G484 - Thermal Physics Flashcards
One Mole
If you have one mole of any object you have
6.002 x 10^23 of them
Avogadro Constant
6.02 x 10^23
What is the mass of one mole of a substance?
The mass of one mole of a substance is equal to its total atomic mass
Molar Mass
Definition
The mass of one mole of a substance
Molar Mass
Equation
Avogadro Constant x mass of one molecule
Brownian Motion
All gas / liquid molecules are in rapid random motion
Experiment to show Brownian Motion
Pour smoke into a cell and trap with a coverslip
Shine a light through the smoke cell
Observe under a microscope
The smoke molecules reflect the light
The movement of the smoke molecules can be observed
The smoke molecules move the way they do because of collisions with air particles which are undergoing Brownian Motion
What are the five assumptions of kinetic gas theory?
- All collisions are completely elastic
- Gravitational force in molecules is negligible
- No intermolecular forces exist except for when a collision is sustained
- A gas consists of a large number of molecules rapid, random motion
- The total volume of gas molecules is negligible when compared with the volume of the container
What is the kinetic model of gas?
A model which describes the motion of gas molecules
Momentum
Formula
Momentum (kg m/s) = Mass (kg) x Velocity (m/s)
Force Formula (momentum)
Force (N) = ChangeInMomentum (kgm/s) / Time (s)
Why does a gas exert pressure?
Gas molecules are in rapid random motion
A gas molecule collides with the wall of the container
There is a change in momentum meaning that the wall has applied a force on the molecule
As F = (mv-mu) / t
Newton’s third law, every force has an equal and opposite reaction, i.e. the molecule has applied a force to the wall
Sum of collisions over an area = pressure
Therefore P = F/A
More collisions per unit time equals greater pressure
Internal Energy
Definition
The internal energy of a body is the total sum of random distributions of kinetic and potential energies of all molecules in the body
Describe the internal energy of an ideal gas
An ideal gas only has internal energy in the form of random kinetic energy
What is the relationship between temperature and average kinetic energy?
Temperature is proportional to average kinetic energy of a single molecule
How does internal energy vary as water is heated at a constant rate?
Internal energy is always increasing
How does potential energy vary as water is heated at a constant rate?
Potential energy change is greatest when the water is changing state
Potential energy increases slightly as temperature increases
How does kinetic energy vary as water is heated at a constant rate?
Kinetic energy increases when temperature increases
How does temperature vary as water is heated at a constant rate?
While the substance is a solid and heat is applied, the kinetic energy of molecules increases so temperature increases
During a state change the applied heat energy is used to break intermolecular bonds so kinetic energy does not increase so temperature doesn’t either
When the substance is a liquid molecules gain kinetic energy so temperature increases
What is temperature?
The average kinetic energy of a single molecule in a body
Gas Pressure
Equation
1/3 x Nm/V x c²
N = total no. of molecules m = mass of one molecule V = volume of container c = mean speed of a molecule
Or
1/3 x p x c² = 1/3xDensityxMeanSpeedSquared
r.m.s. speed
Root mean squared speed
Square root of the mean of the squares
What is absolute temperature measured in?
Kelvin
The SI unit for temperature
What is 0K in degrees Celsius?
