Topic 3 - Thermal Physics

Question 1

Heat

Answer 1

Transfer of energy between a system and its surroundings.

Question 2

Temperature

Answer 2

Temperature is a measure of the average kinetic energy of the particles in a system.

Question 3

Internal energy

Answer 3

total kinetic energy plus potential energy

Question 4

Potential Energy

Answer 4

Potential energy is the energy stored by an object

Question 5

Specific Heat Capacity

Answer 5

The Energy needed to raise the temperature of one unit of mass of material by 1K

Question 6

Specific Latent Heat

Answer 6

Energy per unit mass absorbed or released during a phase change

Question 7

Latent Heat of Fusion

Answer 7

the amount of energy needed to melt or freeze the material at its melting point.

Question 8

Latent Heat of Vaporisation

Answer 8

the amount of energy needed to boil or condense the material at its boiling point.

Question 9

5 Features of an Ideal Gas

Answer 9

• The molecules take up negligible volume.
• All collisions between molecules and the walls of the container are completely elastic.
• no potential energy, only kinetic
• All molecules of a particular gas are identical.
• the molecules have constant random motion

Question 10

Real Gas

Answer 10

Reals gases are non-ideal gases, whose molecules occupy space and have interactions.

Question 11

What happens to potential energy during a phase change?

Answer 11

There is no temperature change, so only the potential energy between the molecules changes.

Question 12

What are 3 conditions under which a real gas behaves similarly to an ideal gas

Answer 12

1) High temperature
2) Low pressure
3) Low density

Question 13

What is the formula for power and how does it relate to Q (heat)?

Answer 13

Formula for power: energy/time , t = time (s)

Q/t = Power , therefore

mc(change in T)/ t = power

Question 14

What is the formula involving pressure, area, and force?

Answer 14

p = F/A

Question 15

If a fixed mass of ideal gas is kept in a -thermally isolated and sealed container- what does this mean for energy?

Answer 15

There is no energy change in the system.

Temperature and Internal energy are constant

Question 16

What energy changes take place when a metal ball is released from rest and falls through a distance of h?

Answer 16

Initially the ball has gravitational potential energy and as it falls the energy is converted into kinetic energy and thermal energy.

Question 17

What happens to temperature if average kinetic energy remains unchanged

Answer 17

We can deduce that temperature is also unchanged.

Question 18

How to find the energy supplied by a heater during a given time period

Answer 18

Multiply the power output of the heater by the time elapsed

power = energy/time

Question 19

7 assumptions of the kinetic model of an ideal gas

Answer 19

1) Newton’s laws of motion are obeyed by the gas particles.
2) Collisions are perfectly elastic.
3) No intermolecular forces act between the particles.
4) Gas molecules are considered point particles / have negligible volume.
5) The motion of the particles is random.
6) The duration of collisions is negligible
7) The effect of gravity is ignored.

Question 20

Why does a decrease in temperature of an ideal gas cause the pressure to decrease?

Answer 20

Lower temperature means that the gas particles have less kinetic energy.

Less kinetic energy means decreased momentum and decreased force on the container walls during collisions.

Since p = F/A , and A (the area of the walls) does not change, pressure is reduced.

Question 21

Distinguish between specific heat capacity and specific latent heat.

Answer 21

Specific heat capacity is the energy required to raise the temperature of one kilogram of a material by one kelvin.

Specific latent heat is the energy required to change the phase of one kilogram of a material without changing its temperature.

Question 22

What happens to energy when two objects at different temperatures are in contact?

Answer 22

When two objects at different temperatures are in contact, the energy gained by the cooler object is equal to the energy lost by the warmer object.

Question 23

What happens to the kinetic energy of an unpowered object as it moves from a lower point in a gravitational field to a higher point

Answer 23

Its kinetic energy is converted into gravitational potential energy.