heat and work

UNIT 4

MODULE 3:

HEAT, WORK AND ENERGY

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JANET BRIGIDA A. CATIPONMHS Teacher, MAE-Chemistry

OBJECTIVES

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1. Define Thermodynamics and Internal Energy

2. Explain the 1st law of thermodynamics.3. Demonstrate that heat can be turned

into work.4. Predict the convention signs of heat and

work in a system.5. Calculate the change in internal energy

within a system.

WHAT IS THIS?3

LET’S PLAY A GAME

“FOUR PICS – ONE WORD”

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HEAT (Q)

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HEAT is the energy transferred from one object to another due to their temperature difference.Where:

Q = Heat in Joule, Caloriem = mass in gramsc = specific heat capacity, J/g-⁰C

∆T = Change in Temperature1 cal = 4.184 Joule

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WORK (W)

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WORK-The energy transferred when an object is moved against force. W = F x D

Where: W = work in JouleF = Force in NewtonD = Distance in Meter

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THERMODYNAMICS

A branch of physics that deals with the physical

laws that relate heat and mechanical work.

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FIRST LAW OF THERMODYNAMICS

CONSERVATION OF ENERGY HEAT AND WORKSYSTEM-SURROUNDINGSINTERNAL ENERGY

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FIRST LAW OF THERMODYNAMICS

Conservation of Energy

• states that the total energy of an isolated system is constant; energy can be transformed from one

form to another, but cannot be created or destroyed. 

• The 1st law of Thermodynamics provides a generalization of this Law Of Conservation Of Energy

in terms of the relationship among the heat transferred to a system.

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SYSTEM-SURROUNDINGS DIAGRAM

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SYSTEM

SURROUNDINGS

BOUNDARY

W < 0 (-)

W >0 (+)

Q > 0(+)

Q < 0 (-)

UNIVERSE

TERMS ON THE SUR-SYS DIAGRAM

• Universe – the observable, physical world.

• System – the limited, defined part of the universe

• Boundary -  the edge of a thermodynamic system across which heat, mass, or work can flow.

• Surroundings – all parts of the universe not included in the defined system.

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ENERGY TRANSFER (HEAT & WORK)

• Open system –allows matter and energy to enter or/and leave the system• Closed system–is a system wherein in no

matter enters or leaves the system. It only interacts with its surroundings in terms of energy transfers.• Isolated system – is a system where

neither matter nor energy enters or leaves the system. 14

INTERNAL ENERGY (U)

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It refers to energy contained within the system.

The internal energy of a system can be changed by (1) heating the system, or (2) by doing work on it, or (3) by adding or taking away matter.

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INTERNAL ENERGY

REMEMBER !!!

• Q > 0 (+) heat transfer to the system• Q < 0 (-) heat transfer from the

system• W < 0 (-) is work done on the system• W > 0 (+) is work done by the

system

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FIRST LAW OF THERMODYNAMICS (∆U)

• The change in the internal energy of a closed system is equal to the amount of heat applied to the system (Q), minus the amount of work(W) done by the system on its surroundings. 

∆U = Q - W18

EXAMPLE

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How To Launch a Chinese Sky Lantern.mp4

UNITS OF HEAT & WORK

• Joule or KiloJoule

• Calorie or Kilocalorie

• BTU

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EXPLANATION TO THE GIVEN EXAMPLE

• Heat (Q) is added to the system, therefore the system absorbs heat causing expansion within the system. (Q is positive)

• Work (W) it expands while it is being heated. (W is negative)

• ∆U, the change in Internal Energy, if the heat added is 30kJ, and the Work done by the system is 15kJ, therefore, ∆U is 45kJ.

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APPLICATION OF THE1ST LAW OF THERMODYNAMICS

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APPLICATION

• Predict the convention signs of heat and work when

1. A gas-filled balloon is heated over a flame

2. Water is heated to the point of vaporization

3. A hot iron bar is placed in cool water. 23

ANSWER

1. Heat is positive and work is negative.

2. Heat is positive and work is negative.

3. Heat is negative and the work is positive.

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APPLICATION

• Added heat of 51 kJ, Work of 15 kJ done by the system• Added heat of 100 kJ, Work of 65

kJ done by the system• Released heat of 65 kJ, Work of

20 kJ done by the system25

Calculate ∆U for each of the following cases:

ANSWER•∆U = +36 kJ•∆U = +35 KJ•∆U = -85 KJ

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SUMMARIZE IT...!!!

• What is Heat, Work and Thermodynamics?• What is the 1st Law of Thermodynamics?

Explain it.• How can you determine the sign of heat and

work if they are done by or on the system?• How to compute for the Internal Energy of a

system?

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EVALUATION

A. Prediction of Convention Signs (2pts each)• A system undergoes a process consisting of the

following two steps:Step 1: The system absorbs 73 J of heat while 35 J of work is done on it.Step 2: The system absorbs 35 J of heat while performing 72 J of work.

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ANSWER

1.Heat is positive and Work is positive.

2.Heat is positive and Work is negative.

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EVALUATION

B. Problem Solving• A system releases 125 kJ of heat while 104 kJ of

work is done on the system. Calculate the change in internal energy (in kJ). (2pts)• Calculate w for a system that absorbs 260 kJ of

heat and for which ∆U= 157 kJ. Is the work done on or by the system? Does the system expand or contract? (4pts)

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ANSWER

1. ∆U = -21kJ2. W = 103 kJ, work

done on the system

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DO NOT FORGET...• Prepare the following materials for

tomorrow’s activity.1. Water2. Beaker3. Tripod4. Burner/alcohol lamp5. Match

• Read about Thermodynamic Engines32

WORDS TO LIVE BY….

Being busy does not always mean real work. The object of all work is

production or accomplishment and to either of these ends there must be

forethought, system, planning, intelligence, and honest purpose, as

well as perspiration. Seeming to do is not doing…

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