Thermal Physics Flashcards

Question

Describe the pressure law and give its equation and graph

Answer 1

the frequency of collisions of the gas molecules per unit area of a container

Answer 2

- Has molecules with negligible volume - Collisions which are elastic - Cannot be liquified - Has no interactions between the molecules (except during collisions) - Obeys the (ideal) gas laws (Boyles law, Charles’ law and Pressure law) (All of these can occur at any temperature or pressure)

Answer 3

When a gas expands, it does work on its surroundings by exerting pressure on the walls of the container it's in

Answer 4

W = pΔV Where: W = work done (J) p = external pressure (Pa) V = volume of gas (m3)

Answer 5

The number of atoms of carbon-12 in 12 g of carbon-12

Answer 6

One mole of any element is equal to the relative atomic mass of that element in grams

Answer 7

- The Boltzmann constant relates the properties of microscopic particles (e.g. kinetic energy of gas molecules) to their macroscopic properties (e.g. temperature) - This is why the units are J K-1 - Its value is very small because the increase in kinetic energy of a molecule is very small for every incremental increase in temperature

Answer 8

A reaction is described as being adiabatic if no heat passes to or from the gas.

Answer 9

in Kelvin.

Answer 10

Gas Laws: - The gas laws are empirical in nature which means they are based on observation and evidence - These are all based on observations of how a gas responds to changes in its environment, namely volume, pressure and temperature from experiment Kinetic Theory: - Kinetic theory is based on theory (as stated in its name) - This means it is based on assumptions and derivations from existing theories - These are then used to explain why the gas laws behave the way they do

Answer 11

- microscopic properties of particles (mass and speed) - macroscopic properties of particles (pressure and volume)

Answer 12

- Molecules of a gas behave as identical (or all have the same mass) - Molecules of gas are hard, perfectly elastic spheres - The volume of the molecules is negligible compared to the volume of the container - The time of a collision is negligible compared to the time between collisions - There are no intermolecular forces between the molecules (except during impact) - The molecules move in continuous random motion - Newton's laws apply - There are a very large number of molecules

Answer 13

1. Determine the change in momentum as a single molecule hits a wall perpendicularly One assumption of the kinetic theory is that molecules rebound elastically This means there is no kinetic energy lost in the collision If the particle hits one side of the wall and rebounds elastically in the opposite direction to their initial velocity, their final velocity is –c The change in momentum is therefore: p = mc Δp = final p – initial p = −mc − (+mc) = −mc − mc = −2mc Where: Δp = change in momentum (kg m s-1) m = mass of the molecule (kg) c = speed of the molecule (m s-1) 2. Calculate the number of collisions per second by the molecule on a wall The time between collisions of the molecule travelling to the opposite facing wall and back is calculated by: dividing a distance of 2L with speed c. Note: c is not taken as the speed of light in this scenario 3. Calculate the force exerted by the molecule on the wall The force the molecule exerts on one wall is found using Newton’s second law of motion: mc (squared), divided by L The change in momentum is +2mc since the force on the molecule from the wall is in the opposite direction to its change in momentum 4. Calculate the total pressure for one molecule The area of one wall is L2 The pressure is defined as the force per unit area: mc (squared) divided by L (cubed) This is the pressure exerted from one molecule in a particular direction 5. Consider the effect of N molecules moving randomly in 3D space The pressure equation still assumes that all the molecules are travelling in the same direction and collide with the same pair of opposite faces of the cube In reality, all molecules will be moving in three dimensions equally and randomly By splitting the velocity into its components cx, cy and cz to denote the amount in the x, y and z directions, c2 can be defined using Pythagoras' theorem in 3D: c2 = cx2 + cy2 + cz2 Since there is nothing special about any particular direction, it can be deduced that: cx2 = cy2 = cz2 Therefore, cx2 can be defined as: Speed in x Direction Equation Where c2 is the sum of the squared speeds of all the molecules c2 = c12 + c22 + c32 +... + cN2 6. Consider the speed of the molecules as an average speed Each molecule has a different speed and they all contribute to the pressure Therefore, the square root of the average of the square velocities is taken as the speed instead This is called the root-mean-square speed or crms crms is defined as: Root Mean Square Speed Equation Therefore N(crms)2 = c12 + c22 + c32 +... + cN2 7. Consider the volume of the box The box is a cube and all the sides are of length l This means L3 is equal to the volume of the cube, V Substituting N(crms)2 and L3 back into the pressure equation obtains the equation: pV= 1/3 Nm (c (rms) squared) This is the pressure parallel to the x (or y or z axis) Multiplying both sides by the volume V gives the final Kinetic Theory of Gases Equation: Kinetic Theory Final Equation Where: p = pressure (Pa) V = volume (m3) N = number of molecules m = mass of one molecule of gas (kg) crms = root mean square speed of the molecules (m s-1)

Answer 14

Small particles (such as pollen or smoke particles) suspended in a liquid or gas are observed to move around in a random, erratic fashion

Answer 15

the existence of molecules in a gas or liquid

Answer 16

A range of speeds No preferred direction of movement

Answer 17

- The observable particles in Brownian motion are significantly bigger than the molecules that cause the motion - In most cases, these were observed as smoke particles in air - The air particles cause the observable motion of the smoke particles that we see - This means that the air particles were small and light and the smoke particles were large and heavy - The small molecules are able to affect the larger particles in this way because: They are travelling at a speed much higher than the larger particles They have a lot of momentum, which they transfer to the larger particles when they collide

Answer 18

- Democritus (2000 years ago) Ancient Greek and Roman philosopher Democritus named the infinitesimally small pieces of matter atomos meaning 'indivisible' This is the etymology of the word 'atom' Both of the two most well-known Greek philosophers, Aristotle and Plato, rejected his theories Due to their influence, Democritus's theories were not accepted until almost 2000 years later - Robert Boyle (1662) Robert Boyle discovered Boyle's Law - Guillaume Amontons (1699) Amontons, and later also by Joseph Louis Gay-Lussac (1809), discovered the Pressure Law - Jacques Charles (1787) This was then followed by Charles who discovered the Charles's Law - Daniel Bernoulli (18th Century) Bernoulli assumed that gases were made up of tiny particles which sparked the beginning of kinetic theory - Robert Brown Brown discovered Brownian Motion, the random motion of particles in a fluid, which helped support kinetic theory - Albert Einstein (1905) In Einstein's miracle year of 1905, he produced a paper on how kinetic theory was used to make predictions for Brownian motion Only then did the atomic and kinetic theory of particles start to become more widely accepted.

Answer 19

a thermodynamic process in which the temperature of a system remains constant.