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

1. 1
   A fixed mass of an ideal gas is sealed in a rigid container and heated from 27 °C to 327 °C. Take 0 °C as 273 K.

   (a)

   By what factor does the mean kinetic energy of the gas molecules increase?

   [1 mark]

   • A12
   • B2.0
   • C4.0
   • D1.0

   (b)

   Which statement describes the internal energy of the ideal gas?

   [1 mark]

   • AThe sum of the random kinetic energies of its molecules, because there are no intermolecular forces
   • BThe sum of the random kinetic and potential energies of its molecules, with the potential energy rising as it is heated
   • CThe kinetic energy of the container and the gas moving as a whole
   • DThe kinetic energy of a single molecule moving at the mean speed

   (c)

   The gas is cooled from 27 °C until the mean kinetic energy of its molecules is half its original value. Calculate the new temperature in °C.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A 0.20 kg block of ice at −20 °C is supplied with energy at a steady rate until it has all melted and the water has then warmed to 20 °C. Take 0 °C as 273 K.

   (a)

   Which statement about the water molecules in the ice at −20 °C is correct?

   [1 mark]

   • AAll the molecules have exactly the same kinetic energy
   • BThey have no kinetic energy because the ice is a solid
   • CThey have no potential energy because they are in fixed positions
   • DTheir kinetic energies differ from molecule to molecule, with a mean value that depends on the temperature

   (b)

   What is the ratio of the mean kinetic energy of the molecules in the ice at −20 °C to that at 0 °C?

   [1 mark]

   • A0.00
   • B1.00
   • C0.93
   • D1.08

   (c)

   Explain why the temperature of the ice stays at 0 °C while it is melting, even though energy is still being supplied.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a physics demonstration, a sample of an ideal gas is cooled towards absolute zero. Take 0 °C as 273 K.

   (a)

   Explain what is meant by absolute zero, and why there cannot be a temperature below 0 K.

   [3 marks]

   (b)

   At 300 K the mean kinetic energy of the molecules is 6.2 × 10⁻²¹ J. Calculate the mean kinetic energy at 75 K, and the temperature at which the mean kinetic energy is 1.0 × 10⁻²¹ J.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A thermal physics class studies a 0.50 kg block of metal that is heated at a steady rate from 300 K until it has completely melted at its melting point of 1000 K. The class then compares the block with a large swimming pool of water.

   (a)

   Explain, in terms of the molecules, how the internal energy of the block changes as it is heated from 300 K to 600 K, and as it then melts at constant temperature.

   [6 marks]

   (b)

   A student claims that a cup of tea (0.30 kg of water at 80 °C) has more internal energy than a swimming pool (5.0 × 10⁴ kg of water at 25 °C) because the tea is hotter. Evaluate this claim.

   [6 marks]

   Total for question 4: 12 marks

End of questions