Temperature and Heat - UCSB · 2020. 1. 1. · Copyright © 2008 Pearson Education Inc., publishing...

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

PowerPoint® Lectures for

University Physics, Twelfth Edition

– Hugh D. Young and Roger A. Freedman

Lectures by James Pazun

Chapter 17

Temperature and Heat


Goals for Chapter 17

• To delineate the three different temperature scales

• To describe thermal expansion and thermal stress

• To consider heat, phase changes, and calorimetry

• To study how heat flows with convection,

conduction, and radiation


Introduction

• Growing up in Pittsburgh, molten steel was a common sight. Still it is imposing at 1500oC.

• The worst common burns you can imagine are steam burns. You have not only water heated to its boiling point but gaseous steam carrying the heat of vaporization. It’s a great deal of energy in a small space.


Measuring temperature

• There are many ways to measure

temperature, but the two devices

mentioned below take advantage of

a gas or liquid sample which

expands if heat is added and

contracts if heat is removed.

• A cylinder of gas will show pressure

rise if volume is kept constant.

• A small container of liquid will see

the liquid increase in volume as

temperatures rise. Mercury was

chosen “early on” because it’s so

dense, a small volume can record

large temperature ranges.


The zeroth law of thermodynamics

• Simply stated? “Heat will always travel from a hot reservoir to a cold one without outside energy forcing an unnatural transfer.”


Thermometers? Just our way of trying to see where the heat is

• The measure of temperature is a way of expressing how much heat one object is holding relative to another.

• There are several examples shown at right. You can base a thermometer on thermal expansion of a gas, differential expansion of bimetal strips, even on something as wild as laser-doppler shift.


The coldest we can ever get?

• Early experiments observed changes in pressure or volume as

temperature changed.

• It was noticed that the linear trends lead to a consistent lowest

temperature that we call “absolute zero”—labeled 0K after Lord Kelvin.

• Refer to Example 17.1.


Conversions are expected

• Values on the temperatures scales (Fahrenheit, Centigrade/Celsius,

and Kelvin) may be readily interconverted. Physics professors will

want values to eventually be in Kelvins because that’s the form in

SI units.

• See Figure 17.7 below.


Thermal expansion—linear

• A change in

length will

accompany a

change in

temperature.

The size of the

change will

depend on the

material.


Changing temperature changes atomic spacing

• Molecules can be visualized as bedsprings and spheres. More

heat (higher temperatures) is reflected by the motion of the

atoms relative to each other.

• See Figure 17.9 below.


Coefficients of expansion


Thermal changes in material length and volume

• Refer to Problem-Solving Strategy 17.1.

• Consult Example 17.2 (change in length).

• Consult Example 17.3 (change in length II).

• Consult Example 17.4 (change in volume).


Thermal expansion we see constantly

• Water is interesting. There are no

other liquids that expand to become

less dense as a solid than they are

as a liquid. This is fortunate, if

lakes were to freeze and dense ice

sink to the bottom, everything in

the water would die as the liquid

became solid from the bottom up.

• Thermal expansion joints allow

roads to expand and contract

without any stress to the material

used to build.

• Refer to Example 17.5.


James Joule and the mechanical equivalent of heat

• Joule knew a mass

above the ground had

potential energy. He

dropped an object on a

cord, turning a paddle

in water monitored by

a very accurate

thermometer.

• His conclusion was to

connect energy

conservation (potential

and kinetic) to heat as

a third form observed.


Specific heat

• A specific heat value reveals

how much temperature will

change when a given amount of

a substance absorbs a given

amount of heat.

• Water is a “benchmark” as one

ml of water will absorb 1 cal of

heat to raise its temperature by

1oC.

• Refer to Example 17.6 and

Example 17.7.


Specific heat values


Phase changes and temperature behavior

• A solid will absorb heat according to its heat

capacity, becoming a hotter solid.

• At the melting point, a solid will absorb its

heat of fusion and become a liquid. An

equilibrium mixture of a substance in both its

liquid and solid phases will have a constant

temperature.

• A cold liquid will absorb heat according to its

heat capacity to become a hotter liquid.

• At the boiling point, a liquid will absorb its

heat of vaporization and become a gas. An

equilibrium mixture of liquid and gas will have

a constant temperature.

• A cold gas can absorb heat according to its heat

capacity and become a hotter gas.


Heats of Fusion and Heats of Vaporization


Using well-behaved water to measure other systems

• Because water is a good thermal sink, is readily available, and reproducibly absorbs 4.184 J for every gram to rise in temperature by 1oC, it is often used to measure another object’s change in heat energy by comparison.

• For example, an unknown metal might be massed, raised to a known temperature (say to 100oC in a boiling water bath), then added to a known amount of cold water. The resulting change in the temperature of the water will allow heat absorbed to be calculated and then the heat capacity of the unknown metal.


Heat calculations

• Follow Problem-Solving Strategy 17.2.

• Refer to Example 17.8 (no phase change).

• Refer to Example 17.9 (changes in both temperature and

phase).

• Refer to Example 17.10 (an example that could be done in a

kitchen).

• Refer to Example 17.11 (combustion, temperature change,

and phase change).


Why, and how well, do materials transfer heat?

• Figure 17.23 illustrates

the model.

• Table 17.5 lists thermal

conductivities. They are

dramatically different,

from very large values

for conductors like

metals to very small

values for insulators

like styrofoam or wood.

• Consider Problem-

Solving Strategy 17.3.


Thermal conductivity


Thermal Conductivity – Lattice Waves – Longitudinal and Transverse


Thermal Conductivity – k (watts/m-Kelvin)


Conduction of heat I

• Consider Example 17.12.

• What makes a picnic

cooler effective?

• Figure 17.25 at right

illustrates the problem.


Conduction of heat II


• This is a good reason not to pick up a metal frying pan by

its bare handle.

• Figure 17.26 below illustrates the problem.


Conduction of heat III


• There are variations of the metal bar problem.

• Figure 17.27 below illustrates the problem.


Convection of heat

• Heating by moving large

amounts of hot fluid,

usually water or air.

• Figure 17.28 at right

illustrates heat moving by

convection.


Radiation of heat

• Infrared lights, hot metal

objects, a fireplace,

standing near a running

furnace … these are all

objects heating others by

broadcast of EM radiation

just lower in energy than

visible red.



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