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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

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 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

Heat 14 (13 of 32)

Heat As Energy Transfer

m

Heat 14 (13 of 32)

Chapter 14 Page 387

Heat 14 (13 of 32)

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?

Heat 14 (13 of 32)

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.

Heat 14 (13 of 32)

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

Heat 14 (13 of 32)

Example 1 A 500-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 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 mmcm Dt mwcw Dt

6. Look up specific heats in tables

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

Calorimetry

m1

m2

TH

TL

Conservation of thermal energy

Final Temperature

Heat 14 (13 of 32)

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

Table of latent heats The following table shows

the latent heats and change of phase temperatures

of some common fluids and gases.

Substance Latent Heat Fusion kJ/kg Melting Point C Latent Heat Vaporization kJ/kg Boiling Point C

Alcohol, ethyl 108 -114 855 78.3

Ammonia 339 -75 1369 -33.34

Carbon dioxide 184 -78 574 -57

Helium 21 -268.93

Hydrogen(2) 58 -259 455 -253

Lead8 24.5 327.5 871 1750

Nitrogen 25.7 -210 200 -196

Oxygen 13.9 -219 213 -183

R134a -101 215.9 -26.6

Toluene -93 351 110.6

Turpentine 293

Water 334 0 2260 100

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

1.0 kg of material from liquid to vapor

Chapter 14 - Page 392

Heat 14 (13 of 32)

Latent Heat

Heat 14 (13 of 32)

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

Heat 14 (13 of 32)

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?

Heat 14 (13 of 32)

Conduction

Heat conduction can be visualized as occurring

through molecular collisions.

The heat flow per unit time is given by

Heat 14 (13 of 32)

Conduction

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)

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?

Heat 14 (13 of 32)

Conduction

Building materials are measured using R-values

rather than thermal conductivity

Where, L is the thickness of the material.

Heat 14 (13 of 32)

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.

Heat 14 (13 of 32)

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.

Heat 14 (13 of 32)

Summary

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)

Internal Energy

Heat 14 (13 of 32)

END