Short Answer Questions — Chapter 3: Thermal Physics
| Property | Heat | Temperature |
|---|---|---|
| Definition | Total thermal energy of a body | Measure of degree of hotness or coldness |
| SI Unit | Joule (J) | Kelvin (K) |
| Depends on | Mass, nature, and temperature of body | Kinetic energy of molecules |
| Instrument | Calorimeter | Thermometer |
| Transfers | From higher to lower temperature | Not "transferred" — it is a state |
Boyle's Law (at constant temperature): The pressure of a given mass of gas is inversely proportional to its volume at constant temperature.
PV = constant → P₁V₁ = P₂V₂
Charles' Law (at constant pressure): The volume of a given mass of gas is directly proportional to its absolute temperature at constant pressure.
V/T = constant → V₁/T₁ = V₂/T₂
(Temperature must be in Kelvin)
Combined Gas Law: P₁V₁/T₁ = P₂V₂/T₂
Specific heat capacity (c) of a substance is the amount of heat required to raise the temperature of 1 kg of the substance by 1°C (or 1 K).
Where:
- Q = heat energy absorbed or released (J)
- m = mass of the substance (kg)
- c = specific heat capacity (J kg⁻¹ K⁻¹)
- ΔT = change in temperature (°C or K)
SI unit: J kg⁻¹ K⁻¹
Examples:
- Water: c = 4200 J kg⁻¹ K⁻¹ (highest among common liquids)
- Iron: c = 470 J kg⁻¹ K⁻¹
- Copper: c = 385 J kg⁻¹ K⁻¹
- Aluminium: c = 900 J kg⁻¹ K⁻¹
Thermal expansion is the increase in the size (length, area, or volume) of a body when its temperature increases. When heated, molecules vibrate faster and push each other farther apart, causing expansion.
Types:
- Linear expansion: increase in length (solids)
- Area expansion: increase in surface area (solids)
- Volume expansion: increase in volume (solids, liquids, gases)
Daily life examples:
1. Railway tracks: Small gaps are left between rails to allow for thermal expansion in summer. Without gaps, tracks would buckle.
2. Telephone/power lines: Sagged (left slightly loose) so they don't snap in winter when they contract.
3. Bridges: Expansion joints are provided so bridges can expand in summer without cracking.
4. Mercury thermometer: Mercury expands when heated, rising up the capillary tube to show temperature.
Latent heat is the heat absorbed or released by a substance during a phase change (change of state) at constant temperature.
The word "latent" means hidden — the temperature does not change even though heat is being absorbed or released.
| Property | Latent Heat of Fusion | Latent Heat of Vaporisation |
|---|---|---|
| Definition | Heat needed to change 1 kg of solid to liquid at melting point | Heat needed to change 1 kg of liquid to gas at boiling point |
| Process | Melting (solid → liquid) | Vaporisation (liquid → gas) |
| For water | 3.36 × 10⁵ J/kg (336 kJ/kg) | 22.6 × 10⁵ J/kg (2260 kJ/kg) |
| Temperature | Remains constant at 0°C for ice | Remains constant at 100°C for water |
Why L_v > L_f for water: Converting liquid to gas requires breaking many more intermolecular bonds than melting solid to liquid.
In a pressure cooker, water boils at a temperature higher than 100°C because:
1. The lid seals the pot completely, trapping steam inside. 2. As cooking proceeds, steam builds up and pressure increases. 3. According to the effect of pressure on boiling point: Higher pressure → higher boiling point. 4. At higher pressure (about 2 atm), water boils at around 120°C instead of 100°C. 5. Food cooks in hotter water/steam → cooks faster.
This is based on the principle that the boiling point of a liquid increases with increase in pressure.
1. When we are hot, sweat glands in the skin secrete sweat (mostly water) onto the skin surface. 2. This water evaporates from the skin surface. 3. During evaporation, water molecules need latent heat of vaporisation (energy to escape as gas). 4. This energy is taken from the skin and the blood near the skin surface. 5. As a result, the skin temperature drops and the body feels cooler.
This is why:
- Evaporation is a cooling process
- We feel cooler in dry weather (sweat evaporates faster) than in humid weather (slow evaporation)
- A wet cloth on the forehead cools a fever
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