Heat
1. Heat and Temperature
Heat
HEAT is a form of ENERGY that flows from a HOTTER body to a COLDER body.
SI unit: joule (J). Also measured in calorie (cal).
Temperature
TEMPERATURE is the measure of the HOTNESS or COLDNESS of a body.
SI unit: kelvin (K). Also measured in °C (Celsius) and °F (Fahrenheit).
Key Difference
| Quantity | Heat | Temperature |
|---|---|---|
| Nature | Energy | Degree of hotness |
| SI unit | Joule (J) | Kelvin (K) |
| Depends on | Mass, material, temperature | Average kinetic energy of molecules |
| Direction of flow | Hot → Cold | — |
2. Thermometers
A THERMOMETER is an instrument used to measure TEMPERATURE.
Types of Thermometers
| Type | Range | Use | Liquid |
|---|---|---|---|
| Clinical thermometer | 35°C to 42°C | Human body temperature | Mercury or alcohol |
| Laboratory thermometer | -10°C to 110°C | General lab use | Mercury |
| Digital thermometer | Wide range | Medical and general | Electronic sensor |
| Maximum-minimum thermometer | Records extremes | Weather stations | Alcohol and mercury |
Reading a Thermometer
- Clinical thermometer has a KINK or CONSTRICTION that prevents mercury from falling back.
- Always hold it at the BULB (not the stem).
- Wait for the mercury level to become STABLE.
Celsius to Fahrenheit Conversion
°F = (°C × 9/5) + 32 °C = (°F - 32) × 5/9
Worked Example (ICSE 2024, 2 marks)
'Convert 40°C to Fahrenheit.' °F = (40 × 9/5) + 32 = 72 + 32 = 104°F.
3. Modes of Heat Transfer
Conduction
Heat transfer through a SOLID without the movement of the material itself.
Mechanism: Heat energy causes atoms/molecules to vibrate more. These vibrations are passed to neighbouring atoms.
Good Conductors: Metals (copper, aluminium, iron, silver). Bad Conductors (Insulators): Wood, plastic, rubber, glass, air, wool.
Convection
Heat transfer through FLUIDS (liquids and gases) by the ACTUAL MOVEMENT of the heated substance.
Mechanism: Heated fluid EXPANDS, becomes LESS DENSE, RISES. Cooler, denser fluid SINKS. This creates CONVECTION CURRENTS.
Examples:
- Boiling water (water circulates).
- Sea breeze and land breeze.
- Room heaters placed near the floor.
Radiation
Heat transfer WITHOUT any medium (can travel through VACUUM).
Mechanism: Heat travels as ELECTROMAGNETIC WAVES (infrared radiation).
Examples:
- Heat from the Sun reaches Earth through space.
- Heat from a fire or room heater.
- Cooking food in a microwave oven.
4. Comparison of Heat Transfer Methods
| Feature | Conduction | Convection | Radiation |
|---|---|---|---|
| Medium needed | YES (solid) | YES (fluid) | NO (can travel through vacuum) |
| How it travels | Molecular vibration | Bulk movement of fluid | Electromagnetic waves |
| Speed | Slow | Moderate | Very fast (speed of light) |
| Direction | Hot to cold | Upward (heated fluid rises) | Straight lines |
| Example | Metal spoon in hot tea | Water boiling | Sun's heat |
5. Conductors and Insulators
Conductors
Materials that allow heat to flow through them EASILY.
- Good: Silver, copper, aluminium, iron, brass.
- Why: Have FREE ELECTRONS that transfer kinetic energy quickly.
Insulators
Materials that do NOT allow heat to flow easily.
- Good: Wood, plastic, rubber, glass, air, wool, thermocol.
- Why: No free electrons. Molecules are tightly bound or widely spaced.
Applications
- Cooking utensils: made of METALS (good conductors) with PLASTIC or WOOD handles (insulators).
- Winter clothing: wool traps air (an insulator) between fibres.
- Double-pane windows: air between panes acts as insulator.
6. Expansion and Contraction
Thermal Expansion
Most substances EXPAND when heated and CONTRACT when cooled.
- Solids: Expands least (firmly held molecules).
- Liquids: Expand more than solids.
- Gases: Expand the MOST.
Applications
- Gaps in railway tracks (allow for expansion in summer).
- Mercury/alcohol in thermometers (expands and rises).
- Tight metal lids can be loosened by warming (lid expands).
7. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Heat vs temperature | 2 marks | High |
| Thermometer reading and conversion | 2-3 marks | Very High |
| Modes of heat transfer | 3 marks | Very High |
| Conductors and insulators | 2 marks | High |
| Expansion and contraction | 2 marks | Medium |
Common Mistakes
- Using heat and temperature INTERCHANGEABLY (they are different concepts).
- Thinking radiation requires a medium (it does NOT — it travels through vacuum).
- Confusing convection current direction (heated fluid RISES, cold SINKS).
- Forgetting the kink in a clinical thermometer.
Self-Test (5 Questions)
Q1. Which mode of heat transfer does NOT require a medium? (1 mark)
- A) Conduction
- B) Convection
- C) Radiation
- D) All need a medium
Q2. Convert 50°C to Fahrenheit. (2 marks)
Q3. 'Why are cooking utensils made of metal with wooden handles?' (2 marks)
Q4. 'In which mode of heat transfer does the substance itself move?' (1 mark)
Q5. 'Why is the bulb of a thermometer placed near the floor in a room?' (2 marks)
Answers
A1. C) Radiation. A2. 122°F. (50 × 9/5 + 32 = 90 + 32 = 122°F.) A3. Metal is a good conductor (heats food quickly). Wood is an insulator (keeps handle cool to hold). A4. Convection (the fluid moves, carrying heat with it). A5. Warm air rises by convection. The bulb near the floor heats the cooler air first, creating a convection current that warms the whole room.
