chapter 17 energy in thermal processes: first law of thermodynamics

Chapter 17

Energy in Thermal Processes:

First Law of Thermodynamics

2

17.1 Thermodynamics – Historical Background Until around 1850, thermodynamics and

mechanics were considered to be two distinct branches of science. Experiments by James Joule and others showed a

connection between the two branches of science. A connection was found to be between the

transfer of energy by heat in thermal processes and the transfer of energy by work in mechanical processes

3

Internal Energy Internal energy is all the energy of a

system that is associated with its microscopic components These components are its atoms and

molecules The system is viewed from a reference

frame at rest with respect to the system

4

Internal Energy and Other Energies Internal energy includes kinetic

energies due to Random translational motion Rotational motion Vibrational motion

Internal energy also includes intermolecular potential energy

5

Heat Heat is a mechanism by which energy is

transferred between a system and its environment due to a temperature difference between them

Heat flows from the one at higher temperature to the one at lower temperature.

Heat is also represented by the amount of energy, Q, transferred by this mechanism

6

Units of Heat Calorie is the unit used for heat

One calorie is the heat necessary to raise the temperature of 1 g of water from 14.5o C to 15.5o C

The Calorie used for food is actually 1 kcal

In the US Customary system, the unit is a BTU (British Thermal Unit)

One BTU is the heat required to raise the temperature of 1 lb of water from 63o F to 64o F

1 cal = 4.186 J

7

17.2 Specific Heat Specific heat, c, is the heat amount of

energy per unit mass required to change the temperature of a substance by 1°C

If energy Q transfers to a sample of a substance of mass m and the temperature changes by T, then the specific heat is

Qc

m T

Q = m c TUnits of specific heat are J/kg°C

8

Some Specific Heat Values

9

More Specific Heat Values

10

Sign Conventions If the temperature increases:

Q and T are positive Energy transfers into the system

If the temperature decreases: Q and T are negative Energy transfers out of the system

11

Specific Heat of Water Water has a high specific heat relative to

most common materials Hydrogen and helium have higher specific heats

This is responsible for many weather phenomena Moderate temperatures near large bodies of water Global wind systems Land and sea breezes

12

Calorimetry One technique for measuring specific

heat involves heating a material, adding it to a sample of water, and recording the final temperature

This technique is known as calorimetry A calorimeter is a device for the

measurement

13

Calorimetry, cont The system of the sample and the water is

isolated The calorimeter allows no energy to enter or leave

the system Conservation of energy requires that the

amount of energy that leaves the sample equals the amount of energy that enters the water Conservation of Energy gives a mathematical

expression of this: Qcold= - Qhot

14

Calorimetry, final The negative sign in the equation is critical for

consistency with the established sign convention

Since each Q = m c T, cunknown can be found

Technically, the mass of the water’s container should be included, but if mw>>mcontainer

it can be neglected

w w wx

x x

m c T Tc

m T T

15

16

17

Phase Changes A phase change is when a substance

changes from one form to another Two common phase changes are

Solid to liquid (melting) Liquid to gas (boiling)

During a phase change, there is no change in temperature of the substance

18

17.3 Latent Heat During phase changes, the energy added to a

substance modifies or breaks the bonds between the molecules or makes different molecular arrangements.

The amount of added energy also depends on the mass of the sample

If an amount of energy Q is required to change the phase of a sample of mass m,

Q mL

19

Latent Heat, cont The quantity L, called the latent heat of

the substance, is the energy required for transforming the substance per unit mass from one phase to another phase

Latent means hidden The value of L depends on the substance

as well as the actual phase change

20

Latent Heat, final The latent heat of fusion, Lf, is used during

melting or freezing The latent heat of vaporization, Lv, is used when

the phase change boiling or condensing The positive sign is used when the energy is

transferred into the system This will result in melting or boiling

The negative sign is used when energy is transferred out of the system This will result in freezing or condensation

21

Sample Latent Heat Values

22

Graph of Ice to Steam

23

Warming Ice, Graph Part A Start with one gram of ice

at –30.0º C During A, the temperature

of the ice changes from –30.0º C to 0º C

Use Q = m cice ΔT Cice= 2090 J/kg º C In this case, 62.7 J of

energy are added

24

Melting Ice, Graph Part B Once at 0º C, the phase

change (melting) starts The temperature stays

the same although energy is still being added

Use Q = m Lf The energy required is 333 J On the graph, the values move

from 62.7 J to 396 J

25

Warming Water, Graph Part C Between 0º C and 100º C,

the material is liquid and no phase changes take place

Energy added increases the temperature

Use Q = m cwater ΔT 419 J are added The total is now 815 J

26

Boiling Water, Graph Part D At 100º C, a phase

change occurs (boiling)

Temperature does not change

Use Q = m Lv This requires2260 J The total is now

3070 J

27

Heating Steam After all the water is converted

to steam, the steam will heat up

No phase change occurs The added energy goes to

increasing the temperature Use Q = m csteam ΔT

In this case, 40.2 J are needed The temperature is going to 120o C The total is now 3110 J

28

29