Thermal Physics

• Kinetic Model of matter • Brownian motion

• Usefulness of KPT

• State of Matters, including Plasma

• Temperature and Temperature Scale

• Processes of Heat Transfer

• Internal Energy and Thermal Properties of matter• Heat capacity, specific heat capacity

• Latent Heat, specific latent heat capacity

DIY

Overview

Thermal Physics

Learning Points

• Recap on Kinetic Model of Matter

• Recap Temperature

• Recap Heat Transfer

• Internal Energy and Thermal Properties of matter

Recap on Temperature

and Heat Transfer

• Linking from Kinetic Particle Theory

• Temperature is a measure of the average kinetic energy per molecule of the substance.

• Temperature

• Human sensations cannot help us objectively determine temperature.

• Temperature is a objective property of an object to describe its“hotness.

• Energy

• Thermal Energy is not a physical object but an abstract mental model. Object do not really contain thermal energy literally!

• Net heat transfer from one body to the other body whenever thereis a temperature difference between these bodies and always flowfrom hotter to colder body.

• Bodies in thermal equilibrium have the same temperature and there is no net heat transfer

Recap on Kinetic Model of Matter

• Solid

• Liquid

• Gas

• Plasma

How would you describe the

difference (in terms of motion)

between solid and gases?

Recap on Kinetic Model of Matter

• Using water as example

• Temperature of gaseous state of material is higher than its solid state.

• Also, for the same material, molecules of gases are much more energetic than molecules of solid (e.g. compare ice with steam) at the same temperature.

Recap on Kinetic Model of Matter

• How do we change state of matter (from solid to liquid)?

• What are we actually doing to the matter when we heat it?

• How do you explain the change of state of matter using this theory?

• Where does all the energy supplied go to?

• Does it remain as “thermal energy” within the material?

Why Heat is a process?

• When energy supplied to heat the object is not retained as thermal energy within the object.

• Instead, it is converted to give the molecules a more energetic “stage” (i.e. it moves faster or move farther apart)

• So what energy is the energy converted to?

Why Heat is a process?

• So “heat” transfer is a process.

• It is a process whereby when the energy is supplied, it is converted to higher energy level of the molecules.

• Technically, there is no thermal energy in the object!

Terminology

Heat??

Thermal Energy??

Internal Energy??

Internal Energy

• Not your Gongfu type of Internal Energy!

Internal Energy

• In thermodynamics, the internal energy of a body is the grand total of all the energies inside a body. i.e. Total of the • kinetic energy due to the motion of molecules

(translational, rotational, vibrational)

• potential energy associated with the vibrational and electric energy of atoms within molecules. It includes the energy in all the chemical bonds, and the energy of the free, conduction electrons in metals.

• Source: wikipedia

Heat vs Internal energy vs Thermal

Energy

∆ total potential energy within the

object •not the same potential energy

as p.e. = mgh (micro, not macro)

∆ total kinetic energy a.k.a

∆themal energy •not the same kinetic energy

as motion of a ball (micro, not macro)

∆average molecular kinetic

energy

X

no. of molecules

∆average molecular potential

energy

X

no. of molecules

Divide by no. of molecules

Macro vs Micro

Object on the ground that has a velocity = 0 m/s

has NO gravitational potential or kinetic energy

as a whole system.

BUT it has internal energy as molecules of the

object are vibrating and moving and there are

bonds between molecules!

Macroscopic Microscopic0 °C

Internal Energy

• Which object has more internal energy?

0 °C

solid

What is the deciding factor/property (in this case)

that causes an object to have more internal energy?

Internal Energy of the object is affected by state of

the object.

0 °C

liquid

Internal Energy

• Which object has more internal energy?

25 °C

solid

30 °C

solid

What is the deciding factor/property (in this case)

that causes an object to have more internal energy?

Internal Energy

• Which object has more internal energy?

25 °C

solid

Remember that temperature can indicate how fast

molecules of the object move.

If they of same temperature, what is the deciding

factor/property (in this case) that determines

which object has more internal energy?

25 °C

solid

Internal Energy

• Which object has more internal energy?30 °C

solid25 °C

solid

What is the deciding factor/property that determines

which object has more internal energy?

Internal Energy of the object is affected by

temperature AND mass of the object.

CHECKPOINTS (5mins think-time)

• Rank in ascending order, in terms of internal energy• (a) a bucket of water at 50oC

• (b) a smaller cup of water at 50oC

• (c) a 120oC spark

• Rank above in terms of molecular kinetic energy

• Rank in terms of internal energy• (a) a bucket of water at 0oC

• (b) a bucket of ice at 0oC

Which Chinese proverb can be an analogy of the comparison between (a) and (c)?

Checkpoint

Correct the following statements.

1. * When heat is absorbed by a substance, the molecules within the substance always gain kinetic energy.

2. *Thermal energy always transfers from a substance of higher internal energy to one of lower internal energy.

3. *Temperature is a measure of the total kinetic energy in a substance (CP Check Yourself Question)

CHECKPOINTS

When heat is absorbed by a substance, the molecules may gain kinetic energy, or potential energy

Thermal energy is always transferred from substance of higher temperature to one of lower temperature.

E.g. When I drop a nail into a basin of water - nail with very high temperature and low internal energy will transfer heat to a basin of water with lower temperature and higher internal energy

Temperature is a measure of the average (not total!) translational kinetic energy of molecules in a substance.

E.g. there is twice as much molecular kinetic energy in 2 liters of boiling water as in 1 liter of boiling water. But the temperatures of both amounts of water are the same because the average kinetic energyper molecule in each is the same.

Internal Energy

• Analogy:

• Class A has 20 students with each score of

40.

• Class B has 5 students with each score of

100.

• Class A has a higher total score of 800 (more

internal energy) even though Class B has

higher quality score (higher temperature)

Transfer of Energy

• The flow of energy is from high temperature to low temperature

• It is NOT from high internal energy to low internal energy.

• E.g. A small hot nail into a large tub of cold water.

• From Kinetic Theory of Matter perspective.

Temperature

• Temperature of an object indicates the AVERAGE kinetic energy of the molecules.

• Previous illustration is a simplistic one. A class would have a variety of scores. Temperature of the “class” will be its average score.

Temperature

• So for an object, even though some molecules will have higher energy level than the rest, the temperature of an object depends on its overall average energy level of the molecules.

Summary

• Heat transfer is a process.

• The energy transfer into object is converted

to raising Internal Energy of the object.

• Internal Energy is the total kinetic, potential,

rotational energy of the molecules.

• Temperature indicates the AVERAGE kinetic

energy of the molecules.

Heat Transfer Processes

Net heat energy will occur from the hotter object to the colder object via three processes:

Conduction, convection, radiation

D.I.Y.

Heat transfer notes