6.2 thermal physics

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Cards (25)

Temperature - average kinetic energy of the particles in a material
Heat - sum of kinetic energy of the particles in a material
Internal energy
Sum of the randomly distributed kinetic energy and potential energy of all the particles
Kinetic energy - determined by the speed and mass of the molecules
Gives the material its temperature
Distribution of particle speed depends on temperature - the higher the temperature, the higher the average kinetic energy
Electrostatic potential energy - due to the separation between the molecules
Raising internal energy
Closed system - doesnt allow transfer of matter
Total internal energy is constant - no work is done
Heating - energy transfer due to temperature difference
Doing work - energy transfer as a result of exerting a force on another object
Changing states
Internal energy changes, but kinetic energy stays constant
Potential energy is altered not the kinetic energy
First law of thermodynamics
Change of internal energy of an object - total energy transfer due to work done and heating
U=Q+W
Internal energy = heat transferred+ work done
Specific heat capacity
The energy required to raise the temperature of 1kg of the substance by 1K or 1°C
energy change = mass x specific heat capacity x change in temperature
$Q=$ $mc\Delta\theta$
Continuous-flow heating - when a fluid flows continuously over a heating element
Using continuous-flow calorimeter
Energy is transferred to the fluid
Water flows until at a constant temperature
Record flow rate and measure temperature difference between thermometers
Record current and potential difference
Energy supplied is $Q=$ $mc\Delta\theta+$ $H$
H is heat lost to surroundings
Repeat changing only potential difference of the power supply and flow rate to keep temperature change constant
$c=$ $Q_2-Q_1/(m_2-m_1)\Delta\theta$
Q is electrical energy supplied over time
$Q=$ $V/t$ to find Q
c is the specific heat capacity of water
Specific latent heat
The energy needed to change state
Energy is required to break bonds
The larger the mass of the substance the more energy it takes to change state
Specific latent heat of fusion or vaporisation is the quantity of thermal energy require to change the state of 1kg of substance
Energy change = mass of substance x specific latent heat
$Q=$ $ml$
Boyle’s Law
Relationship between pressure and volume
At a constant temperature
p ∝ 1/V p = kV
Ideal gas - obeys Boyle’s law with complete precision
Perfect gas - under conditions that Boyle’s law is valid enough to describe its behaviour
Charles’ Law
Relationship between temperature and volume
At a constant pressure
Temperature measured in Kelvin
V ∝ T V = kT
Gay-Lussac’s Law
Pressure law
Relationship between pressure and temperature
At a constant volume
p ∝ T p = kT
Avagadro’s Law
Volume is proportional to the number of particles
At a constant pressure and temperature
More particles = more frequent collisions
V ∝ N V = kN
Absolute zero
Measured in Kelvin
Point of zero particle vibration
0C = 273K
0K = -273C
Molecular mass
Molecular mass = sum of the masses of all the atoms in a molecule
Relative to the mass of a carbon-12 atom
Relative molecular mass
Carbon-12 has a relative mass of 12
Molar mass
Mole = at a fixed pressure and temperature a fixed volume of gas will contain the same number of gas molecules regardless of what the gas is
One mole of gas contains the same number of particles
Avogadro’s constant ( $N_A$ ) = $6.02\times10^{23}$ particles per mole
Molar mass = mass 1 mole of a substance would have
Equal to relative molecular mass of a substance
Number of molecules = number of moles (n) x Avogadro’s constant
$N=$ $nN_A$
Ideal gas equation
Constant = $\frac{pV}{T}$
The constant is dependent on the quantity of gas used
Measured in moles, n
Constant becomes nR where R is the molar gas constant
$8.31Jmol^{-1}K^{-1}$
Ideal gas equation: pV=nRT
Works for gases at low pressures and high temperature
Work done
Work is done to expand or contract a gas at constant pressure
Must be an energy transfer - usually heat
Work done $W=$ $p\Delta V$
Work done in changing the volume of gas at a constant pressure
Boltzmann’s constant
Botlzmann’s constant, k = the gas constant for one particle of gas
Equivalent to $\frac{R}{N_A}$
$1.38\times10^{-23}JK^{-1}$
Equation of state of an ideal gas: $pV=$ $NkT$
Combination of $N=$ $nN_A$ and $k=$ $\frac{R}{N_A}$ $\rightarrow\ Nk=$ $NR$
Core practical - Boyle’s law experiment
Oil traps a pocket of air in a sealed tube with fixed dimensions
A tyre pump used to increase the pressure on the oil
Bourden gauge records the pressure
As pressure increases, more oil is pushed into the tube and the oil level rises
Air is compressed
Volume occupied by air in the tube reduced
... boyle's law experiment
Measure the volume of air when the system is at atmospheric pressure
Gradually increases pressure keeping temperature constant
Record the pressure and volume of air as it changes
Multiply the pressure and the volume at different points to prove the same value
Plot graph of pressure against 1/volume
Straight line
Core practical - Charles’ law experiment
Capillary tube sealed at bottom containing a drop of concentrated sulphuric acid halfway
Traps the column of air between the bottom and the acid drop
A beaker filled with near-boiling water, and the length of air increases
As water cools, the length of the air column decreases
Regularly record the temperature of water and the air column length
Repeat twice with fresh near-boiling recording the length at the same temperatures and find the average
...charles' law experiment
Plot length against temperature
Line of best fit is a straight line
Length of the air column is proportional to the temperature
Volume of air column is equal to the volume of the cylinder
Proportional to the cylinder's length
Volume is proportional to temperature
Brownian motion
1827, Robert Brown noticed pollen grains in the water moved in zigzag random motion
Brownian motion = random movement of any particles suspended in a fluid is a result of collisions with fast, randomly-moving particles in the fluid
Supports kinetic particle theory of different states of matter
Evident when large, heavy particles are moved by smaller, lighter particles travelling at high speeds
Evidence air is made up of tiny atoms/molecules moving quickly
Empirical versus theoretical
Empirical laws are based on observations and evidence
Predict what will happen but don't explain why
Gas laws and the ideal gas equation are based on observations of properties, and can be proven with experiments
Theories are based on assumptions and derivations from knowledge and theories already in place
Kinetic theory
Kinetic theory assumptions
Ideal gas obeys assumptions
Follows 3 gas laws
Have internal energy dependent only on kinetic energy
Postulates:
Random - random motion with a range of velocities
Travel in straight lines outside of collisions
Identical - all particles have identical mass / spherical shape
Follow newtons laws
Volume - is negligible compared to the container
Attraction - there is a negligible force of attraction between particles
Time - the time spent in contact with the container walls is negligible
Elastic - all the collisions are elastic
Changing understanding of gas behaviour
Democritus, ancient Greek and Roman philosophers 2000 years ago
1662, Robert Boyle discovered the relationship between pressure and volume at a constant temperature
1787, Jacques Charles discovered that the volume of gas is proportional to temperature at a constant pressure
1699, Guillaume Amontons discovered that temperature is proportional to pressure at a constant volume
1809, rediscovered by Joseph Louis Gay-Lussac
...changing understanding of gas behaviour
18th century, Daniel Bernoulli explained Boyle's law by assuming gases were made up of tiny particles
1827, robert brown discovered Brownian motion supporting kinetic theory
Scientific community only accepts new ideas when they can be independently validated
Most thought kinetic theory was a useful hypothetical model until Einstein used Brownian motion, when atomic and kinetic theory became widely accepted