19.3 Content
19.3.1 Heat and Temperature
Temperature is a measure of the average kinetic energy of particles in a substance.
Heat is energy transferred between objects due to a temperature difference.
- Temperature measures “how hot” (°C or K)
- Heat measures “energy transferred” (J)
An object can have high temperature but low heat capacity (e.g., a spark) or low temperature but transfer a lot of heat (e.g., melting ice).
19.3.2 Interactive: Energy Bar Chart - Heating
Visualizing thermal energy changes:
19.3.3 Specific Heat Capacity
When heat is added to a substance (without changing phase), temperature rises:
\[Q = mc\Delta T\]
where: - \(Q\) = heat energy (J) - \(m\) = mass (kg) - \(c\) = specific heat capacity (J/kg·K) - \(\Delta T\) = temperature change (K or °C)
Specific heat capacity (\(c\)) is the energy needed to raise 1 kg by 1°C.
19.3.4 Common Specific Heat Capacities
| Material | \(c\) (J/kg·K) | Notes |
|---|---|---|
| Water | 4186 | Very high—good coolant |
| Aluminium | 900 | Moderate |
| Copper | 390 | Low—heats quickly |
| Iron | 450 | Moderate |
| Ice | 2090 | About half of water |
19.3.5 Latent Heat
During a phase change, temperature remains constant while heat is absorbed or released:
\[Q = mL\]
where: - \(L_f\) = latent heat of fusion (solid ↔︎ liquid) - \(L_v\) = latent heat of vaporization (liquid ↔︎ gas)
19.3.6 Interactive: Phase Change Energy
Energy changes during melting:
Key observation: Temperature stays at 0°C throughout melting—all energy goes into breaking molecular bonds.
19.3.7 Latent Heat Values for Water
| Phase Change | \(L\) (J/kg) | Process |
|---|---|---|
| Fusion | \(3.34 \times 10^5\) | Melting/Freezing |
| Vaporization | \(2.26 \times 10^6\) | Boiling/Condensing |
19.3.8 Heat Transfer Modes
Conduction: Transfer through direct molecular contact
- Requires matter (works in solids, liquids, gases)
- Faster in metals (free electrons)
Convection: Transfer by fluid motion
- Requires a fluid (liquid or gas)
- Driven by density differences
Radiation: Transfer by electromagnetic waves
- No medium required
- All objects emit thermal radiation
19.3.9 Thermal Conduction
For a slab of material:
\[P = \frac{kA\Delta T}{L}\]
where: - \(P\) = power (rate of heat transfer) (W) - \(k\) = thermal conductivity (W/m·K) - \(A\) = cross-sectional area (m²) - \(\Delta T\) = temperature difference (K) - \(L\) = thickness (m)
19.3.10 Thermal Conductivity Values
| Material | \(k\) (W/m·K) | Classification |
|---|---|---|
| Copper | 400 | Excellent conductor |
| Aluminium | 240 | Good conductor |
| Glass | 0.8 | Poor conductor |
| Wood | 0.15 | Insulator |
| Air | 0.025 | Excellent insulator |