Chapter 9 - Thermal Energy

Sections 9.1, 9.2, & 9.3

ObjectivesBy the end of class today, you will be able to:

Define and describe thermal energy

Describe the relationship between temperature and kinetic energy

PS F-11, 15, 17

9.1 - Thermal Energy

All matter is made up of constantly moving atoms or molecules

The motion of a substance’s molecules determines if it is a solid, a liquid, or a gas

When molecules are made to move faster, they gain kinetic energy.

9.1 - Thermal Energy

In general, the warmer an object is, the more kinetic energy its atoms and molecules possess.

We can also say that the warmer an object gets, the more thermal energy it contains.

9.1 - Thermal Energy

What is thermal energy?

The total energy of all an object’s atoms and molecules

Thermal energy consists of both the potential and kinetic energy of the particles in a substance as they move around.

9.2 - Temperature

Why do we measure an object’s temperature?

To be able to tell how warm or cold it is.

A thermometer measures by expansion or contraction of a liquid, usually colored alcohol.

9.2 - Temperature

Temperature Scales -

Celsius Scale - O° is the freezing point of H₂O and 100° is the boiling point of H₂O.

Fahrenheit Scale - 32° is the freezing point of H₂O and 212° is the boiling point of H₂O.

9.2 - TemperatureTemperature is related to the random motion of atoms/molecules in a substance.

Temperature is related to the average KE of molecular motion.

A substance with a high temperature has molecules with high average KE

The molecules are moving very fast if the object has a high temperature

Which box has a larger temperature?

9.2 - TemperatureConcept Check Question:

True or False -

Cold is the absence of moving molecules.

False!

Cold refers to very-slow moving molecules, not their absence. Without molecules, the concept of temperature is meaningless!!!

9.3 - Absolute Zero

In principle, there is no upper limit to temperature.

As thermal energy keeps increasing, a solid object melts to a liquid and then evaporates into a gas.

9.3 - Absolute Zero

However, there is a definite limit at the lower end of the temperature scale.

This limit is called absolute zero!

9.3 - Absolute ZeroIn degrees Celsius, this temperature is -273°!

At this temperature, molecules have lost all available KE.

No more energy can be taken from a substance at absolute zero.

It can’t get any colder!!!!

9.3 - Absolute Zero

The absolute temperature scale is called the Kelvin scale.

Absolute zero is 0 K. (Kelvin)

There are no negative numbers on the Kelvin scale.