Section 8 - Thermal physics Flashcards
What is the internal energy?
In a body it is the sum of all the randomly distributed kinetic and potential energies of all the particles in a body
How can you change the internal energy?
Increased - by heating it or doing work to transfer energy to the system
Decreased - by cooling it or by doing work to remove energy from the system
What is specific heat capacity?
The amount of energy required to raise the temperature of 1kg of a substance by 1K without a change in state
What is specific latent heat?
The quantity of thermal energy required to change the state of 1kg of a substance, without changing the temperature
What is Boyle’s law?
pV=constant
* At a constant temperature and with a fixed mass, the pressure and volume of a gas are inversely proportional
* A (theoretical) gas that obeys Boyle’s law at all temperatures is called an ideal gas
What is Charles’s Law?
v/t = constant
* At a constant pressure and with a fixed mass, the volume of a gas is directly proportional to its absolute temperature
What is The Pressure Law?
p/T = constant
* At a constant volume and with a fixed mass, the pressure of a gas is directly proportional to its absolute temperature
What is the experiment to investigate Boyle’s Law?
- The oil traps a pocket of air in a sealed tube with fixed dimensions
- A tire pump is used to increase the pressure on the oil and the Bourdon gauge records the pressure
- As the pressure increases, the oil level will rise, and the air will compress - the volume occupied by air in the tube will reduce
- Measure the volume of air when the system is at atmospheric pressure, then gradually increase the pressure, keeping the temperature constant
- Write down the pressure and the volume of air as it changes
- Multiplyng these together should result in the same value
- If you plot a graph of p against 1/V you should get a straight line
What is the experiment to investigate Charles’s Law?
- A capillary tube is sealed at the bottom and contains a drop of conc H2SO4 acid halfway up the tube which traps a column of air between the bottom of the tube and the acid drop
- The beaker is filled with near-boiling water, and the length of the trapped column of air increases - as the water cools, the length of the air column decreases
- Regularly record the temperature of the water and the air column length as the water cools
- Repeat with fresh near-boiling water teice more,m letting the tube adjust to the new temperature between each repeat - recorf the length at the same temperatures each time and take an average
- If you plot your average results on a graph of length against temperature and draw a line of best fit, you will get a straight line - shows the length of the air column is proportional to temperature
- The colume of the column of air is equal to the volume of a cylinder, whch is proportional to its length
What is molecular mass?
Sum of all the masses of all the atoms that make up a molecule
How do you calculate the number of molecules?
N=nNA
* Where n is the number of moles, N the number of molecules and NA avogadro’s constant
When does the ideal gas equation work well?
For gases at low pressures and fairly high temperatures
What must be done for a gas to expand or contract at constant pressure?
Work must be done
* This usually involves the transfer of heat energy
* The work energy in changing the volume of a gas at constant pressure is given by work done = pΔV
Draw the derivation for the pressure on one wall of a box in the x direction
Draw the derivation for the general equation of the pressure of a gas
What is the root mean square speed or Crms
- c̄² is the means square speed which is the average of the square speeds of all the particles with units m²s-²
- so the sqare root of it gives you the average speed with units ms^-1
What are the assumptions for the kinetic theory?
R - Random motion of particles
A - Attraction between particles = none
V - Volume of the molecules is negligible
E - Elastic collisions
D - Duration of collisions is negligible
What is an ideal gas?
Obeys Boyle’s law with perfect precision
What is a perfect gas?
A real gas under conditions that Boyle’s law is a valid enough description of its behaviour
What are empirical laws?
Based on observations and evidence which means they can predict what will happen but they don’t explain why - gas laws and ideal gas equation
What are theories?
A theory is based on assumptions and derivations from knowledge and theories we already had - kinetic theory
How has our understanding of gases developed?
- Democritus had ideas 2000 years ago
- Robert Boyle discovered the relationship between pressure and volume at a constant temperature
- Jacques Charles discovered that the volume of a gas is proportional to temperature at a constant pressure
- The pressure law was discovered by Guillaume Amontons who noticed that at a constant volume, temperature is proportional to pressure, it was then re-discovered by Joseph Louis Gay-Lussac
- Daniel Bernoulli explained Boyle’s law by assuming that gases were made up of tiny particels - the beginning of kinetic theory
- Robert Brown discovered Brrownian motion which helped support kinetic theory
What is Brownian motion?
- Botanist Robert Brown noticed that pollen grains in water moved with a zigzag random motion
- This supports the kinetic particle theory as it states that the random motion is a result of collisions with fast, randomly-moving particles in the fluid
When you need to find the temperature of a gas and you’re given the Crms, which set of equations do you use?
Kinetic theory equations
