Thermal Physics and Gases Flashcards
When is energy transferred between two objects?
When one exerts a force on the other and makes it move so has done work on the other object. One object is hotter than the other so energy transfer by heating takes place by conduction, convection or radiation.
Why do brake pads heat up when applying brakes to a moving vehicle?
The work done by the frictional force between the brake pads and the wheel heats the brake pads, which gain energy from the kinetic energy of the vehicle. The temperature of the brake pads increases as a result and the internal energy of each pad increases.
Define internal energy
The sum of the random distribution of the kinetic and potential energies of the object’s molecules
Define thermal energy
Internal energy of an object due to temperature
When does the internal energy of an object increase?
When energy is transferred by heating the object. Work is done on the object, eg by electricity
When is the internal energy of an object constant?
Either there is no energy transfer by heating and no work is done. Or the energy transferred by heating and work done balance each other out.
First law of thermodynamics
When work is done on or by an object and/or energy is transferred by heating, the change of internal energy of the object equals the total energy transfer due to work done and heating.
Define thermal equilibrium
No overall heat transfer occurs between two objects at the same temperature.
Define absolute zero
The lowest possible temperature, the temperature at which an object has minimum internal energy.
Define specific heat capacity
The energy needed to raise the temperature of 1kg of a substance by 1K without a change of state
Melting point
The temperature at which a pure substance melts
Boiling point
The temperature at which a pure liquid at atmospheric pressure boils.
Specific latent heat of fusion
The energy needed to change the state of 1kg of a solid to a liquid without a change in temperature. If not specific then any mass.
Specific latent heat of vaporisation
The energy needed to change the state of 1kg of a liquid to a gas without a change in temperature. If not specific then any mass.
Sublimation
The change of state when a solid changes to a gas directly.
Temperature against time graph for pure solid heated at constant rate.
Straight diagonal line up to melting point, then flat line, then straight diagonal line as liquid.
What kind of energy changes when a substance changes state?
Its potential energy.
What are the energy changes when something falls?
The gravitational potential energy of the falling object is converted into internal energy when it hits the bottom. The gain of internal energy is q=mcT
Define pressure
The force per unit area a gas exerts normally (right angles) on a surface.
Boyle’s law + explain
pV=constant
For fixed mass of gas at constant temperature.
The pressure is increased by reducing the volume because the gas molecules travel less distance between impacts at the walls so more impacts per second so pressure is greater.
Charles’s law
V/T = constant
For a fixed mass of gas at a constant pressure
What is the work done on a gas?
Pressure times change in volume
So area under pressure against volume graph
Pressure law + explain
p/T=constant
For a fixed mass of gas at a constant volume.
The pressure is increased by raising the temperature as the average speed of the molecules is increased so impacts on container walls are harder and more frequent so pressure increases.
Brownian motion
The random and unpredictable movement of a particle caused by molecules of the surrounding substance colliding at random with the particle.
Define Avogadro constant
The number of atoms in 12g of the carbon isotope C-12
What is 1 atomic mass unit, u?
1/12 of the mass of a C-12 atom
What is a mole?
The quantity of a substance that contains the Avogadro number of identical particles of the substance
Molarity
The number of moles in a certain quantity of a substance
Molar mass
The mass of one mole of a substance
Ideal gas equation
pV/T = constant
For one mole the constant is R (8.31J/mol/K)
Ideal gas criteria
The particles are identical, all collisions are elastic, the particles take up negligible volume compared to the container, it obeys Boyle’s law at all temperatures.
Number of molecules against speed graph
Bell curve but right end doesn’t touch x-axis
Number of molecules against speed graph for higher temperature
Bell curve with lower peak. Crosses other graph once and then remains above it.
Root mean square speed
The square root of the mean value of the square of the molecular speeds of the molecules in a gas.
Assumptions about molecules for kinetic theory equation
Volume of each negligible compared with the volume of the gas. They don’t attract each other (otherwise reduced force of impact on container surface). They move in continual random motion. The collisions between them and with walls are elastic. Each collision with the walls is of much shorter duration than the time between impacts.
Part 1 of deriving pV=1/3NmCrms^2
Consider one molecule mass m in a rectangular box and the dimensions and components of velocity in 3D. If molecule moves only in x direction and hits a face normally, the change in mv is -2mu. Time between collisions with same face is 2x/u. Force on molecule is change in mv/t. Force in face is - that. Pressure on that face is force over area. Get p=mu^2/volume
Part 2 of deriving pV=1/3NmCrms^2
Consider N molecules. Total pressure is sum of individual pressures so sum of mu^2/V. So p=Nmu^2/V (u is mean speed). Motion is random so could be in y and z direction. So p=Nmv^2/V and p=Nmw^2/V. So p=Nm/3V x (u^2+v^2+w^2). The bracket is Crms^2 as velocity squared is the sum of the squares of the components. Sub in.
What does the internal energy of an ideal gas depend on?
Only the kinetic energy of its molecules.
Is 3/2 x nRT
Mean KE of a molecule in an ideal gas
3/2 x kT
How to investigate Boyle’s law
Clamp a syringe to a stand and put plunger all way in. Attach plunger to mass holder. Record volume in syringe for different masses. Calculate pressure due to to mass using weight/area. Correct pressure by adding pressure of air. Plot graph of 1/vol against corrected pressure. Should be straight line through 0. Law relies on ideal gas, constant temperature, no resistance to plunger.
How to investigate Charles’ law
Clamp syringe to stand. Record volume of gas in it at room temp. Submerge in beaker of water. Vary temperature of water and record volume when plunger stops moving for each temperature. Repeat for hot and very cold temperatures to get large range and reduce error in calculation for gradient. Plot graph of volume against temp (K) to get straight line should go through 0K. Relies on fixed pressure, same mass of gas, no resistance to plunger.
Total kinetic energy for n moles of ideal gas
3/2 x nRT
Also is internal energy
