Thermal Physics — Solutions

Full worked solutions, with method and final answer for every part.

Question 1

(a)Q = mcΔT, with ΔT = 100 − 20 = 80°C.

Q = 0.50 × 4200 × 80 = 168 000 J

Question 2

(a)Q = mL.

Q = 0.20 × 334 000 = 66 800 J

(b)The energy supplied during melting goes into breaking the intermolecular bonds that hold the solid structure together (increasing potential energy), rather than increasing the kinetic energy of the particles, so the temperature stays constant.

Energy breaks bonds between particles rather than raising their kinetic energy/temperature

Question 3

(a)At constant temperature, P1V1 = P2V2.

P2 = (100 × 0.006) ÷ 0.004 = 150 kPa

(b)Boyle's Law.

Boyle's Law

Question 4

(a)Compressing the gas reduces the volume, so the same number of particles collide with the container walls more frequently in a given time. More collisions per second means a greater average force on the walls, so pressure increases.

Smaller volume → more frequent particle collisions with the walls → greater pressure

Question 5

(a)Q = mcΔT.

2 × 900 × 50 = 90 000 J

Question 6

(a)Power = energy ÷ time.

45 000 ÷ 60 = 750 W

Question 7

(a)Boyle's Law: P1V1 = P2V2.

P2 = (200×3×10⁻³) ÷ (2×10⁻³) = 300 kPa

Question 8

(a)Using Q = mcΔT with m = 0.3 kg, c = 4200, ΔT = 20: 0.3 × 4200 × 20.

= 25 200 J, which matches the given value, confirming consistency

Question 9

(a)Raising the temperature increases the average kinetic energy of the gas particles, so they move faster. Faster particles collide with the container walls more frequently and with greater force per collision, increasing the pressure.

Higher temperature → faster particles → more frequent, harder collisions with the walls → higher pressure

Question 10

(a)As steam condenses on the skin, it releases its latent heat of vaporisation (a large amount of extra energy) in addition to then cooling down like water would, delivering significantly more total energy to the skin than boiling water at the same temperature.

Steam releases additional latent heat as it condenses, transferring more total energy than water at the same temperature

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