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Heat 14 (01 of 32)

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

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.

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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 As Energy Transfer

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

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(c) Specific heat

of the material

(Q) Thermal

energy added

(DT) Change in

temperature

(m) Mass of

the object

Heat As Energy Transfer

m

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Chapter 14

Page 387

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Heat As Energy Transfer

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 As Energy Transfer

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 As Energy Transfer

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?

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Heat As Energy Transfer

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)

Calorimetry

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.

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Calorimetry

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

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Mug massmm = 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.

1. Draw sketch of problem.

2. List given information.

3. List what is to be found:

Heat Gain or Loss:

Q = mc Dt

Copper: cm = 390 J/kg C0

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

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:

QT =mmcmDt + mwcw Dt

Copper: cm = 390 J/kg C0

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

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

Calorimetry

m1

m2

TH

TL

Conservation of thermal energy:

Final Temperature:

Heat 14 (13 of 32)

(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.

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Table of latent heats

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

Latent 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

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Latent 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

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Latent 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

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Latent 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?

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Conduction

x

Heat conduction can be visualized as occurring through molecular collisions.

The heat flow per unit time is given by:

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Conduction

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.

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Conduction

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)

Conduction

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?

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Conduction

Chapter 14

Page 397

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

Where, L is the thickness of the material.

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Summary

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.

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Summary

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.

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Summary

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

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

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Internal energy of an ideal (monatomic) gas:

kinetic energy in terms

of the temperature

Internal Energy

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END