Internal energy and absolute zeroEdexcel International A Level Physics: Subtopic test

1. 1
   A 0.20 kg block of ice at 0 °C is placed in a warm room. It absorbs energy from the air, melts completely and forms water that is still at 0 °C. A physics class is asked to describe what has changed for the molecules in the sample.

   (a)

   Which statement defines the internal energy of a substance?

   [1 mark]

   • AThe kinetic energy of the substance moving as a whole
   • BThe sum of the random kinetic and potential energies of its molecules
   • CThe kinetic energy of its molecules only
   • DThe energy needed to raise its temperature by 1 K

   (b)

   Which row describes what happens to the molecules as the ice melts at 0 °C?

   [1 mark]

   • AMean kinetic energy rises; potential energy is unchanged
   • BMean kinetic energy falls; potential energy rises
   • CBoth mean kinetic energy and potential energy rise
   • DPotential energy rises; mean kinetic energy is unchanged

   (c)

   Explain why the temperature of the sample does not change while the ice is melting, although it is absorbing energy.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A cryogenics laboratory stores liquid nitrogen, which boils at 77 K, and uses it to cool samples. Technicians compare the behaviour of nitrogen molecules at the temperature of the cold store with their behaviour at room temperature, 300 K.

   (a)

   What is the boiling point of liquid nitrogen, 77 K, in degrees Celsius?

   [1 mark]

   • A+350 °C
   • B−77 °C
   • C−196 °C
   • D−350 °C

   (b)

   How does the mean kinetic energy of nitrogen molecules at 300 K compare with that at 77 K?

   [1 mark]

   • A3.9 times larger
   • B0.26 times as large
   • C2.0 times larger
   • DUnchanged

   (c)

   A technician says that absolute zero is the temperature at which the molecules have no energy at all. Explain what is meant by absolute zero in terms of the molecules, correcting the statement if necessary.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A sealed, rigid flask contains a fixed mass of gas that behaves as an ideal gas. At 27 °C the internal energy of the gas is 1.5 kJ. The flask is then heated until the gas is at 327 °C.

   (a)

   Calculate the ratio of the mean kinetic energy of the molecules at 327 °C to that at 27 °C.

   [3 marks]

   (b)

   Calculate the internal energy of the gas at 327 °C and explain why the internal energy changes by this factor.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A 0.050 kg sample of ice at −20 °C is heated steadily until it has become steam above 100 °C. A thermometer shows that the temperature rises steadily, except for two flat regions, one at 0 °C and one at 100 °C. Two students then make claims. Student A says: 'Raising the temperature of a gas from 10 °C to 20 °C doubles the average kinetic energy of its molecules.' Student B says: 'At absolute zero the molecules have zero kinetic energy, so a substance has no internal energy at all.'

   (a)

   Explain, in terms of the kinetic and potential energy of the molecules, what happens to the internal energy of the sample during the whole heating process, including the two flat regions.

   [6 marks]

   (b)

   Evaluate the claims made by Student A and Student B.

   [6 marks]

   Total for question 4: 12 marks

End of questions