Thermal Physics

Question 1

When does energy transfer occur? How does internal energy increase?

Answer 1

When one object does work on the other object, or when one object is hotter than the other, resulting in energy transfer by heating due to a temperature difference.

Question 2

What is the first law of thermodynamics?

Answer 2

The total change of the internal energy of an object is equal to the total energy transfer due to work done and heating.

Question 3

What is the definition of internal energy?

Answer 3

The sum of the random distributions of kinetic and potential energies of the molecules.

Question 4

What is the meaning of temperature?

Answer 4

It measures the degree of hotness an object has

Question 5

When is thermal equilibrium attained?

Answer 5

When two objects are at the same temperature.

Question 6

What is absolute zero?

Answer 6

Zero kelvin. Minimum internal energy

Question 7

What is degrees to kelvin change?

Answer 7

+273.

Question 8

What is the specific heat capacity of a substance?

Answer 8

The energy needed to raise the temperature of a unit mass of the substance by 1k without change of state.

Question 9

what is the unit of c (specific heat capcacity )

Answer 9

JKG^-1K^-1

Question 10

What energy transfer occurs in an inversion tube?

Answer 10

Potential energy is converted into internal energy when it hits the bottom of the tube.

Question 11

What is the specific latent heat of fusion?

Answer 11

The energy needed to change the state of a unit mass of substance from solid to liquid without change of temperature.

Question 12

What is the specific latent heat of vaporisation?

Answer 12

The energy needed to change the state of a unit mass of substance from liquid to vapour without change of temperature.

Question 13

What is the meaning of the term ‘latent’

Answer 13

Hidden

Question 14

What is latent heat of fusion?

Answer 14

The energy needed to melt a solid at its melting point

Question 15

What is latent heat of vaporisation?

Answer 15

The energy needed to vaporise a liquid at its boiling point.

Question 16

What is the name of the process of a solid turning directly into a gas?

Answer 16

Sublimation

Question 17

What are the energy changes during heating and melting?

Answer 17

During heating, particles gain kinetic energy, however during melting, temperature doesn’t change, so particles are gaining potential energy.

Question 18

Which formula should be used to find latent heat of fusion or vaporisation?

Answer 18

Q=ML. L = specific latent heat of fusion/vaporisation.

Question 19

What is Boyle’s law?

Answer 19

PV = constant.

Question 20

What is charles’ law?

Answer 20

V/T = constant.

Question 21

What is the pressure law?

Answer 21

P/T = constant.

Question 22

What is work done to change volume of a gas formula?

Answer 22

W = PV.

Question 23

How were the gas laws discovered originally?

Answer 23

By experiment rather than by theory.

Question 24

What is brownian motion?

Answer 24

The irregular motion of small particles suspended in a liquid or a gas, caused by the bombardment of the particles by molecules in the medium.

Question 25

What is avogadro's constant?

Answer 25

The number of atoms in exactly 12g of carbon-12 isotope.

Question 26

What is a mole?

Answer 26

An amount of substance that contains Na particles (Avogadro number)

Question 27

What is the definition of an ideal gas?

Answer 27

A gas that obeys Boyle's law.

Question 28

What is the ideal gas equation?

Answer 28

PV = nRT. n = number of moles of substance. Also written as PV = NkT.

Question 29

What is the theoretical explanation for Boyle's law?

Answer 29

Pressure increases when volume decreases because gas molecules travel less distance between impacts at walls. This leads to more impacts per second, and pressure is therefore greater.

Question 30

What is the explanation for the pressure law?

Answer 30

Average speed of the molecules is increased by raising the temperature. Impacts on the container walls are harder and more frequent, so pressure is increased.

Question 31

What happens to velocity vs number of molecules graph when temperature of gas is raised?

Answer 31

Molecules move faster on average, due to higher kinetic energy, so the root mean square speed increases. Curve becomes flatter and broader because the greater the temperature, the more molecules are moving at higher speeds. Peak is slightly lower and more to the right.

Question 32

What are the assumptions made in kinetic theory of gases?

Answer 32

Molecules are point molecules - their volume is negligible. Molecules don't attract each other - this would reduce the force of their impacts on container surface. Molecules move in continual random motion Elastic collisions only Duration of collisions is very short compared to time spent between collisions. All molecules of gas are identical

Question 33

Difference between heat and temperature?

Answer 33

Heat is a measure of energy. When you heat an object you are transferring energy to it by there being a temperature difference.

Question 34

Why is there no change in temperature during melting/evaporation.

Answer 34

During state changes, energy is used to break bonds, not raise the temperature.

Question 35

How do you answer questions involving two objects transferring thermal energy between each other, and one is melting, for example ice in water which is melting

Answer 35

Equate shc energy formula of water including its temperature change to that of the ice, but add on latent heat of fusion to equation on rhs. Use the fact that they will both reach the same temperature in the end. One equation in terms of T (final temperature) should be written.

Question 36

What is difference between ideal gas and perfect gas?

Answer 36

Ideal gas obeys boyle's law whereas a perfect gas almost obeys boyle's law.

Question 37

What is difference between kelvin and degrees scale?

Answer 37

Kelvin is an absolute scale which starts at zero. Degrees is a relative scale.

Question 38

What is isobaric and what is isothermic?

Answer 38

Constant pressure and constant temperature.

Question 39

How do you get the mass of a molecule?

Answer 39

Mass of one mole / Avogadro's constant.

Question 40

Why is it a MEAN kinetic energy?

Answer 40

Motion of the particles is random, so each kinetic energy will be different.