Thermal Physics

Question 1

What is the internal energy of a body?

Answer 1

The sum of the randomly distributed kinetic and potential energies of the particles in the body.

Question 2

What is the 1st Law of Thermodynamics?

Answer 2

The increase in internal energy of a system is equal to the heat added to the system minus the work done by the system:

ΔU = ΔQ - ΔW

Question 3

Decribe the changes to the internal energy of a substance during a change of state.

Answer 3

Its internal energy changes but its kinteic energy (and therefore temp.) stays the same, because only the potential energies of the particles change.

Question 4

How can you use the continuous flow method to calulate specific heat capacity?

Answer 4

c = t(I₁V₁ - I₂V₂)/ΔT(m₁ - m₂)

Question 5

What is the specific latent heat of fusion or vaporisation?

What is the specific heat capacity of a substance?

Answer 5

• The thermal energy needed to change the state of 1 kg of a substance.
• The energy needed to raise the temperature of 1 kg of a substance by 1 K.

Question 6

State and explain Boyle’s Law.

Answer 6

p ∝ 1/V

for a given no. of particles at a fixed temp.

increased volume = greater distance between particles & walls = fewer collisions btwn. particles and walls = smaller avg. force per second = lower pressure

Question 7

State and explain Charles’ Law.

Answer 7

V ∝ T

for a given no. of particles at a constant pressure

increased temp. = increased avg. KE = increase speed = more frequent collisions & higher mtm. change per collision = greater force per area = greater pressure therefore volume must increase to keep pressure constant

Question 8

State and explain the pressure law.

Answer 8

p ∝ T

for a given no. of particles at a fixed volume

increased T = increased avg. KE = increased speed = more frequent collisions and higher mtm. change per collision = greater pressure

Question 9

What is absolute zero?

Answer 9

The temperature at which all particle motion ceases and the pressure of a gas drops to zero.

Question 10

What is the work done by an expanding gas?

Answer 10

ΔW = pΔV

Question 11

How does Brownian motion provide evidence for the existence of atoms?

Answer 11

• Random movement of particles suspended in fluid.
• Random motion is the result of collisions with fast, randomly-moving particles which make up the fluid.

Question 12

What are the simplifying assumptions needed for the kinetic theory model?

Answer 12

• Particles move randomly and continually
• The motion of the particles follows Newton’s Laws
• Collisions between particles or particles and the walls are perfectly elastic.
• Except suring collisions, particles always move in straight lines.
• Any forces which act during the collisions last for much less time than the time between collisions.

Question 13

Derive the kinetic theory model equation for an ideal gas.

Answer 13

Cube with side length L, containing N particles of mass m each.

1. A particle moving with velocity c₁ collides normally & elastically with one of the sides. Δmtm. = 2mc₁
2. time btwn. collisions: Δt = 2L/c₁
3. avg. force on one wall, F = mc₁²/L
4. avg. pressure exerted by one particle, p = mc₁²/L³ = mc₁²/V
5. if all N particles travelled parallel to x-axis, total pressure on wall would be: m/L³ * (c₁² + c₂² + c₃² + …)
6. from 3D Pythagoras, c² = c_x² + c_y² + c_z² and becuase the motion is random, c_x² = c_y² = c_z², so c_x² = c²/3
7. Therefore, true pressure parallel to the x axis is: 1/3 * m/V * (c₁² + c₂² + c₃² + …)
8. Substituting in N*c_rms² = c₁² + c₂² + c₃² + … gives: pV = 1/3 * N * c_rms² * m