Thermal Physics

Question 1

Define internal energy (U).

Answer 1

The sum of the randomly distributed kinetic and potential energies of atoms or molecules in a substance.

U= Ek + Ep

Question 2

Define specific heat capacity and give its units.

Answer 2

The thermal energy required to raise the temperature of a 1kg body of mass by 1K.

J kg-1 K-1

Question 3

Define latent heat of fusion.

Answer 3

Energy required to change the unit mass of a substance from a solid to liquid while at a constant temperature.

Question 4

Define latent heat of vaporisation.

Answer 4

Energy required to change the unit mass of a substance from a liquid to gas while at constant temperature.

Question 5

What is absolute zero and why can’t there be a temperature lower than it?

Answer 5

0K- theoretical lowest possible temperature.

A body would have minimum internal energy (No Ek as molecules stop moving, only Ep)

Question 6

What is thermal equilibrium?

Answer 6

• No net transfer of thermal energy between objects involved.
• Same temperature.

Question 7

Celsius to Kelvin?

Answer 7

C° + 273 = K

Question 8

Why must thermometers be the same temperature as the object being measured?

Answer 8

• No net transfer of thermal energy must take place.

- Accurate reading of temp.

Question 9

Why is the difference between C and K omitted when discussing high temperature of objects (eg stars)?

Answer 9

The difference between C and K is negligible at high temperatures.

Question 10

Why does a thermometer placed in a hot cup of water not give the initial reading of the temperature of the water?

Answer 10

• Net transfer of thermal energy from the water to the thermometer.
• Reduces the temperature of the water.
• When there is no net transfer of thermal energy, the water and thermometer will be the same temperature (lower than initial).

Question 11

What is Brownian motion?

Answer 11

• Gas molecules travel in random directions at random velocities.
• Transfer of momentum between particles so collisions are elastic.
• Can be observed by smoke particles under a microscope.

Question 12

Describe the arrangement, forces and motion of solid atoms.

Answer 12

• Fixed, regular, 3D
• Strong electrostatic forces
• Vibrate around equilibrium position

Question 13

Describe the arrangement, forces and motion of liquid atoms.

Answer 13

• Random arrangement
• Weaker attraction between particles
• Flow between each other in random direction

Question 14

Describe the arrangement, forces and motion of gas atoms.

Answer 14

• Random arrangement
• Almost no forces of attraction
• Move at high speeds in random directions

Question 15

Why is ice less dense than water?

Answer 15

• When water freezes, strong intermolecular forces hold water molecules in hexagonal ring structures.
• Spaced further apart so fewer molecules in space -> less dense

Question 16

Explain why the volume of water increases as temperature increases.

Answer 16

• As temp increases, particles gain more kinetic energy so move at higher speeds.
• Higher speeds = more collisions which cause the liquid to expand.

Question 17

Why is the thermodynamic scale of temperature used?

Answer 17

It doesn’t rely on the properties of a substance.

Question 18

Define heat.

Answer 18

Thermal energy transferred from a body of high temp to low temp.

Question 19

Why do all molecules possess Ek and Ep?

Answer 19

Ek - random movement.

Ep- chemical bonds between molecules and bonds within nuclei.

Question 20

Why does internal energy change as temperature changes?

Answer 20

As temp increases, molecules gain more Ek -> higher internal energy

Question 21

Why does internal energy change during phase changes?

Answer 21

• Ek stays the same as temperature stays the same.

- Ep increases as the mean separation of particles increase -> increase in internal energy

Question 22

Why does temperature stay constant during phase changes?

Answer 22

Thermal energy is converted into potential energy.

Potential energy increases as mean separation of particles increase.

Question 23

Explain why 1kg of water has more internal energy than 1kg of ice.

Answer 23

Same Ek because same temperature.

Different Ep. Ep is higher in the water due to larger mean separation of molecules so higher internal energy.

Question 24

Define specific latent heat.

Answer 24

Energy required to change the state per unit mass of a substance while at a constant temperature.

Question 25

Equation including energy, mass and specific latent heat.

Answer 25

Q = m L