Thermodynamics

Question 1

Internal energy

Answer 1

• Internal energy = potential energy of interatomic bonds + kinetic energy of all bonds
• Temperature = average kinetic energy of the particles
• change in potential energy doesn’t increase temperature, but does lead to changes in state

Question 2

Finding SHC

Answer 2

• Energy = IV*change in time = mc+change in temperature
• use an electric heater and a stop watch to find energy, then use mass and change in temperature to get c (SHC)
• energy could be absorbed by the heater itself, or lost to surroundings if not insulated well enough
• Thermometer readings may not be 100% accurate

Question 3

Ideal gas

Answer 3

• rules for ideal gas: Motion of particles is random, all collisions are entirely elastic, Intermolecular forces are negligible, volume of particles is negligible compared to total gas volume, collisions take very small amounts of time
• internal energy of an ideal gas = random kinetic energy of molecules, no potential energy

Question 4

Changes of state

Answer 4

• plasma is made if ions, so it can be manipulated by magnetic fields
• when ice melts all the energy it absorbs goes into overcoming bonds, not kinetic energy so temperature doesn’t increase even though thermal energy is absorbed
• Latent heat of fusion = energy required to go from solid to liquid
• latent heat of vaporisation = energy required to go from liquid to gas
• energy change = latent heat*change in mass

Question 5

Absolute zero

Answer 5

• 0k = lowest theoretical temperature
• at close to 0k, objects can have superconductivity (no electrical resistance)
• lowest naturally ocurring temperature is about 1k, in the Boomerang Nebula (we have achieved lower)
• absolute zero means no energy at all, however quantum mechanics says there is a minimum amount of energy everything has

Question 6

Pressure

Answer 6

• pressure in fluid = densitygchange in height
• total pressure = atmospheric pressure + pgh
• atmospheric pressure = 1.01*10^5 Pa

Question 7

Gas laws

Answer 7

• Boyle’s law = pressure is directly proportional to volume (at a constant temperature)
• P1V1 = P2V2
• if volume is constant, pressure is directly proportional to temperature
• at absolute zero, gas exerts no pressure as there is no random movement so no force is exerted on the container walls

Question 8

Ideal gas equation

Answer 8

• PV = nRT, n = number of moles, R = 8.31
• PV = NkT, N = number of molecules, k= 1.38*10^-23
• R = 6.022*10^23 * k
• (P1V1)/T1 = (P2V2)/T2

Question 9

Kinetic Model evidence

Answer 9

• Brownian motion = random movements of particles in all direction, observed with small seeds in water
• Brownian Motion is continuous so collisions are elastic
• gasses are easily compressible so particles have negligible volume compared to gas volume

Question 10

Formula for pressure, volume and speed

Answer 10

• PV = 1/3 Nm*
• side length of box = d, mass of particle = m, speed of particle = c
• equal chance of moving in all three directions, so c = x + y + z, x= c/3
• momentum in x-direction = mx, momentum after colliding with wall = -mx, change in momentum = 2mx
• particle travels full side length and back before next collision so distance 2d at speed x, in time 2d/x
• force = change in momentum/time, = 2mx*x/2d = mx^2/d
• pressure on wall exerted by one particle = (mx^2/d)/d^2 = mx^2/d^3 = mx^2/volume of box
• total pressure of all particles = (m/v)(mean square speed of all particles number of particles) = (m/v)(N)
• P=Nm/V
• as x = c/3, =/3, so PV = 1/3Nm

Question 11

Combining the pressure and ideal gas formula

Answer 11

• PV = 1/3Nm = NkT
• 1/3m = kT, 1/2m = 3/2kT
• 1/2m = average kinetic energy of particles
• as k is a constant, this tells us average kinetic energy of particles is proportional to Temperature