-273.15
Zeroth Law of Thermodynamics
If object A is in thermal equilibrium with object B, and object B is in thermal equilibrium with object C, then object C is also in thermal equilibrium with object A
A hot thing will pass its heat to a cod thing until they are both at the same temperature
Specific Heat Capacity
Definition
The amount of energy required to raise the temperature of 1kg of a material by 1 kelvin
Density
Formula
Density = Mass of One Molecule x Number of Molecules in 1m³
Measured in kg/m³
Internal Energy Formula (SHC)
E = mc∆θ
E = internal energy (J) m = mass (kg) c = specific heat capacity (J/kg/K) θ = change in temperature (K)
Internal Energy Formula (heat capacity)
E = C∆θ
E = internal energy (J) C = heat capacity θ = change in temperature
Experiment to find the SHC of water
Fill a beaker with water
Measure the mass of the water
Wrap in insulation
Put a thermometer and heater into the water
Come t the heater to a power supply with an ammeter and a voltmeter
At 30 second intervals record the voltage, current, and change in temperature from the first reading
Plot time on the x axis and change in temperature in the y axis
Rearrange the internal energy equation into the form y=mx + c
E = Pt
θ = P/mc t
Gradient = P/mc
Calculate gradient from the graph
Calculate an average voltage and current to work out the average power
Substitute gradient, mass and power into
c = P/(gradient x mass) to find the specific heat capacity
Specific Latent Heat of Fusion
Definition
Energy required to change 1kg of a substance from a solid state at its melting point to a liquid state at the same temperature
Specific Latent Heat of Vaporisation
Definition
Energy required to change 1kg of a substance from a liquid state at its boiling point to a gas state at the same temperature
Latent Heat Formula
E = ml
E = energy required/supplied (J) m = mass of substance (kg) l = specific latent heat (J/kg)
Boyle’s Law
Pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature
T ∝ 1/V
P1V1 = P2V2
Charle’s Law
Temperature is proportional to volume when pressure is constant
T ∝ V
V1/T1 = V2/T2
Pressure Law
Pressure is proportional to temperature when volume is constant
P ∝ T
P1/T1 = P2/T2
Charle’s Law Experiment
Attach a thermometer and a tube containing mercury to a ruler
Measure the distance from the bottom of the tube to the first bubble of mercury
Put the ruler into a beaker of water
Add ice to reduce the temperature and take readings of volume at different temperatures
Heat on a bunsen burner to take readings at higher temperatures
Plot the results on a graph
The line of best fit should be straight and intersect the y axis at 0 kelvin
Boyle’s Law Experiment
Vary the amount of liquid in a sealed glass tube to vary the pressure of the system
Record the pressure of the gas and the volume of the gas in the tube
For volume measure the length of the tube containing gas as cross sectional area is constant
Plot on a graph
The line of best fit should be a curve
What is absolute 0?
The temperature at which a substance has minimal internal energy
What is thermal equilibrium?
Areas that are the same temperature are in thermal equilibrium
Ideal Gas Equations
PV = nRT
Or
PV = NkT
P = pressure V = volume n = number of moles N = number of molecules R = ideal gas constant k = Boltzmann constant
What is the relationship between average kinetic energy per molecule and absolute temperature?
Average kinetic energy per molecule is proportional to absolute temperature
E = 3/2 kT
k = Boltzmann constant E = average kinetic energy per molecule T = absolute temperature
Ideal Gas Constant
R = 8.31 (J/mol/K)
Boltzmann Constant
k = 1.38 x 10^-23 (J/K)
Observations and Conclusions from the Smoke Cell Experiment
- movement of smoke particles caused by randomly moving air molecules
- smoke particles are continuously moving because the air particles are continuously moving
- smoke particles are visible but air molecules are not so the air particles must be much smaller
- small movements of smoke particles due to large numbers of air molecules hitting from all sides
Kilowatt Hour
The energy used by a 1kW device in 1 hour
Melting
A solid becomes a liquid at a fixed temperature
Potential energy increases
Average kinetic energy does not increase
Boiling
A liquid becomes a gas at a fixed temperature
Potential energy increases
Average kinetic energy does not increase
Evaporation
Liquid turns to a gas below the boiling point
Temperature is proportional to average kinetic energy per molecule
Some molecules have higher KE
High enough for them to leave the liquid
Solid
Molecules held in a fixed pattern
Vibrate about fixed positions
Liquid
Molecules closely, randomly packed together
Free to move
Gas
Molecules widely separated
Move rapidly
What happens to the motion of molecules in a solid after a small increase in temperature?
Greater amplitude of vibration
Greater frequency of vibration
