These questions revise the lessons in heat. Use c for water = 4200 J kg⁻¹ °C⁻¹ and l for boiling water = 2.26 × 10⁶ J kg⁻¹.
Try each question on paper first.
Questions
Question 1
0.40 kg of water at 60°C is mixed with 0.10 kg of water at 20°C. Find the final temperature, assuming no heat is lost.
Answer
Heat lost = heat gained: 0.40(60 − θ) = 0.10(θ − 20). Then 24 − 0.4θ = 0.1θ − 2, so 26 = 0.5θ and θ = 52°C.
Question 2
How much energy heats 1.5 kg of water from 25°C to 65°C?
Answer
θ = 40°C. Q = mcθ = 1.5 × 4200 × 40 = 252 000 J.
Question 3
A 1000 W heater supplies the energy in Question 2. How long does it take, ignoring heat loss?
Answer
t = Q ÷ P = 252 000 ÷ 1000 = 252 s, which is 4.2 minutes.
Question 4
0.30 kg of steam at 100°C condenses into water at 100°C. How much energy is released?
Answer
Q = ml = 0.30 × 2.26 × 10⁶ = 6.78 × 10⁵ J. The temperature stays at 100°C, so mcθ is not used.
Question 5
A gas at 150 kPa has a volume of 300 cm³. It expands at constant temperature until the pressure is 100 kPa. Find the new volume.
Answer
Boyle’s law: V₂ = P₁V₁ ÷ P₂ = 150 × 300 ÷ 100 = 450 cm³.
Question 6
A car tyre has a pressure of 220 kPa at 27°C. After a drive its temperature is 57°C. Assume the volume is constant. Find the pressure.
Answer
T₁ = 300 K and T₂ = 330 K. P₂ = 220 × 330 ÷ 300 = 242 kPa.
Question 7
A solid melts at 80°C. On a heating graph, a flat line at 80°C lasts from 4 to 10 minutes. Explain why the temperature does not rise and what the heat is doing.
Answer
The solid is melting. The energy supplied is used to overcome the forces between particles, increasing their potential energy. The kinetic energy stays constant, so the temperature stays at 80°C.
Question 8
250 cm³ of gas at 27°C is heated to 177°C at constant pressure. Find the new volume.
Answer
T₁ = 300 K and T₂ = 450 K. V₂ = 250 × 450 ÷ 300 = 375 cm³.
If you got these wrong
- Question 1: revise explaining thermal equilibrium.
- Questions 2 and 3: revise using specific heat capacity.
- Questions 4 and 7: revise using specific latent heat and interpreting temperature-time graphs.
- Questions 5, 6 and 8: revise applying gas laws under stated conditions.
Log the failing step in the mistake log and paper error review, or set a timed round with the timed original practice session builder. To have a teacher go through your answers, see online one-to-one Physics tuition.