Thermal Properties of Matter

I. Temperature, Heat, Internal Energy and the 0th Law of Thermodynamics

A. Temperature- measure of hot and cold- T is directly related to the average KE of the particles- T KE ̴̴�� avg

What is kinetic energy KE?- KE = ½ mv2 = p2/2m- higher KE --> faster particles --> higher T- lower KE --> slower particles --> lower T

Absolute temperature scale: Kelvin scale, KWhat does absolute mean?

At absolute zero, 0 K, the particles reach minimum speedIf the Kelvin temp doubles, the Keavg doubles.If the Kelvin temp triples, the Keavg triples.

(atoms and molecules)

It just sounds so sad!

The Celsius temperature scale, C, has the same size degreesas the Kelvin scale, but it is not “absolute.”

0 0C = water freezes/melts100 0C = water boils/condenses

T (K) = T (0C) + 273

Ex. At what Kelvin temps does water melt and condense?

Ex. What is 0 K in Celsius degrees?

300 K 400 K

Place in contact.

In which direction does thermal energy (heat) flow?

What 1 factor determines the direction of energy flow?

Ex. Suppose you have two objects at different temps.

The Second Law of Thermodynamics (Study of Heat): Heat flows spontaneously (naturally) from hotter to colder,

and not the other way around.

T

300 K400 K

B. What is Heat?

definition: Heat is energy that is transferred from one body to another because of a ΔT

Ex. Name 3 basic ways that energy can be transferred:conduction, convection, radiation

Which way ….

…can transfer energy in a vacuum?

…must involve direct contact between two bodies?

…relies on density differences?

Can heat be transferred by all three of these ways?

radiation

conduction

convection

So heat is the transfer of energy. Question: What kind of energy?

C. Answer: Internal energy = the sum total of all the energy of all the molecules in an object.

The internal KE of molecules occurs b/c they can randomly -translate (move in 1 direction)-rotate (spin)-vibrate

The internal PE of molecules is there b/c they exert electrical forces on other molecules --> bonding

Internal energy = total PE of bonds + random KE of the molecules

random KEand PE

gravitational PE

external KE

of molecules

X

X X

For any object:

Ex. Which body has molecules with a higher avg KE?

300 K 310 K

300 K

Ex. Which body has more internal energy?

310 K

Ex. The mechanical equivalent of heat.

4.184 J of mechanical (motional) work done by the falling weight was transferred to the internal KE energy of water, raising the temperature of 1 gram of it by 1 0C.

4.184 J = 1 calorie

Experiment carried out by the English brewer: James Prescott

Joule

Ex. Is it correct to say the “heat in a body?”No. Heat refers to the transfer of energy, or the amount of

energy that is transferred between…. 1. two bodies, or between2. a body (or “system” of bodies) and its

environment. due to a ΔT.

This type of energy transfer is called non-mechanical,because the bodies themselves are not exerting forceson each other.

In IB, heat is called thermal energy. This means that, in IB, it is also very naughty to use the words, “thermal energy in a body.”

Trick IB question: Who has more thermal energy in her, a mommy polar bear at 20 0C or her cub at 22 0C?

Answer: The mom may have more internal energy, but thermalenergy is heat, which is really the transfer of energy

And what color is their skin?

D. Thermal Equilibrium300 K

400 K

Two objects are said to be in thermal equilibrium if,when placed in contact with each other,

1/ no energy flows from one to the other, and2/ their temperatures do not change.

Place in contact. Wait. What happens eventually?

Suppose you start outwith two bodies atdifferent temps -->

They will reach the same temperature!

AC

Ex. Suppose C and A are two objects in equilibrium:

BC

Then you bring C over to B, and they just happen to be in equilibrium, too.

What does this tell you about A and B?The 0th (yes, the zeroth!) Law of Thermodynamics:

If two systems (like A and B above) are in thermal equilibriumwith a third system (like C above), then they are in equilibrium witheach other.

This allows the concept of temperature to be useful, because C could be used like a thermometer.