Thermal Physics and Gases

Question 1

When is energy transferred between two objects?

Answer 1

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.

Question 2

Why do brake pads heat up when applying brakes to a moving vehicle?

Answer 2

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.

Question 3

Define internal energy

Answer 3

The sum of the random distribution of the kinetic and potential energies of the object’s molecules

Question 4

Define thermal energy

Answer 4

Internal energy of an object due to temperature

Question 5

When does the internal energy of an object increase?

Answer 5

When energy is transferred by heating the object. Work is done on the object, eg by electricity

Question 6

When is the internal energy of an object constant?

Answer 6

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.

Question 7

First law of thermodynamics

Answer 7

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.

Question 8

Define thermal equilibrium

Answer 8

No overall heat transfer occurs between two objects at the same temperature.

Question 9

Define absolute zero

Answer 9

The lowest possible temperature, the temperature at which an object has minimum internal energy.

Question 10

Define specific heat capacity

Answer 10

The energy needed to raise the temperature of 1kg of a substance by 1K without a change of state

Question 11

Melting point

Answer 11

The temperature at which a pure substance melts

Question 12

Boiling point

Answer 12

The temperature at which a pure liquid at atmospheric pressure boils.

Question 13

Specific latent heat of fusion

Answer 13

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.

Question 14

Specific latent heat of vaporisation

Answer 14

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.

Question 15

Sublimation

Answer 15

The change of state when a solid changes to a gas directly.

Question 16

Temperature against time graph for pure solid heated at constant rate.

Answer 16

Straight diagonal line up to melting point, then flat line, then straight diagonal line as liquid.

Question 17

What kind of energy changes when a substance changes state?

Answer 17

Its potential energy.

Question 18

What are the energy changes when something falls?

Answer 18

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

Question 19

Define pressure

Answer 19

The force per unit area a gas exerts normally (right angles) on a surface.

Question 20

Boyle’s law + explain

Answer 20

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.

Question 21

Charles’s law

Answer 21

V/T = constant

For a fixed mass of gas at a constant pressure

Question 22

What is the work done on a gas?

Answer 22

Pressure times change in volume

So area under pressure against volume graph

Question 23

Pressure law + explain

Answer 23

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.

Question 24

Brownian motion

Answer 24

The random and unpredictable movement of a particle caused by molecules of the surrounding substance colliding at random with the particle.

Question 25

Define Avogadro constant

Answer 25

The number of atoms in 12g of the carbon isotope C-12

Question 26

What is 1 atomic mass unit, u?

Answer 26

1/12 of the mass of a C-12 atom

Question 27

What is a mole?

Answer 27

The quantity of a substance that contains the Avogadro number of identical particles of the substance

Question 28

Molarity

Answer 28

The number of moles in a certain quantity of a substance

Question 29

Molar mass

Answer 29

The mass of one mole of a substance

Question 30

Ideal gas equation

Answer 30

pV/T = constant

For one mole the constant is R (8.31J/mol/K)

Question 31

Ideal gas criteria

Answer 31

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.

Question 32

Number of molecules against speed graph

Answer 32

Bell curve but right end doesn’t touch x-axis

Question 33

Number of molecules against speed graph for higher temperature

Answer 33

Bell curve with lower peak. Crosses other graph once and then remains above it.

Question 34

Root mean square speed

Answer 34

The square root of the mean value of the square of the molecular speeds of the molecules in a gas.

Question 35

Assumptions about molecules for kinetic theory equation

Answer 35

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.

Question 36

Part 1 of deriving pV=1/3NmCrms^2

Answer 36

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

Question 37

Part 2 of deriving pV=1/3NmCrms^2

Answer 37

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.

Question 38

What does the internal energy of an ideal gas depend on?

Answer 38

Only the kinetic energy of its molecules.

Is 3/2 x nRT

Question 39

Mean KE of a molecule in an ideal gas

Answer 39

3/2 x kT

Question 40

How to investigate Boyle’s law

Answer 40

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.

Question 41

How to investigate Charles’ law

Answer 41

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.

Question 42

Total kinetic energy for n moles of ideal gas

Answer 42

3/2 x nRT

Also is internal energy