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Physics A Level > 3.6.2 Thermal Physics > Flashcards

3.6.2 Thermal Physics Flashcards

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1
Q

Define internal energy

A

Sum of the randomly distributed KE and PE of the particles in a body.

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2
Q

What is absolute zero?

A

Lowest possible temperate, no kinetic energy. 0K.

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3
Q

What is the triple point of water?

A

273K, the temperature at which pure water exists in thermal equilibrium with ice and water vapour.

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4
Q

Define latent heat.

A

Energy required to change the state of a substance eg. melting or boiling.

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5
Q

Define specific latent heat.

A

Energy required to change the state of a substance without change of temperature.

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6
Q

What is the equation for specific latent heat?

A

Q = mL

Q = energy, J
L = specific latent heat, Jkg-1
m = mass, kg

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7
Q

What is the latent heat of fusion?

A

Melting or freezing.

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8
Q

What is the latent heat of vapourisation?

A

Boiling or condensing.

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9
Q

Define pressure.

A

The force per unit area that it exerts normally (at 90°) to the surface. Pa or Nm-2.

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10
Q

What is pressure affected by?

A

Temperature, volume of the gas particles, mass of the gas particles.

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11
Q

What is Boyle’s law?

A

Pressure is inversely proportional to volume.

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12
Q

What is the equation for Boyle’s law?

A

P1 x V1 = P2 x V2

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13
Q

What is Charles’s law?

A

For a fixed mass of gas at constant pressure, volume is proportional to temperature.

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14
Q

What is Gay-Lussac’s law?

A

For a fixed mass of a gas at constant volume, P/T = constant.

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15
Q

How do we increase the internal energy of a gas?

A

Increase the KE (increase temp)
Increase the PE (increase work done).

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16
Q

What is the equation for work done?

A

W = P∆V

W = work done, J
P = pressure, Pa
V = volume, m3

17
Q

Which ideal gas equation do we use for moles?

A

PV = nRT

18
Q

Which ideal gas equation do we use for molecules?

A

PV = NkT

19
Q

What are the general steps to derive the kinetic theory equation?

A

1) calculate force exerted by one particle
2) many particles
3) convert to pressure
4) 3 dimensions
5) RMS

20
Q

What is the actual derivation for the kinetic theory formula?

A

ρ1 = mu
ρ2 = -mu (after elastic collision)
∆ρ = -mu - mu = -2mu

Distance between collisions = 2L
Number of collisions per second = u/2L
t = s/u = 2L/u
∆ρ/∆t = -2mu^2/2L
= -mu^2/L

(N’s 2nd L) F = ∆ρ/∆t
Force on particle = -mu^2/L
(N’s 3rd L) Force on wall = mu^2/L

Lots of particles = N
Particles don’t all move at same speeds so use mean squared speed (bar)u^2
F = Nm(bar)u^2/L

P = F/A = Nm(bar)u^2/AL
= Nm(bar)u^2/V

Overall mean squared speed= (bar)L^2 = (bar)u^2 + (bar)w^2 + (bar)v^2
Because on average they will be at the same speed
(bar)u^2 = (bar)v^2 = (bar)w^2
(bar)c^2 = 3(bar)u^2
(bar)u^2 = (bar)c^2/3

(bar)c^2 = root mean speed
P = 1/3 Nm(bar)c^2/V
P = 1/3 Nm(crms)^2/V

21
Q

What is step one for the derivation of the kinetic theory formula? (Calculate force exerted by one particle.)

A

ρ1 = mu
ρ2 = -mu (after elastic collision)
∆ρ = -mu - mu = -2mu

Distance between collisions = 2L
Number of collisions per second = u/2L
t = s/u = 2L/u
∆ρ/∆t = -2mu^2/2L
= -mu^2/L

(N’s 2nd L) F = ∆ρ/∆t
Force on particle = -mu^2/L
(N’s 3rd L) Force on wall = mu^2/L

22
Q

What is step two of the kinetic theory equation? (Many particles)

A

Lots of particles = N
Particles don’t all move at same speeds so use mean squared speed (bar)u^2
F = Nm(bar)u^2/L

23
Q

What is step 3 of the kinetic theory derivation? (Convert to pressure)

A

P = F/A = Nm(bar)u^2/AL
= Nm(bar)u^2/V

24
Q

What is step 4 of the kinetic theory equation? (3 dimensions)

A

Overall mean squared speed= (bar)L^2 = (bar)u^2 + (bar)w^2 + (bar)v^2
Because on average they will be at the same speed
(bar)u^2 = (bar)v^2 = (bar)w^2
(bar)c^2 = 3(bar)u^2
(bar)u^2 = (bar)c^2/3

25
Q

What is step 5 of the kinetic theory equation? (RMS)

A

(bar)c^2 = root mean speed
P = 1/3 Nm(bar)c^2/V
P = 1/3 Nm(crms)^2/V

26
Q

What assumptions do we make in the kinetic theory model?

A
  • All particles are identical
  • Gas has a large number of particles
  • Particles have negligible volume compared with volume of the container
  • Particles move at random
  • Particles obey Newtonian mechanics
  • All collisions are perfectly elastic
  • Particles move in straight lines between collisions
  • Forces that act during collisions last much sorter times than the times between collisions
27
Q

What is Brownian motion?

A

Pollen grains on water move randomly, resulting from collisions with the fast, randomly moving particles in the water.

28
Q

What is Brownian motion evidence for?

A

Existence of atoms.

Physics A Level (15 decks)

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