of just touching to find temperature?
Answer: A — Our sense of touch is unreliable and cannot give an exact value
Our sense of touch can be fooled (a metal object feels colder than a wooden one at the same temperature) and cannot give a number. A thermometer gives an accurate, exact temperature reading.
Q31. medium
Sea breeze (cool air from the sea to the land during the day) is an example of heat transfer by:
Answer: A — Convection
During the day, land heats up faster than the sea. The warm air over the land rises (convection) and cooler air from the sea flows in to replace it, creating the sea breeze - a large-scale convection current.
Q33. medium
Woollen clothes keep us warm in winter mainly because wool:
Answer: A — Traps air, which is a poor conductor of heat (insulator)
Wool traps a lot of air within its fibres. Air is a poor conductor (good insulator) of heat, so it slows the loss of body heat to the cold surroundings, keeping us warm.
Q35. medium
The dark inner walls of a solar cooker are painted black because black surfaces:
Answer: A — Absorb heat (radiation) well
Black surfaces are good absorbers of heat radiation, so the inside of a solar cooker is painted black to soak up the sun's heat and cook the food efficiently.
Q37. medium
The shiny silver coating inside a thermos flask (vacuum flask) helps to:
Answer: A — Reduce heat loss/gain by reflecting radiation
A thermos flask keeps drinks hot or cold by reducing all three types of heat transfer. Its shiny silvered walls reflect heat radiation, and the vacuum between its double walls stops conduction and convection.
Q39. medium
When ice melts into water, the ice:
Answer: A — Absorbs heat from its surroundings
To melt, ice must absorb heat from its surroundings (this is why the surroundings feel cooler near melting ice). Melting is the change from solid to liquid, which needs heat energy.
Q41. medium
Handles of soldering irons, cooking pans and kettles are made of insulating material so that:
Answer: A — They do not become too hot to hold
Insulating handles (plastic, wood, bakelite) do not conduct heat well, so they stay cool enough to hold safely even when the metal part is very hot.
Q43. medium
The reading of a thermometer should be taken with the eye kept:
Answer: A — Level with the top of the mercury (to avoid parallax error)
To read a thermometer correctly, the eye should be at the same level as the top of the mercury column, looking straight at it, to avoid a parallax error.
Q45. medium
Which of these is a GOOD conductor of heat?
Answer: A — Aluminium
Aluminium (a metal) is a good conductor of heat, which is why it is used for cooking pots. Wood, plastic and wool are insulators (poor conductors).
Q47. medium
Ventilators are placed near the CEILING of a room because:
Answer: A — Warm, used air rises and escapes through them (convection)
Warm, stale air rises to the top of a room. Ventilators near the ceiling let this warm air out, while fresh cooler air enters through windows lower down - a use of convection for ventilation.
Q49. medium
During a hot day, houses with white or light-coloured walls stay cooler because light colours:
Answer: A — Reflect most of the sunlight (heat)
Light/white surfaces reflect most of the sun's heat radiation instead of absorbing it, so light-coloured houses stay cooler inside during hot weather.
Q51. medium
The SI unit of temperature is the:
Answer: A — Kelvin (K)
The SI unit of temperature is the kelvin (K). In everyday life, temperature is usually given in degrees Celsius (°C). The joule is the unit of energy (heat).
Q53. medium
When you touch a hot object, heat flows:
Answer: A — From the object into your hand
Because the object is hotter than your hand, heat flows from the hotter object into your cooler hand, which is why you feel warmth (or get burned if it is very hot).
Q55. medium
A room warms up on a sunny day because sunlight brings heat by:
Answer: A — Radiation (through space and the window)
The Sun's heat travels to Earth and into a room mainly by radiation (it can pass through the vacuum of space and through glass windows), warming the objects inside.
Q58. medium
Heat is a form of:
Answer: A — Energy
Heat is a form of energy that flows from a hotter body to a colder body. Its SI unit is the joule.
Q60. medium
On the Celsius scale, the boiling point of water is:
Answer: A — 100°C
Water boils at 100°C and freezes at 0°C on the Celsius scale at normal atmospheric pressure.
