Temperature and Thermal Energy:

Part 2

Physics

In this lesson, we will discuss the

following:

Heat

Specific Heat Capacity

Thermal Expansion

Phases of Matter

Definition of Heat

Heat is the flow of thermal energy from one substance to another.There is no HEAT without

FLOWING energy!

Definition of Heat

Thermal energy always spontaneously moves from an object with a higher temperature to an object with a lower temperature.

Hot Object Cold ObjectHeat

Definition of Heat

Thermal energy can be made to flow from lower temperature to higher temperature, but an input of energy is necessary to make it happen.

Cold Object Hot Object

Heat

Abbreviation for Heat

Heat is abbreviated Q.

Q

Acceptable Units for Heat

Because heat is just the transfer of thermal energy, the acceptable units for heat are the same as the acceptable units for thermal energy.

Heat is a Scalar Quantity

• Heat is a scalar quantity, and as a result, heat has no direction.

• Heat can, however, be positive or negative.– If an object’s heat is positive, thermal energy is

flowing into the object.– If an object’s heat is negative, thermal energy is

flowing out of the object.

Definition of Specific Heat Capacity

• Specific heat capacity is defined as the quantity of heat required to change the temperature of 1 unit mass of a substance by 1 degree Celsius.

• Specific heat capacity is sometimes called thermal inertia, or thermal sluggishness.

• Specific heat capacity is sometimes shortened to specific heat.

Definition of Specific Heat Capacity• Different substances have different specific heat

capacities.• If a substance has a large specific heat capacity, it has a

large thermal inertia. In other words, you must put a lot of energy into that substance to increase its temperature a little bit. In addition, you must take out a lot of energy to lower its temperature just a little bit. Water has a very high specific heat capacity.

• If a substance has a small specific heat capacity, you do not need to put a lot of energy into that substance to increase its temperature a lot, and you do not need to take out much energy to lower its temperature a lot.

Abbreviation for Specific Heat Capacity

We will abbreviate specific heat capacity as C.

C

Acceptable Units for Specific Heat

Capacity

Acceptable units for specific heat capacity are in the text box to the right.

Kkg

Calories

Ckg

Calories

Kkg

calories

Ckg

calories

Kkg

Joules

Ckg

Joules

Specific Heat Capacity is a Scalar Quantity

Specific heat capacity is a scalar quantity and has no direction.

Heat Formula Involving Specific Heat Capacity

TmCQ

Heat

Mas

sSpecific Heat Capacity

Change in Temperature

Using Specific Heat Capacity in Calculations

Using Specific Heat Capacity in Calculations

Click HERE if the web

page at the right does not

open.

Definition of Thermal Expansion

Thermal expansion is the increase in volume of a substance when the temperature of the substance is increased (usually).

Definition of Thermal Contraction

Thermal contraction is the decrease in volume of a substance when the temperature of the substance is decreased (usually).

Definition of Thermal Expansion/Contraction

• Substances typically expand when heated because the particles (atoms and molecules) in the substance move faster and spread apart when heated.

• Substances typically contract when cooled because the particles in the substance move slower and get closer together when cooled.

Typical Behavior for Substances for Changes in Temperature

• Usually, substances expand when heated.

• Usually substances contract when heated.

Bridges and sidewalks

have expansion

joints for this very reason.

The Interesting Thermal Expansion of Water

• At most temperatures, water expands when heated and contracts when cooled.

• Water does something interesting between the temperatures of 00 C and 40 C.

• Imagine you start with water at 50 C, and you want to freeze the water. You must lower the temperature of the water.

The Interesting Thermal Expansion

of Water

• When the water reaches 40 C, the water begins to freeze and turn into a microscopic slush.

• As the water freezes, its molecules arrange themselves into a highly-ordered crystalline structure.

• And, water molecules actually move farther apart when they form crystals!

• As a result, when water cools between 40 C and 00 C, it actually expands!

The Interesting Thermal Expansion

of Water

• Liquid water is densest at 40 C.

• Thus, 40 C water will sink to the bottom of any body of liquid water.

• And, water at 40 C will expand whether it is heated or cooled!

The Interesting Thermal Expansion

of Water

• In addition, ice at 00 C is less dense than liquid water.

• In other words, ice floats in a glass of water or on a lake or pond.

Five Known Phases of Matter

• There are currently five known phases of matter:– Bose-Einstein Condensate– Solid– Liquid– Gas– Plasma

• In this class, we will talk primarily about solids, liquids, and gases. We will discuss plasmas a little bit when we talk about stars.

Solid• A solid has a definite shape and a definite volume.

• A solid has a definite shape and volume because the molecules (or atoms) making up a solid are close together and do not move around a lot. The molecules are stuck in one place.

• We say that the molecules in a solid do not translate very quickly, but they can still vibrate and rotate.

Liquid• A liquid has a definite

volume but does not have a definite shape.

• A liquid has a definite volume because the molecules in a liquid are CLOSE enough together that they attract one another.

• A liquid does not have a definite shape because the molecules in a liquid are free to move around (they are not stuck in one place). The molecules in a liquid move around faster than the molecules in a solid.

Gas• A gas does not have a

definite volume or a definite shape.

• A gas does not have a definite volume because the molecules in a gas are so far apart that they do not pull on each other very much at all; the molecules can move as far apart as they want.

• A gas does not have a definite shape because its molecules are free to move around, and they move around really fast.

Melting and Freezing

• Melting is a change of phase from solid to liquid.

• Freezing is a change of phase from liquid to solid.

Evaporation and Condensation

• Evaporation is a change of phase from liquid to gas.

• Condensation is a change of phase from gas to liquid.

Sublimation and Deposition

• Sublimation is a change of phase from solid directly to gas (without changing to liquid in the middle).

• Deposition is a change of phase from gas to solid (without changing to liquid in the middle.

Dry ice undergoes sublimation!

Energy and Phase Changes

Solid Liquid Gas

You must add energy to go this way.

You must take out energy to go this way.