Thermal Physics

 

1    a    Difference: Evaporation occurs at all temperatures, whereas boiling of pure water occurs
at a fixed temperature (100 °C at a pressure of 100 kPa).                                                     [1]

      Similarity: Both processes involve molecules escaping from the water surface.                    [1]

b    i     The molecules of water move faster.                                                                            [1]

ii    Internal energy = kinetic energy + potential energy

      The kinetic energy of the molecules increases because they move faster.                        [1]

      The potential energy does not change much because the separation between the
molecules remains the same.                                                                                        [1]

      Hence, the internal energy increases due to increase in the kinetic energy of the
molecules.                                                                                                                  [1]

c		Kinetic energy of the particles	Potential energy of the particles	Internal energy
	An aluminium block increasing its temperature from room temperature to 300 °C.	+	0	+
	Water boiling at 100 °C and changing into steam at 100 °C.	0	+	+
	Water at 0 °C changing into ice at –15 °C.	–	–	–

      One mark for each correct row.                                                                                         [3]

2    a    The temperature of the metal increases at a steady rate, therefore it must be heated at a
steady rate.                                                                                                                       [1]

b    E = mcΔθ                                                                                                                         [1]

      E = 800 × 10^–3 × 600 × (30 – 20)                                                                                       [1]

      E = 4.8 × 10³ J                                                                                                                  [1]

c    From the graph it takes 90 s for the temperature to change from 20 °C to 30 °C.                   [1]

      power =                                                                                                             [1]

      power = 53.3 W » 53 W                                                                                                    [1]

3    E = (VI)t                                                                                                                                [1]

E = 9.5 × 5.2 × (5.0 × 60) = 1.48 × 10⁴ J                                                                                  [1]

E = mcΔθ                                                                                                                               [1]

c =  =                                                                                                 [1]

c » 2.0 × 10³ J kg^–1 K^–1                                                                                                           [1]