Heat, Temperature and Transfer of Heat
Heat, Temperature and Transfer of Heat
Heat gives 1 to 2 questions in the Part-III science paper. The chapter is short but the examiner favours it because the heat-versus-temperature distinction is a genuine misconception that a class 7 teacher must be able to correct.
Heat and Temperature Are Not the Same
| Point | Heat | Temperature |
|---|---|---|
| What it is | A form of energy | Degree of hotness or coldness |
| SI unit | Joule | Kelvin |
| Common unit | Calorie | Degree Celsius |
| Depends on mass | Yes | No |
| Measured by | Calorimeter | Thermometer |
A bucket of warm water at 40 degrees contains far more heat than a cup at the same 40 degrees, though the temperature is identical. That example settles the distinction and is asked in exactly that form.
✗ A larger object at the same temperature has a higher temperature | ✓ It has more heat energy, but the same temperature
Celsius to Fahrenheit: F = (9/5)C + 32 | Kelvin = Celsius + 273
A clinical thermometer reads roughly 35 to 42 degrees Celsius and has a kink to stop the mercury falling back. A laboratory thermometer has a wider range and no kink. Mercury thermometers are now being replaced by digital ones because mercury is toxic.
The Three Modes of Transfer
| Mode | How it works | Happens in |
|---|---|---|
| Conduction | Particle to particle, without the particles moving | Mainly solids |
| Convection | The heated fluid itself moves | Liquids and gases |
| Radiation | No medium needed at all | Even through vacuum |
Heat from the Sun reaches Earth by radiation, since space is a vacuum and the other two modes need a medium. That is the most asked single fact in the topic.
- Metals are good conductors; wood, plastic, air and water are poor conductors, called insulators.
- Land and sea breezes are convection currents, as is the circulation of air in a room.
- Dark surfaces absorb and radiate heat better than light ones, which is why light clothes suit summer.
- A thermos flask blocks all three modes at once — vacuum stops conduction and convection, silvering stops radiation.
The thermos flask question is popular precisely because it requires all three modes in one answer.
Expansion on Heating
Most substances expand on heating and contract on cooling. Gaps are left between rail tracks and in bridges for this reason, and electric wires are strung slightly loose in summer.
Water is the famous exception: between 0 and 4 degrees Celsius it contracts on heating and expands on cooling. This anomalous expansion is why ice floats and why fish survive under a frozen lake — the coldest water rises and freezes at the top, leaving liquid water below.
सूर्य की ऊष्मा विकिरण से आती है — निर्वात में चालन और संवहन संभव नहीं।
TRE pointer: The heat-versus-temperature table is the highest-value half of this topic, and the bucket-and-cup example is the version examiners use. The Sun's heat reaches Earth by radiation fact is asked almost every year, and the anomalous expansion of water is the reasoning question behind why fish survive winter. Because these are explanation questions, guessing is expensive when a wrong answer costs −1/3.
60-Second Recap
- Heat is energy measured in joules; temperature is degree of hotness measured in kelvin.
- More mass at the same temperature means more heat, not higher temperature.
- Kelvin = Celsius + 273; a clinical thermometer has a kink, a lab thermometer does not.
- Conduction in solids, convection in fluids, radiation without any medium.
- Solar heat reaches Earth by radiation; a thermos blocks all three modes.
- Water contracts between 0 and 4 degrees, which is why ice floats.
Frequently Asked Questions
What is the difference between heat and temperature?
Heat is a form of energy and depends on the mass of the body, while temperature only measures how hot something is and does not depend on mass. A bucket and a cup of water at the same temperature hold very different amounts of heat energy.
How does the Sun's heat reach the Earth?
By radiation, which is the only mode of heat transfer that needs no medium. Space between the Sun and the Earth is a vacuum, so conduction and convection, both of which require particles, cannot operate there. This is the most frequently asked fact in the topic.
Why does ice float on water?
Because of the anomalous expansion of water: between zero and four degrees Celsius water expands as it cools, so ice is less dense than the water beneath it. This also means a frozen lake keeps liquid water below the ice, allowing fish to survive winter.
How does a thermos flask keep liquids hot?
By blocking all three modes of heat transfer at once. The vacuum between its double walls prevents conduction and convection, and the silvered surfaces reflect radiation back. The stopper prevents heat loss by evaporation, which completes the design.