Physics Notes: Heat, Capacity & Measurement 🌡️
1. Heat (Q)
Definition: Heat is the transfer of thermal energy between two bodies due to temperature difference. It is measured in Joules (J). Heat flows from hotter to colder bodies.
1.1 Formula for Heat Transfer (No Phase Change)
Q = m × c × Δθ
Where: m = mass in kg, c = specific heat capacity in J/kg°C, Δθ = temperature change in °C
Example 1: A 2 kg block of Aluminium (c = 900 J/kg°C) is heated from 25°C to 55°C. Calculate the heat absorbed.
Solution: Δθ = 55 - 25 = 30°C
Q = 2 × 900 × 30 = 54,000 J
Example 2: 500 g of water is heated from 20°C to 80°C (c = 4200 J/kg°C).
Solution: m = 0.5 kg, Δθ = 60°C
Q = 0.5 × 4200 × 60 = 126,000 J
GCE Question 1: A 3 kg iron block is heated from 30°C to 90°C (c = 460 J/kg°C). Calculate Q.
Solution: Δθ = 60°C
Q = 3 × 460 × 60 = 82,800 J
2. Heat Capacity and Specific Heat Capacity
Specific Heat Capacity (c)
Definition: Heat required to raise 1 kg of a substance by 1°C
Unit: J/kg°C
Heat Capacity (C)
Definition: Heat required to raise the temperature of the whole substance by 1°C
Formula: C = Q / Δθ or C = m × c
Unit: J/°C
GCE Question 2: A body absorbs 5000 J to raise its temperature by 10°C. Calculate its heat capacity.
Solution: C = Q / Δθ = 5000 / 10 = 500 J/°C
Example 3: A 2 kg copper block (c = 390 J/kg°C) is heated by 15°C. Calculate heat absorbed.
Q = 2 × 390 × 15 = 11,700 J
3. Heat Conductors and Insulators
Heat Conductor
Material that allows heat to pass through quickly. E.g., metals like Copper, Silver, Aluminium.
Heat Insulator
Material that resists heat flow. E.g., Wood, Plastic, Wool, Air.
GCE Question 3: Why are metals used for cooking utensils?
Answer: Metals are good conductors of heat; they transfer heat quickly to cook food efficiently.
4. Thermometer and its Types 🌡️
Definition:
Device used to measure temperature. Works on the principle that physical properties change uniformly with heat.
Types (Cameroon Standard)
- Mercury-in-Glass: Mercury expands; range -10°C to 150°C
- Alcohol-in-Glass: Coloured alcohol; range -100°C to 80°C
- Clinical Thermometer: Measures body temperature; range 35°C to 43°C
GCE Question 4: Explain why alcohol thermometers are used in very low temperatures.
Answer: Alcohol freezes at a much lower temperature than mercury, allowing measurement in very cold conditions.
5. Practice Questions
Question 5: 1.5 kg of water is heated from 25°C to 75°C (c = 4200 J/kg°C). Calculate heat absorbed.
Solution: Δθ = 50°C, Q = 1.5 × 4200 × 50 = 315,000 J
Question 6: A 500 g metal block absorbs 2000 J to increase its temperature by 10°C. Find its specific heat capacity.
Solution: m = 0.5 kg, c = Q / (m × Δθ) = 2000 / (0.5 × 10) = 400 J/kg°C
Question 7: State one difference between conductor and insulator.
Answer: Conductors allow heat to pass easily; insulators resist heat flow.