define// thermal physics

Question 1

internal energy of a gas

Answer 1

internal energy is the sum of the randomly distributed kinetic energies and potential energies of the particles in a body

Question 2

Define specific heat capacity

Answer 2

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.

Question 3

Define absolute zero

Answer 3

Lowest possible temperature where all the particles have the minimum possible kinetic energy.
This occurs at 0 K or -273° C

Question 4

Define specific latent heat

Answer 4

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

Question 5

Boyle’s Law

Answer 5

For a fixed mass of a gas at a constant temperature, the pressure and volume of a gas are inversely proportional.

Question 6

Charle’s Law

Answer 6

For a fixed mass of gas at constant pressure, the volume V of a gas is directly proportional to its absolute temperature.

Question 7

State the pressure law

Answer 7

For a fixed mass at a constant volume, the pressure of a gas is directly proportional to its absolute temperature.

Question 8

define molecular mass

Answer 8

The molecular mass is the mass of a given molecule (Mr).

Question 9

define molar mass

Answer 9

Molar mass M is the mass of a chemical compound divided by its amount-of-substance measured in moles.

Question 10

Define the avogadro constant

Answer 10

It refers to the number of particles in one mole of a substance.

6.023x10^23

Question 11

Define the molar gas constant

Answer 11

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

Question 12

Define Boltzmann constant

Answer 12

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.

Question 13

define the work done on a gas of a constant pressure

Answer 13

Work done= pressure x change in volume

Question 14

State the assumptions made about an ideal gas in kinetic theory

Answer 14

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

Question 15

state assumptions for an ideal gas

Answer 15

• molecules are not too close
• no imf
• pressure is not extremely high

Question 16

define the internal energy of an ideal gas in kinetic theory

Answer 16

Kinetic energy of atoms.

Question 17

Describe how the internal energy of a gas can be increased and reduced

Answer 17

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)

Question 18

Describe how the internal energy of a gas can be increased and reduced

Answer 18

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)

Question 19

Describe how a continuous-flow calorimeter works

Answer 19

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

Question 20

Describe what happens to the potential energies and the average kinetic energy of the particles in a substance as that substance changes state

Answer 20

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

Question 21

Explain why kinetic theory is theoretical rather than empirical

Answer 21

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.

Question 22

What is Brownian motion?

Answer 22

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.

Question 23

How does Brownian motion provide evidence for the existence of atoms?

Answer 23

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.

Question 24

Derivation of kinetic theory equation

Answer 24

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.

1. Determine the change in momentum as a single molecule hits a wall perpendicularly
   P=mc
   ΔP= -mc -(mc)= -2mc
2. Calculate the number of collisions per second by the molecule on the wall.
   t= 2L (distance from wall to wall and back) /v
3. Calculate the force exerted by the molecule on the wall
   F= Δp/Δt= 2mc/(2L/c) = mc^2/L
4. Calculate total pressure for one molecule
   Pressure=F/A= mc^2/L^3
5. Consider effects of N molecules moving in 3D
   C^2=cx^2+cy^2+cz^2 all equal therefore Cx^2=1/3c^2
6. Consider the speed of molecules as average speed
   C rms is taken as each molecule has different speed
7. Consider volume of the box
   L^3=V hence pV=1/3Nm(crms)^2

Question 25

Explain how knowledge and understanding of the behaviour of gases has changed over time

Answer 25

• Democritus, has ideas about gases and theorised a very small piece of matter that everything is made up from (atoms). This is rejected by Plato and Aristotle which were very influential hence his theories weren’t accepted.

-Boyle discovers relationship between pressure and volume at constant temperature (1622)
-Guillaume Amontons (1699) and later Joseph L. Gay-Lussac (1809), discovered relationship between temperature and pressure at constant volume. [pressure law]
-Jaqueline Charles (1787) discovered the relationship between the volume of a gas and its temperature at constant pressure.
-Bernoulli sparked the beginning of the kinetic theory by assuming gases are made by tiny particles
-Robert Brown, a botanist, discovers Brownian motion backs up the kinetic theory with his discovery by giving proof that air is made up by tiny atoms moving quickly and randomly.
-Albert Einstein (1905) writes a paper on how kinetic energy theory was used to make predictions on Brownian motion. Atomic and kinetic theory become more widely accepted since.

Question 26

Why was Bernouli’s work on kinetic theory not accepted early on?

Answer 26

New theories can only be accepted if independently validated. Initially the kinetic theory was only thought about as a hypothetical model as atoms existed wasn’t accepted. It became accepted only once Einstein used kinetic theory to make predictions for Brownian motion.