1

Chapter 5

TEMPERATURE

AND HEAT

Dr. Babar Ali

2

CHAPTER OUTLINE

Temperature & Heat

Kinetic Theory of Matter

Specific Heat Calculating Heat

Phase Changes Latent Heat Heat Transfer Thermal Expansion Thermal Properties

of Water

3

TEMPERATURE

Temperature is a measure of how hot or cold a substance is.

Thermometer is an instrument used for measuring temperature, and is based on thermometric properties of matter.

Three scales are used for measuring temperature.

4

TEMPERATURE SCALES

32 - 212

Fahrenheit

Celsius

0 - 100

Kelvin

273 - 373

5

Example 1:

The melting point of silver is 961ºC. What is this temperature in Kelvin?

TK = TC + 273

TK = 961 + 273 = 1234 K

6

Example 2:

Pure iron melts at 1800 K. What is this temperature in Celsius?

TC = TK - 273

TC = 1800 - 273 = 1527 C

7

Example 3:

On a winter day, the temperature is 5 ºF. What is this temperature on the Celsius scale?

TC = [(TF + 40) ÷ 1.8]- 40

TC = [(5 + 40) ÷ 1.8]- 40 = -15

See also Eq. 5.2 (a, b)

8

KINETIC THEORY OF MATTER

All matter is composed of tiny particles (molecules) that are in constant motion.

9

KINETIC THEORY OF MATTER

Solid particles are closely packed with little motion. Solids have definite volume and definite shape.

Liquid particles are loosely packed and have some motion. Liquids have definite volume, but indefinite shape.

Gas particles are far apart from one another and have great motion. Gases have indefinite shape and volume.

10

KINETIC THEORYOF MATTER

Increase in T increases the motion of molecules in any state.

For example, evaporation of liquids results from loss of fast moving molecules @ surface.

11

TEMPERATURE & HEAT

Heat is the Ethermal that is transferred from one body to another because of T difference between the bodies.

Heat flow occurs from high T to low T.

T (temperature) is the average Ekinetic of molecules in a substance. See, Kinetic Molecular Theory - Heat

12

CONCEPTCHECK 1

Consider a cup of hot coffee and a frozen lake.

Which has higher T?

Hot coffee has a higher

temperature than a frozen lake

Which has more heat?

Frozen lake has more heat because it contains many more particles

13

CONCEPTCHECK 2

On a cold day when someone leaves a door open, it is often said that the person is letting in the cold air. Is this correct?

No, heat travels from higher to lower T. heat (H) is going out rather than cold in!

14

MEASURING HEAT

Heat (H) is a physical quantity that can be measured. The science of heat measurement is known as calorimetry.

Quantity of H is to: 1) the amount of substance 2) its T, and 3) the nature of the substance.

The SI unit of H is joules (J). Another convenient unit for H is calorie.

15

Concept Check

It takes 5× as much heat to boil 5 kg of water than 1 kg.

Which container requires more heat to reach the same

temperature?

Mass and heat are directly

proportional

16

Concept Check

Although the same amount of heat is added to both containers, the container with less water gets hotter.

Using the same amount of H, which container reaches a

higher T?

Mass and temp. are indirectly proportional

17

SPECIFIC HEAT

Different materials absorb or lose H differently. i.e., the filling of a hot apple pie may be too hot to

eat, whereas the crust is not.

The Specific heat of a substance is the amount of H required to T of 1 gram of that substance by 1 C. Units are J/gC (or cal/gC)

18

CALCULATINGHEAT

Heat changes are measured using a calorimeter. Conservation of E: the amount of heat lost by one

body is gained by another.

Mass of substance

Specific heat of substance

Change in its temperature

Heat = × ×

H = m × c × T

H gained or lost is based on three quantities:

19

Example 1:

m = 20 g

∆T = 75 ºC

c = 0.055 cal/g◦C

H = ???H = 82.5 cal

H = m × c × ∆T

H= (20 g)(0.055 cal/gºC)(75 ºC)

How much H must be supplied to 20 g of tin to raise its T from 25C to 100C? Specific heat (c) of tin is 0.055 cal/g ºC.

20

Example 2:

Calculate the specific heat of a solid if 1638 J of heat raises the temperature of 125 g of the solid from 25.0 to 52.6 C.

