Thermal Physics

Question 1

Specific heat capacity

Answer 1

The amount of energy needed to raise the temperature of 1kg of a substance by 1 degree Celsius or 1 kelvin

Question 2

Specific latent heat

Answer 2

The amount of internal energy needed to be gained or lost to change the state of 1kg of a substance without changing its temperature

Question 3

Pressure law

Answer 3

For a fixed mass of an ideal gas, the pressure is directly proportional to the absolute temperature provided volume remains constant

Question 4

Pressure law explanation

Answer 4

Temperature increases giving molecules more kinetic energy

The rate of change of momentum per collision increases

Stronger collisions concentrated in a fixed area

Collisions more frequent

Question 5

Boyle’s law

Answer 5

For a fixed mass of an ideal gas, pressure is inversely proportional to volume provided the absolute temperature remains constant

Question 6

Boyle’s law explanation

Answer 6

Increasing volume means molecules must travel further until they collide again with walls

So fewer collisions per second so lower pressure

P=F/A

Lower rate of changing of momentum so lower force

Question 7

Charles’ law

Answer 7

For a fixed mass of an ideal gas, the volume is directly proportional to the absolute temperature provided pressure stays constant

Question 8

Charles’ law explanation

Answer 8

As temperature increases, the average KE of molecules increases

The mean speed of molecules is now higher

Molecules must travel further before they collide

Rate of change of momentum per collision is higher
The force of collisions is now higher

The volume must increase to decrease frequency of collisions

Question 9

What is an ideal gas?

Answer 9

A gas that follows the 3 gas laws at all temperatures, pressures and volumes

Question 10

How to calculate work done on a gas

Answer 10

W=PΔV

Question 11

What’s an empirical law?

Answer 11

Predicts what will happen but not why

Question 12

What’s a theory?

Answer 12

Predicts what will happen and why

Question 13

What is Brownian motion?

Answer 13

Pollen grains in water follow a random jittery motion

This is due to the random bombardment of smaller particles that move rapidly

Atoms are too small to be observed directly

This is the random motion of larger particles due to collisions from unseen smaller particles.
This gives evidence for kinetic theory - gasses being made of tiny particles.

Question 14

What are the assumptions for kinetic theory?

Answer 14

About motion:
-Motion follows Newtons laws.
-Collisions between molecules and molecules and molecules and wall are elastic.
-No intermolecular forces unless during collision
-Forces are instantaneous: time of collisions is negligible compared to time between collisions
-Molecules move rapidly/randomly

About particles
-Large number of molecules in the gas.
-Volume of molecules is negligible compared to volume of container.
-All molecules are identical

Question 15

Kinetic theory equation

Answer 15

PV=1/3Nmc̄^2

Question 16

What is the root mean square speed?

Answer 16

The square root of the mean of the squares of the values of speed

Question 17

Define the Avogadro constant

Answer 17

The number of atoms in 12g of carbon-12

Question 18

Define the internal energy of a gas

Answer 18

The sum of the potential and kinetic energies of all the molecules

Question 19

What happens to particles in a gas when they are heated?

Answer 19

The average kinetic energy of each particle increases

Question 20

How to calculate the number of molecules?

Answer 20

N=nNa

Na=Avogadro’s constant

Question 21

How is pressure created?

Answer 21

-Walls exert a force on molecules during collision
-So molecules exert an equal and opposite force on the walls
-This creates pressure