Summary
Temperature — measured by a thermometer — is the reliable indicator of how hot or cold an object is, and heat always flows from a hotter body to a colder one through three processes: conduction (solids), convection (liquids and gases), and radiation (requires no medium).
Chapter 3 establishes that our sense of touch is unreliable for judging temperature, making the thermometer essential. The clinical thermometer (range 35°C–42°C) measures body temperature; the normal human body temperature is 37°C. The laboratory thermometer (range –10°C to 110°C) is used for other substances. Heat always flows from a hotter body to a colder one by three modes: conduction transfers heat through solids from the hotter end to the colder end; convection moves heat through liquids and gases via rising hot fluid and descending cool fluid; and radiation needs no medium, which is how the sun's heat reaches Earth. Conductors (aluminium, iron, copper) allow heat to pass easily; insulators (wood, plastic) do not. Dark surfaces absorb more heat than light surfaces, and wool keeps us warm because it is a poor conductor that traps air between its fibres.
Key points & formulas
- 01Our sense of touch is unreliable for judging temperature — the same water can feel hot to one hand and cold to the other simultaneously.
- 02Temperature is the reliable measure of hotness; it is measured by a thermometer.
- 03Clinical thermometer range: 35°C to 42°C; normal human body temperature is 37°C.
- 04Laboratory thermometer range: generally –10°C to 110°C; used for substances other than the human body.
- 05Heat flows from a hotter object to a colder object; the three modes are conduction, convection, and radiation.
- 06Conductors (aluminium, iron, copper) allow heat to pass easily; insulators (wood, plastic) do not — water and air are also poor conductors.
- 07Dark-coloured surfaces absorb more heat; light-coloured surfaces reflect more heat — hence light clothes are preferred in summer and dark in winter.
- 08Wool is a poor conductor of heat and traps air between its fibres, preventing body heat from escaping to cold surroundings.
Frequently asked questions
01Why is our sense of touch unreliable for judging temperature?
The same water can feel hot to one hand and cold to the other at the same time (as Activity 3.1 demonstrates), so touch gives inconsistent readings depending on the prior condition of each hand.
02What is the range of a clinical thermometer and why?
35°C to 42°C, because the human body temperature normally does not go below 35°C or above 42°C.
03What is the normal temperature of the human body?
37°C. However, slight variations above or below are normal, since 37°C represents the average body temperature of a large number of healthy persons.
04Why can a clinical thermometer not be used to measure the temperature of hot milk or other hot objects?
Its range is only 35°C–42°C. Temperatures beyond this range would cause the thermometer to break.
05What is the range of a laboratory thermometer?
Generally –10°C to 110°C, making it suitable for measuring temperatures of substances other than the human body.
06What is conduction?
The process by which heat is transferred from the hotter end to the colder end of an object. In solids, heat is generally transferred by conduction.
07What is convection?
The mode of heat transfer in liquids and gases where the hot fluid rises and cooler fluid moves in to take its place, setting up currents that spread heat throughout the substance.
08What is radiation, and why is it different from conduction and convection?
Radiation is the transfer of heat that requires no medium — it can occur whether a medium is present or not. This is how the sun's heat reaches Earth across empty space.
09Why do we wear light-coloured clothes in summer and dark-coloured clothes in winter?
Dark surfaces absorb more heat (useful for warmth in winter), while light-coloured surfaces reflect most of the heat that falls on them (keeping us cooler in summer).
10Why do woollen clothes keep us warm in winter?
Wool is a poor conductor of heat, and air is trapped between the wool fibres. This trapped air prevents the flow of heat from our body to the cold surroundings.
11What is the difference between a conductor and an insulator of heat?
Materials that allow heat to pass through them easily — such as aluminium, iron, and copper — are conductors. Materials that do not allow heat to pass through easily — such as plastic and wood — are insulators.
12What is sea breeze and when does it blow?
During the day, land heats faster than water, so the air over land becomes hotter and rises. Cooler air from the sea rushes in to take its place — this is sea breeze, which blows during the day.
13What is land breeze?
At night, water cools more slowly than land, so cooler air from the land moves towards the sea. This is called land breeze, which blows at night.
14Why does a clinical thermometer have a kink near the bulb?
The kink prevents the mercury from falling on its own when the thermometer is removed from the mouth, so the reading remains stable long enough to be noted.
15Why must the mercury level in a clinical thermometer be brought below 35°C before use?
To clear the previous reading. If the mercury is not reset below 35°C, the thermometer may show a falsely high or stale temperature reading.
16Is the NCERT Class 7 Science Chapter 3 Heat PDF free to read?
Yes — the full chapter PDF is free to read and download on cbseprepmaster.com with no sign-up required.
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