Hc =

m x Tm = 125 g

∆T = 27.6 ºC

c = ???

H = 1638 J

c = 0.475 J/g ºC

1638 J=

125 g x 27.6 Co

21

PHASE CHANGES

When matter releases or absorbs E without a change in T, phase change occurs

Liquid

Gas

Solid

Condensation

Freezing

Evaporation

Melting

Sublimation Deposition

Arrows up: E inArrows down: E out

22

T vs. HFor Water

When heat is added to ice, it absorbs the H without a ΔT causing a phase change

Similarly, when heat is added to hot water, a phase change occurs without an in T

23

LATENT HEATOF FUSION

The quantity of H required to melt 1 g of solid is called the latent heat of fusion (Lf).

Lf for ice is 80 cal/g

24

LATENT HEATOF VAPORIZATION

The quantity of H required to evaporate 1 g of liquid is called the latent heat of vaporization (Lv).

Lv for water is 540 cal/g

25

HEAT DURING A PHASE CHANGE

The amount of H, released or absorbed during phase change, depends on the amount of substance and its latent heat.

H = m Lf H = m Lvand

26

Example 1:

m = 50g

Lf = 80 cal/g

H = ??? H = 4000 cal

How much heat is required to melt 50 g of ice at 0C?

H = m × Lf

H = 50 g × 80 cal/g

27

Example 2:

m = 50g

Lv= 540 cal/g

H = ??? H = 27000 cal

How much heat is required to vaporize 50 g of water at 100C?

H = m × Lv

H = 50 g × 540 cal/g

28

Example 3:

Calculate the amount of H required to change 20 g of ice at 0C to water at 10C.

melting ice

H warming water

HTotalH = +

Solid-Liquid

Liquid

29

Example 3:

= m × Lf

= 20 g × 80 cal/g = 1600 cal

= m × c × ∆T

= 20 g × 1.0 cal/gºC × 10 ºC = 200 cal

melting ice

H

warming water

H

Htotal = 1600 cal + 200 cal = 1800 cal

30

HEAT TRANSFER

When there is a T difference between two objects, heat flows from warmer cooler

Heat (Energy) Transfer occurs by one of 3 methods:

1. Conduction

2. Convection

3. Radiation

31

1. Conduction

Conduction is transfer of heat by contact between two objects through molecular collisions

Metals are good conductors of heat, while glass and wood are poor conductors (insulators).

32

2. Convection

Convection is transfer of heat by actual motion of molecules

Liquids and gases transfer heat mainly by convection.

33

3. Radiation

Radiation is transfer of heat without molecules

Heat from the sun reaches the earth through space by radiation.

34

THERMAL EXPANSION

All matter expands when heated, due to molecular motion which causes them to separate from each other. the more molecular motion, the more

expansion (volume)

Gases > Liquids > Solids

35

THERMAL EXPANSIONOF SOLIDS

Solids expand upon in T, based on three factors:

1) increase in temperature

2) the original length

3) type of material

36

THERMAL EXPANSIONOF SOLIDS

A practical use of thermal expansion of metals is use of bimetallic strip in operation of a thermostat.

37

THERMAL EXPANSIONOF WATER

When water at 0C is heated, it first contracts and then expands

Water has its smallest volume at 4C.

water has its highest density @ 4ºC.

38

More on WATER

Thermal behavior of water is important in preserving aquatic life in colder climates.

As water cools on surface, it sinks due to its high density, allowing for water circulation

@ T < 4C, it becomes less dense and rises to the surface

Freezing occurs at top, allowing for marine life to exist below the surface.

39

THERMAL CIRCULATIONS

The difference in the specific heat of water and land cause the land to warm up more quickly during the day.

The less dense warm air rises, and is replaced by cool air from the sea, causing a sea breeze.

40

THERMAL CIRCULATIONS

During the night, the land cools faster than the sea, reversing the process, and causing a land breeze.

41

THE END

Some Symbols Used

• all• there is or there exists• Change• infinity• multiplication• proportional to• less than or equal to• greater than or equal to• plus or minus• degrees• therefore• ≈ approximately equal to• decreases• increases

• E – Energy• H – Heat• e- - electron• p+ - proton

F. Morales Unit 1 42