5.1 Thermal Physics

Question 1

Thermal Equilibrium

Answer 1

When both objects have the same average kinetic energy

Question 2

Absolute scale of temperature

Answer 2

It’s the thermodynamic scale, and it does not depend on the properties of any particular substance.

Question 3

How to change from kelvin to Celsius?

Answer 3

add 273 to the Celsius to turn to kelvin

Question 4

What is Brownian Motion?

Answer 4

Gas molecules move in random haphazard motion

Question 5

How do you observe Brownian motion in a lab?

Answer 5

• Put smoke in a brightly illuminated glass jar and observe particles using microscope
• Smoke particles appear as bright specs moving haphazardly from side to side

Question 6

What is internal energy?

Answer 6

Sum of the random distribution of kinetic energy + potential energy of particles in a system

Question 7

What is absolute zero?

Answer 7

Lowest limit for temperature

Temperature at which a substance has minimum internal energy

Question 8

What happens as temperature rises?

Answer 8

Increase in internal energy of a body

Question 9

What happens when a substance changes phase in terms of energy?

Answer 9

Its internal energy changes but kinetic energy remains same so temperature stays the same. Internal Potential energy increases.

Question 10

Define specific heat capacity

Answer 10

Amount of energy needed to raise the temperature of 1Kg of the substance by 1K

Question 11

Describe an electrical experiment to determine the specific heat capacity of a metal or liquid?

Answer 11

1. Heat substance with heater (need temp rise of 10k to get accurate C)
2. With ammeter and voltmeter attached to heater use E=W=VIt

Equipment:

• electric heater
• solid (metalcylindar) or liquid
• insulating material (like np) and lid
• Digital thermometer

Question 12

Define Specific latent heat of fusion or vaporization

Answer 12

The quantity of thermal energy required to change state of 1kg of a substance

Question 13

Describe an experiment to determine SLH of a solid or liquid

Answer 13

For a solid:

1. Put a heating coil and equal masses of ice in two funnels above beakers
2. Turn on one heating coil for 3 minutes. Record the energy transferred in the 3 minutes. Don’t turn on other coil-it’s there so you can measure how much ice melts due to the ambient temperature of the room
3. At the end of the 3 minutes, measure mass of the water collected in the beaker. Subtract one form the other to get the mass of ice, m, that melted solely due to the presence of the heater.
4. E=mL, so to find the SLH of fusion for water, L=E/m

For a liquid- boil water in a distilling flask, condense the vapour given off and E/m of condensed water collected

Question 14

Describe a solid in terms of spacing, ordering and motion

Answer 14

-The molecules are arranged in a
regular, 3-dimensional structure.

• The atoms and molecules are bonded to each other by strong, attractive forces due to the electrical charges of the electrons and protons in the atoms.
• The molecules are relatively close to each other (~ 3.0 x 10^-10 m) and vibrate about fixed positions. The higher the temperature of the solid, the greater is the amplitude of vibration of the molecules.

Question 15

Describe a liquid in terms of spacing, ordering and motion

Answer 15

• The molecules are still in contact and vibrating, but they are free to move around randomly and this is why a liquid flows and has no fixed shape.
• The attractive forces between the molecules are weaker than in a solid and not strong enough to hold them in fixed positions.
• The separation of the molecules is about the same as it is in solids (~ 3.0 x 10^-10 m)

Question 16

Describe a gas in terms of spacing, ordering and motion

Answer 16

• The intermolecular forces are negligible and so the molecules are completely free of each other. They whiz around in rapid, random motion, colliding with the walls of the containing vessel and with each other. It is this molecular bombardment of the container walls which gives rise to gas pressure.
• On average, the molecules are about ten times further apart (~ 3.0 x 10^-9 m) than they are in solids and liquids. This is why a gas occupies a much larger volume than the same mass of liquid.

Question 17

State how the observations from smoke experiemnt lead to conclusions about the nature and properties of the molecules of a gas

Answer 17

• movement of smoke particles caused by (being hit by) randomly moving air molecules
• smoke particles are continuously moving because the air molecules are continuously moving
• smoke particles are visible but air molecules are not hence air molecules must be (very) small.
• small movement of smoke particles is due to the large numbers of air molecules hitting from all sides

Question 18

What is a mole?

Answer 18

One mole is the amount of substance containing 6.02×10^23 particles

Question 19

What are the 4 assumptions for the model of kinetic theory of gases?

Answer 19

large number of molecules in random, rapid motion

particles (atoms or molecules) occupy negligible
volume compared to the volume of gas

all collisions are perfectly elastic and the time of
the collisions is negligible compared to the time
between collisions

negligible forces between particles except during
collision

Question 20

Explain how The kinetic theory of gases can be used to understand how gas in a container produces pressure.

Answer 20

As the collisions between atoms and the wall of the container are perfectly elastic, the atoms rebound from the wall at the same speed they travel in at.

This makes their change in momentum, p, (given by p = m(v-u)) equal to 2mv. As the change in momentum is equal to force multiplied by time, the average force exerted on the atom by the wall is given by 𝐹=2mv/2

Using Newton’s third law, the atom also exerts an equal and opposite force on the wall of the container. The total pressure on the wall is equal to the sum of the force of each collision between atoms in the gas and the wall, and the area of the wall.

Question 21

Explain how the internal energy of an ideal gas is related to its temperature.

Answer 21

Ideal gas has Zero potential energy,

Ke is proportional to T.

Question 22

State Boyle’s law

Answer 22

pV=constant

Question 23

State pressure law

Answer 23

p/T=constant

Question 24

State Charles’ Law

Answer 24

At constant pressure Volume of gas is directly proportional to absolute temperature

Question 25

How do you investigate Boyles law?

Answer 25

1. Oil confines parcel of air in sealed tube with fixed dimensions
2. Tyre pump increases pressure, Bourdon gauge records the pressure.
3. Volume will decrease as air compresses
4. Not down pressure and volume of air

Question 26

How do you investigate pressure law

Answer 26

1. Immerse flask of air in beaker of water (submerge as much as possible)
2. Connect stopper to bourdon gauge using short tube (volume of tube much smaller then flask)
3. Record pressure and temp
4. Heat water, then remove heat (stir for uniform temperature, allow time for heat to transfer to air)
5. Record pressure and temperature
6. heat again and repeat till bp reached
7. Repeat twice all over again (with fresh water)
8. plot P against T, LOBF, estimate absolute zero by extrapolation up to x axis