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Heat. Physics Lecture Notes. Heat 14 (01 of 32). Heat. 1) Heat As Energy Transfer. 2) Internal Energy. 3) Specific Heat. 4) Calorimetry. 5) Latent Heat. 6) Heat Transfer: Conduction. Heat 14 (02 of 32). Heat As Energy Transfer. Heat is random thermal Energy.

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Heat 14 01 of 32

Heat

Physics Lecture Notes

Heat 14 (01 of 32)


Heat 14 01 of 32

Heat

1) Heat As Energy Transfer

2) Internal Energy

3) Specific Heat

4) Calorimetry

5) Latent Heat

6) Heat Transfer: Conduction

Heat 14 (02 of 32)


Heat 14 01 of 32

Heat As Energy Transfer

Heat is random thermal Energy

Unit of heat: calorie (cal)

1 cal is the amount of heat necessary to raise the temperature of 1 g of water by 1 Celsius degree.

1 kcal is the amount of heat necessary to raise the temperature of 1 kg of water by 1 Celsius degree.

Heat 14 (13 of 32)


Heat 14 01 of 32

The apparatus below is used to determine the mechanical equivalent of heat:

Heat As Energy Transfer

Heat is a form of energy and can be equated to mechanical energy.

Heat 14 (13 of 32)


Heat 14 01 of 32

Heat As Energy Transfer equivalent of heat:

Definition of heat:

Heat is thermal energy transferred from one object to another because of a difference in temperature.

The sum total of all the energy of all the molecules in a substance is its internal (or thermal) energy.

Temperature: measures molecules’ average kinetic energy

Internal energy: total energy of all molecules

Heat: transfer of energy due to difference in temperature

Heat 14 (13 of 32)


Heat 14 01 of 32

( equivalent of heat:c) Specific heat

of the material

(Q) Thermal

energy added

(DT) Change in

temperature

(m) Mass of

the object

Heat As Energy Transfer

m

Heat 14 (13 of 32)


Heat 14 01 of 32

Chapter 14 equivalent of heat:

Page 387

Heat 14 (13 of 32)


Heat 14 01 of 32

Heat As Energy Transfer equivalent of heat:

Problem:

A 50 g piece of cadmium is at 20 oC. If 400 J of heat

is added to the cadmium, what is its final temperature

Heat 14 (13 of 32)


Heat 14 01 of 32

Heat As Energy Transfer equivalent of heat:

Problem:

A 100 g lead bullet traveling at 300 m/s is stopped by a large tree. Half the kinetic energy of the bullet is transformed into heat energy and remains with the the bullet while the other half is transmitted to the tree. What is the increase in temperature of the bullet?

Heat 14 (13 of 32)


Heat 14 01 of 32

Heat As Energy Transfer equivalent of heat:

Problem:

A 3.0 kg block of iron is dropped from rest from the top of a cliff. When the block hits the ground it is observed that its temperature increases by 0.50 oC. Assume that all the potential energy is used to heat the block. How high is the cliff?

Heat 14 (13 of 32)


Heat 14 01 of 32

Heat As Energy Transfer equivalent of heat:

Problem:

A 1.5 kg copper block is given an initial speed of 30 m/s on

a rough horizontal surface Because of friction, the block finally comes to rest. If the block absorbs 85% of its initial kinetic energy in the form of heat, Calculate its increase in temperature?

Heat 14 (13 of 32)


Heat 14 01 of 32

Calorimetry equivalent of heat:

Problem:

A 0.40 kg iron horseshoe that is initially at 500 oC is dropped into a bucket containing 20 kg of water at 22 oC. What is the final equilibrium temperature?

Neglect any heat transfer to for from the surroundings.

Heat 14 (13 of 32)


Heat 14 01 of 32

Calorimetry equivalent of heat:

Problem:

A 200 g block of copper at a temperature of 90 oC is dropped into 400 g of water at 27 oC. The water is contained in a 300 g glass container. What is the final temperature of the mixture

Heat 14 (13 of 32)


Heat 14 01 of 32

Mug mass equivalent of heat:mm = 0.500 kg

Coffee massmc = 0.200 kg

Initial temperature of coffee and mug:t0 = 200C

Final temperature of coffee and mug:tf = 960C

Total heat to raise

temperature

of coffee (water) and mug to 960C.

Example 1:A500-g copper coffee mug is filled with 200-g of coffee. How much heat was required to heat cup and coffee from 20 to 960C?

1. Draw sketch of problem.

2. List given information.

3. List what is to be found:


Example 1 cont how much heat needed to heat cup and coffee from 20 to 96 0 c m m 0 2 kg m w 0 5 kg

Heat Gain or Loss: equivalent of heat:

Q = mc Dt

Copper: cm = 390 J/kg C0

Coffee (water): cw = 4186 J/kg C0

Example 1(Cont.):How much heat needed to heat cup and coffee from 20 to 960C?mm = 0.2 kg; mw = 0.5 kg.