Q6. hard
A clinical thermometer has a small BEND (kink/constriction) in the tube. Its purpose is to:
Answer: A — Stop the mercury from falling back, so the reading can be taken after removing the thermometer
The kink prevents the mercury from flowing back down on its own once the thermometer is taken out of the mouth. This lets you read the body temperature calmly. You must shake the thermometer to bring the mercury back.
Q7. hard
Why should a clinical thermometer be given a few JERKS (shaken) before use?
Answer: A — To bring the mercury level below the normal reading (past the kink)
Because of the kink, mercury stays at the last reading. Shaking (jerking) the thermometer forces the mercury back below the scale, so it can rise correctly for the next measurement.
Q9. hard
A key difference between a laboratory thermometer and a clinical thermometer is that the laboratory thermometer:
Answer: A — Has no kink (constriction) and covers a wider range
The laboratory thermometer has no kink (so mercury falls back on its own and must be read in place) and covers a wider temperature range. The clinical thermometer has a kink and a narrow range.
Q11. hard
When a hot cup of tea is left on a table, it cools down because:
Answer: A — Heat flows from the hot tea to the cooler surroundings
Heat flows from the hotter tea to the cooler surroundings (air, cup, table), so the tea loses heat and cools until it reaches the temperature of the surroundings.
Q14. hard
Materials that allow heat to pass through them easily are called:
Answer: A — Good conductors of heat (like metals)
Good conductors of heat, such as iron, copper and aluminium, allow heat to pass through easily. This is why cooking pots are made of metal.
Q15. hard
Materials that do NOT allow heat to pass through them easily are called:
Answer: A — Insulators (poor conductors) - like wood, plastic and wool
Insulators (poor conductors of heat) such as wood, plastic, wool, air and cloth do not let heat pass easily. This is why cooking pan handles are made of plastic or wood.
Q18. hard
When water is heated in a pot, the water at the bottom gets hot first and RISES, while cooler water sinks. These moving flows are called:
Answer: A — Convection currents
Convection currents form when hot (lighter) water rises and cold (denser) water sinks, circulating heat throughout the pot. The same process occurs in the air and the oceans.
Q20. hard
Which method of heat transfer does NOT require any material medium?
Answer: A — Radiation
Radiation does not need any medium - it can pass through a vacuum (like space). Conduction needs a solid, and convection needs a liquid or gas.
Q22. hard
Why do we prefer to wear LIGHT-coloured clothes in summer and DARK-coloured clothes in winter?
Answer: A — Dark colours absorb more heat (warm in winter); light colours reflect heat (cool in summer)
Dark surfaces absorb more heat radiation, keeping us warmer in winter. Light/white surfaces reflect more heat, keeping us cooler in summer. This is why summer clothes are usually light-coloured.
Q24. hard
Gaps are left between railway tracks (and in bridges) to allow for:
Answer: A — Expansion of the metal in hot weather
Metal expands when heated. Small gaps between rail sections give room for this expansion in hot weather, preventing the tracks from buckling (bending out of shape).
Q26. hard
A thermometer works on the principle that a liquid (like mercury) ______ when heated.
Answer: A — Expands (rises up the tube)
When the thermometer bulb is heated, the mercury inside expands and rises up the thin tube. The height of the mercury against the scale gives the temperature reading.
Q28. hard
On the Celsius scale, water freezes at ____ and boils at ____.
Answer: A — 0°C and 100°C
On the Celsius scale, the freezing point of water is 0°C and the boiling point is 100°C (at normal atmospheric pressure). On the Fahrenheit scale, these are 32°F and 212°F.
Q30. hard
A metal spoon and a wooden spoon are both at room temperature. The metal spoon feels colder because:
Answer: A — Metal conducts heat away from your hand faster than wood
Both are at the same temperature, but metal (a good conductor) carries heat away from your warm hand quickly, so it feels cold. Wood (an insulator) does not, so it feels warmer. This shows why touch is unreliable.
Q32. hard
A land breeze (from land to sea at night) occurs because:
Answer: A — At night, land cools faster than the sea, so cooler land air flows toward the warmer sea
At night, the land cools faster than the sea. The air over the warmer sea rises, and cooler air from the land flows out to the sea, producing a land breeze - the reverse of the daytime sea breeze.
Q34. hard
Why do we feel warmer wearing TWO thin sweaters than ONE thick sweater of the same total thickness?
Answer: A — The layer of air trapped between the two sweaters gives extra insulation
Two thin sweaters trap a layer of air between them. Since air is a poor conductor of heat, this extra trapped air layer provides better insulation than a single thick sweater.
Q36. hard
Which surface is the BEST absorber (and best emitter) of heat radiation?
Answer: A — A dull black surface
Dull black surfaces are the best absorbers and emitters of heat radiation. Shiny, light-coloured or silvery surfaces are poor absorbers but good reflectors of heat.
Q38. hard
A thermos (vacuum) flask keeps liquids hot for long by preventing heat transfer through all three methods. The VACUUM between its walls stops:
Answer: A — Conduction and convection (which need a medium)
The vacuum (no air) between the double walls stops conduction and convection, since both need a material medium. The silvered walls reduce radiation. Together they keep the contents hot (or cold) for hours.
Q40. hard
Which statement about heat and temperature is CORRECT?
Answer: A — Heat is a form of energy that flows; temperature measures the degree of hotness
Heat is a form of energy that flows from a hotter body to a colder one. Temperature is a measure of how hot or cold something is. They are related but not the same.
Q42. hard
Birds fluff up their feathers in cold weather to:
Answer: A — Trap more air (a poor conductor) and stay warm
Fluffing feathers traps a thicker layer of air around the bird's body. Since air is a poor conductor of heat, this insulation reduces heat loss and keeps the bird warm in the cold.
Q44. hard
Why should a clinical thermometer NOT be held by its bulb while reading?
Answer: A — The heat from your hand can change the reading
Holding the bulb transfers heat from your warm hand into it, which can raise the mercury and give a wrong reading. The thermometer should be held by the stem, away from the bulb.
Q46. hard
In a room, a heater placed on the floor warms the whole room because warm air:
Answer: A — Rises and cooler air sinks, setting up convection currents
Air heated by the heater becomes lighter and rises; cooler air moves down to replace it and gets heated in turn. These convection currents circulate warm air throughout the room.
Q48. hard
Which correctly matches the heat-transfer method with its example?
Answer: A — Conduction - metal spoon in hot tea; Convection - boiling water; Radiation - sunlight
A metal spoon heating up in tea is conduction (through a solid); boiling water is convection (currents in a liquid); the Sun warming us is radiation (through empty space).
Q50. hard
Why does a cold soft-drink bottle taken out of a fridge soon become covered with water droplets on the outside?
Answer: A — Water vapour in the warm air condenses on the cold surface
The cold bottle cools the air touching it, causing the invisible water vapour in the warm air to condense into tiny water droplets on the outside. This shows air holds water vapour that condenses when cooled.
Q52. hard
A digital thermometer is now preferred over a mercury thermometer mainly because:
Answer: A — It contains no toxic mercury and is safer if it breaks
Digital thermometers do not use mercury, which is poisonous and hazardous if the glass breaks. They are safe, quick and easy to read, so they have largely replaced mercury clinical thermometers.
Q54. hard
Which statement about thermal expansion is TRUE?
Answer: A — Solids, liquids and gases generally expand on heating and contract on cooling
As a general rule, all three states of matter - solids, liquids and gases - expand when heated and contract when cooled. Gases expand the most for a given temperature rise.
Q56. hard
Which of the following is the BEST way to keep a house cool in summer using knowledge of heat?
Answer: A — Use light-coloured walls, good ventilation, and reflective roofs
Light colours reflect heat, ventilation removes warm air by convection, and reflective roofs bounce away the sun's radiation - all reduce heat gain and keep the house cooler. Black surfaces and closed ventilators would make it hotter.
Q59. hard
Why is the base of cooking pots often made of copper or aluminium?
Answer: A — These metals are good conductors of heat and spread heat quickly to the food
Copper and aluminium are excellent conductors of heat, so they quickly transfer heat from the flame to the food, allowing food to cook evenly and fast.