Heat and Its Transfer
NCERT Class 7 • Science • Chapter "Heat" | NCERT Class 9 • Science • Chapter "Matter in Our Surroundings"
Heat and Its Transfer
Heat is a form of energy that flows from a hotter body to a colder one. Temperature measures how hot or cold a body is, not how much heat it contains, and keeping the two apart is the first step to understanding every question in this topic.
Heat, Temperature and Measurement
- Heat always flows from higher to lower temperature until both bodies reach the same temperature.
- A large body at a low temperature can hold more heat than a small body at a high temperature, so heat and temperature are not the same.
- Temperature is measured with a thermometer; a clinical thermometer has a kink that stops mercury from flowing back.
- The Celsius scale takes the melting point of ice as zero and the boiling point of water as one hundred.
- The Kelvin scale is the SI scale, and zero kelvin is the lowest possible temperature.
- Most substances expand on heating and contract on cooling, which is why gaps are left between rail lines and in bridges.
- Water is unusual because it expands on freezing, which is why ice floats.
The Three Modes of Transfer
- Conduction transfers heat through a solid from particle to particle without the particles themselves moving from place to place.
- Metals are good conductors while wood, plastic, air and water are poor conductors, called insulators.
- Convection transfers heat in liquids and gases by the actual movement of the heated material, which rises while colder material sinks.
- Sea breeze, land breeze, the working of a chimney and the boiling of water are all examples of convection.
- Radiation transfers heat without any medium, which is how the sun's heat reaches the earth.
- Dark and rough surfaces absorb and emit radiation better, while light shiny surfaces reflect it.
- This is why light coloured clothes are worn in summer and dark ones in winter.
| Mode | Medium Needed | Example |
|---|---|---|
| Conduction | Solid, particles fixed | Spoon in hot tea |
| Convection | Liquid or gas, material moves | Sea breeze, boiling water |
| Radiation | No medium needed | Heat of the sun |
Convection — the transfer of heat in fluids by the actual movement of the heated part of the fluid.
Exam me kaise aata hai
- Heat from the sun reaches us by — radiation
- Heat transfer in a metal spoon is by — conduction
- Sea breeze is an example of — convection
- Which surfaces absorb radiation best — dark and rough surfaces
UPSC/State PSC ke liye
- Water expands on freezing, so ice is less dense and floats, which lets aquatic life survive under a frozen surface.
- Radiation is the only mode that works through vacuum, which is why a thermos flask uses a vacuum gap and silvered walls together.
Yahan confuse hote hain
✗ Heat and temperature are the same thing | Heat is energy transferred; temperature is degree of hotness | ✓
✗ Convection can happen in solids | Convection needs a fluid whose material can move | ✓
✗ All substances contract on freezing | Water expands on freezing, which is why ice floats | ✓
Ek nazar me
- Heat flows from hot to cold; temperature measures the degree of hotness.
- Most substances expand on heating; water expands on freezing.
- Conduction in solids, convection in fluids, radiation without a medium.
- Dark rough surfaces absorb and emit radiation best.
Ab practice karein
Light, sound aur heat se har exam me questions pakka aate hain. ExamAtlas ke mock test se apni taiyari abhi jaanchein.