4. Recall applicable formula or law:

5. Decide that TOTAL heat is that required to raise temperature of mug and water (coffee). Write equation.

QT =mmcmDt + mwcw Dt

6. Look up specific heats in tables:


Example 1 cont how much heat needed to heat cup and coffee from 20 to 96 0 c m c 0 2 kg m w 0 5 kg

Q equivalent of heat:T =mmcmDt + mwcw Dt

Copper: cm = 390 J/kg C0

Coffee (water): cw = 4186 J/kg C0

Example 1(Cont.):How much heat needed to heat cup and coffee from 20 to 960C?mc = 0.2 kg; mw = 0.5 kg.

7. Substitute info / solve problem:

Water: (0.20 kg)(4186 J/kgC0)(76 C0)

Dt = 960C - 200C = 76 C0

Cup: (0.50 kg)(390 J/kgC0)(76 C0)

QT = 78.4 kJ

QT= 63,600 J + 14,800 J


Heat 14 01 of 32

Calorimetry equivalent of heat:

m1

m2

TH

TL

Conservation of thermal energy:

Final Temperature:

Heat 14 (13 of 32)


Heat 14 01 of 32

( equivalent of heat:m) Mass of

the object

(L) Latent heat

of the fusion or

vaporization

(Q) Thermal

energy added

Latent Heat - Stored / Hidden

Energy is required for a material to change phase,

Even though its temperature is not changing.

Heat 14 (13 of 32)


The water problem
The water problem equivalent of heat:


Heat 14 01 of 32

Table of latent heats equivalent of heat:

The following table shows the latent heats and change of phase temperatures of some common fluids and gases.


Heat 14 01 of 32

Latent Heat equivalent of heat:

Heat of fusion, Lf: heat required to change 1.0 kg of material from solid to liquid

Heat of vaporization, Lv: heat required to change 1.0kg of material from liquid to vapor

Chapter 14 - Page 392

Heat 14 (13 of 32)


Heat 14 01 of 32

Latent Heat equivalent of heat:

Q2= mLf

Q4= mLv

3.33 x 105 J

22.6 x 105 J

1 kg

Ice

-50 oC

150

100

50

0

-50

Q1= mcIDt

Q3= mcWDt

Q5= mcSDt

1.05 x 105 J

4.19 x 105 J

1.01 x 105 J

Heat 14 (13 of 32)


Heat 14 01 of 32

Latent Heat equivalent of heat:

Heat required to convert 1 kg of ice

at -50 oC to steam at 150 oC

Q1 = 1.05 x 105 J

Q2 = 3.33 x 105 J

Q3 = 4.19 x 105 J

Q4 = 22.6 x 105 J

Q5 = 1.01 x 105 J

3.22 x 106 J

Heat 14 (13 of 32)


Heat 14 01 of 32

Latent Heat equivalent of heat:

Problem:

A large block of ice at 0 oC has a hole chipped in it, and 400 g of aluminum pellets at a temperature of 30 oC are poured into the hole. How much of the ice melts?

Heat 14 (13 of 32)


Heat 14 01 of 32

Conduction equivalent of heat:

x

Heat conduction can be visualized as occurring through molecular collisions.

The heat flow per unit time is given by:

Heat 14 (13 of 32)


Heat 14 01 of 32

Conduction equivalent of heat:

Chapter 14

Page 396

The constant k is called the thermal conductivity.

Materials with large k are called conductors; those with small k are called insulators.

Heat 14 (13 of 32)


Heat 14 01 of 32

Conduction equivalent of heat:

Problem:

A window has a glass surface of 1.6 x 103 cm2 and a thickness of 3.0 mm. Find the rate of heat transfer by conduction through this pane when the temperature of the inside surface of the glass is 20 oC and the outside temperature is 40 oC.

Heat 14 (13 of 32)


Heat 14 01 of 32

Conduction equivalent of heat:

Problem:

A glass window pane has an area of 3.0 m2 and a thickness of 0.60 cm. If the temperature difference between its faces is 25 oC, how much heat flows through the window per hour?

Heat 14 (13 of 32)


Heat 14 01 of 32

Conduction equivalent of heat:

Chapter 14

Page 397

Building materials are measured using R−values rather than thermal conductivity:

Where, L is the thickness of the material.

Heat 14 (13 of 32)


Heat 14 01 of 32

Summary equivalent of heat:

Internal energy U refers to the total energy of all molecules in an object. For an ideal monatomic gas,

Heat is the transfer of energy from one object to another due to a temperature difference. Heat can be measured in joules or in calories.

Specific heat of a substance is the energy required to change the temperature of a fixed amount of matter by 1° C.

Heat 14 (13 of 32)


Heat 14 01 of 32

Summary equivalent of heat:

In an isolated system, heat gained by one part of the system must be lost by another.

Calorimetry measures heat exchange quantitatively.

Energy in involved in phase changes even though the temperature does not change.

Heat of fusion: amount of energy required to melt 1 kg of material.

Heat of vaporization: amount of energy required to change 1 kg of material from liquid to vapor.

Heat 14 (13 of 32)


Heat 14 01 of 32

Summary equivalent of heat:

Heat transfer takes place by conduction, convection, and radiation.

In conduction, energy is transferred through the collisions of molecules in the substance.

Heat 14 (13 of 32)


Heat 14 01 of 32

Internal energy of an ideal (monatomic) gas: equivalent of heat:

kinetic energy in terms

of the temperature

Internal Energy

Heat 14 (13 of 32)


Heat 14 01 of 32

END equivalent of heat: