define// thermal physics Flashcards
internal energy of a gas
internal energy is the sum of the randomly distributed kinetic energies and potential energies of the particles in a body
Define specific heat capacity
The specific heat capacity (c) of a substance is the amount of energy needed to raise the temperature of 1kg if the substance by 1 K.
Define absolute zero
Lowest possible temperature where all the particles have the minimum possible kinetic energy.
This occurs at 0 K or -273° C
Define specific latent heat
The specific latent heat (l) of fusion or vaporisation is the quantity of thermal energy required to change the state of 1 kg of a substance. (without changing temperature)
ΔQ= ml
Boyle’s Law
For a fixed mass of a gas at a constant temperature, the pressure and volume of a gas are inversely proportional.
Charle’s Law
For a fixed mass of gas at constant pressure, the volume V of a gas is directly proportional to its absolute temperature.
State the pressure law
For a fixed mass at a constant volume, the pressure of a gas is directly proportional to its absolute temperature.
define molecular mass
The molecular mass is the mass of a given molecule (Mr).
define molar mass
Molar mass M is the mass of a chemical compound divided by its amount-of-substance measured in moles.
Define the avogadro constant
It refers to the number of particles in one mole of a substance.
6.023x10^23
Define the molar gas constant
The constant of proportionality that relates the energy to temperature, when a mole of particles at the state temperature is considered.
R = 8.31 JK-1mol-1
Define Boltzmann constant
The Boltzmann constant is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas.
define the work done on a gas of a constant pressure
Work done= pressure x change in volume
State the assumptions made about an ideal gas in kinetic theory
1- Molecules are points: the V of the molecule is insignificant compared to the volume of the ideal gas
2- All collisions between gas molecules and their container are elastic: there is no loss of kinetic energy
3- No intermolecular forces (would make pressure lower)
4- Molecules move in straight lines but in random directions
5- The time taken for a collision is much shorter than time between collisions
6- Any sample of an ideal gas contains a very large number of molecules
state assumptions for an ideal gas
- molecules are not too close
- no imf
- pressure is not extremely high
define the internal energy of an ideal gas in kinetic theory
Kinetic energy of atoms.
Describe how the internal energy of a gas can be increased and reduced
The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa)
Describe how the internal energy of a gas can be increased and reduced
The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa)
Describe how a continuous-flow calorimeter works
A continuous-flow heating is when a fluid flows continuously over a heating element. As it flows heat is transferred to the fluid.
It is assumed that the heat transferred is constant.
The rise in temperature of the fluid is measured using the electric thermometers and is calculated by:
Δθ = T2 – T1
Time is recorded throughout the experiment so that the mass can be found using the flow rate. Current (I) and potential difference (V) are recorded.
The experiment is repeated with different flow rates and current and pd, Δθ remains constant.
For the first flow rate, the electrical energy supplied to the fluid in time t1 is:
I1V1t1 = Q1 = m1cΔθ + H
H= thermal energy lost to the surroundings, assumed to be the same
The second flow rate is:
I2V2t2 = Q2 = m2cΔθ + H
Subtracting the first flow rate equation from the second gives the equation:
I2V2t2 – I1V1t1 = Q2 – Q1 = (m2 – m1)cΔθ
Rearranging this for the specific heat capacity of the fluid, c gives the final equation:
Continuous Flow Equation
C= (Q2-Q1)/(m2-m1) Δθ
Describe what happens to the potential energies and the average kinetic energy of the particles in a substance as that substance changes state
When a substance changes state, its internal energy changes but the kinetic energy and temperature remains constant. This is due to potential energy changing.
I.e. liquid to gas, Ep increases
Explain why kinetic theory is theoretical rather than empirical
Kinetic theory is a theory as suggested by the name, this means it is based on assumptions and derivations from existing theories. This is unlike gas laws which are empirical as they are based on observation and scientific evidence.
What is Brownian motion?
Brownian motion of particles is the phenomenon when small particles (i.e. smoke or pollen) suspended in a liquid or gas are observed to move around at random, erratic fashion. It can be observed on a microscope.
How does Brownian motion provide evidence for the existence of atoms?
The particles are said to have random motion hence:
-They have a range of speeds
-No preferred direction of movement
The particles in Brownian motion are significantly bigger than the particles causing the motion. The collisions cause larger particles to change speed and direction randomly, providing proof for the behaviour of molecules in a gas.
Small particles are able to affect the larger particles because:
-travelling at higher speeds
-have a lot of momentum which is transferred to the larger particles when they collide.
Derivation of kinetic theory equation
Take into account one molecule in a cub shaped container that collides at regular intervals with the walls. By calculating the pressure exerted by one molecule, the total pressure exerted by a total of N molecules.
- Determine the change in momentum as a single molecule hits a wall perpendicularly
P=mc
ΔP= -mc -(mc)= -2mc - Calculate the number of collisions per second by the molecule on the wall.
t= 2L (distance from wall to wall and back) /v - Calculate the force exerted by the molecule on the wall
F= Δp/Δt= 2mc/(2L/c) = mc^2/L - Calculate total pressure for one molecule
Pressure=F/A= mc^2/L^3 - Consider effects of N molecules moving in 3D
C^2=cx^2+cy^2+cz^2 all equal therefore Cx^2=1/3c^2 - Consider the speed of molecules as average speed
C rms is taken as each molecule has different speed - Consider volume of the box
L^3=V hence pV=1/3Nm(crms)^2
