Download Section 12.1 Temperature and Thermal Energy

Mr. Borosky
Physics Section 12.1 Notes
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Chapter 12 Thermal Energy
In this chapter you will:
Learn how temperature relates to the potential and kinetic
energies of atoms and molecules.
Distinguish heat from work.
Calculate heat transfer and the absorption of thermal energy.
Sections
Section 12.1: Temperature and Thermal Energy
Section 12.2: Changes of State and the Laws of Thermodynamics
Section 12.1 Temperature and Thermal Energy
Objectives
Describe thermal energy and compare it to potential and kinetic
energies.
Distinguish temperature from thermal energy.
Define specific heat and calculate heat transfer.
Read intro paragraph p. 313
Energy – ability of an object to change itself or its surroundings.
Thermodynamics – study of heat
THERMAL ENERGY
Read Section.
Thermal Energy – the measure of the internal motion of an object’s
particles.
From Old Book
Caloric – an invisible fluid added to a body when it was heated;
this idea came from Scientists in the 18th Century. This Caloric
Theory could explain observations such as expansion when objects
are heated, but it could not easily explain why hands warm up when
rubbed together.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 12.1 Notes
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In the Mid 19th Century the caloric theory was replaced by Kinetic
Molecular Theory.
Kinetic Molecular Theory – this theory is based on the assumption
that matter is made up of many tiny particles that are always in
motion. In a hot body the particles move faster and thus have a
higher energy than particles in a cooler body.
Thermal Energy – is also called internal energy. It is the Sum of
Kinetic Energy (KE) and Potential Energy (PE) of the internal
motion of particles that make up an object.
THERMAL ENERGY AND TEMPERATURE
Read Section.
Temperature only depends on the average Kinetic Energy of the
particles in the object.
The thermal energy in an object is proportional to the number of
particles in it.
Temperature, however, is not dependent on the number of particles in
an object.
EQUILIBRIUM AND THERMOMETRY
Read Section.
Conduction – transfer of kinetic energy when particles collide. It
is most common in solids. It is the principle on which household
thermometers work.
Temperature – measure of hotness of an object on a quantitative
scale. In gases it is proportional to the average kinetic energy of
the particles. It does NOT depend on the number of particles in a
body.
Thermal Equilibrium – state in which the rate of energy flow between
2 or more bodies is equal and the objects are at the same
temperature.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 12.1 Notes
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Thermometer – device used to measure temperature. It gets placed in
contact with an object and allowed to come to thermal equilibrium
with that object.
The operation of a thermometer depends on some property such as
volume that changes with temperature.
TEMPERATURE SCALES: CELSIUS AND KELVIN
Read Section.
3 Temperature Scales
1. Fahrenheit (F)(we use this on an every day basis in the US)
2. Celsius (C)
3. Kelvin (K)
The Celsius and Kelvin scales are usually used in science.
Figure 12-6 on p. 316 is helpful for conversions from one
temperature scale to another.
Temperature
Scale
Celsius
Kelvin
Fahrenheit
Absolute
Zero
-273 C
0 K
-459.4 F
Water
Freezes
0 C
273 K
32 F
Body
Temperature
37 C
310 K
98.6 F
Water
Boils
100 C
373 K
212 F
Equations to use to do temperature conversions.
1. K = C + 273
2. C = K – 273
3. C = (F – 32) / 1.8
4. F = C (1.8) + 32
or
C (9/5) + 32
5. F = (K – 273)(1.8) + 32
6. K = ((F – 32) / 1.8) + 273
Numbers 5 and 6 are not really needed you can use the earlier
equations as an intermediate step before you get the final answer.
Generally materials contract as they cool and expand when they warm
up. Hence the potholes in the winter time.
Temperatures do not appear to have an upper limit but they do have a
lower level.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 12.1 Notes
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Absolute Zero – lowest possible temperature at which gas would have
zero volume. Lowest temperature possible at this point all thermal
energy would be removed from the gas. This temperature is at 0 K or
–273 C.
The Kelvin Temperature scale is based on Absolute Zero. Absolute
Zero is at 0 K on the Kelvin Scale. Each interval on the Kelvin
scale is equal to 1 C.
Kelvin – is the unit or interval on the Kelvin Scale.
On the Celsius and Fahrenheit scale temperatures are in degrees.
the Kelvin Scale temperatures are in Kelvins.
On
Example Problem C to K p. 246
K = C + 273 = 25 + 273 = 298 K
Example Problem K to C p. 246
C = K – 273 = 4.22 – 273 =
-268.78 C
Do Practice Problems p. 317 # 1-2
HEAT AND THE FLOW OF THERMAL ENERGY
Read Section.
One way to increase the temperature of an object is to place it in
contact with a hotter object.
Heat – energy that flows as a result of a difference in temperature.
The symbol for heat is Q. Heat is a form of energy thus it is
measured in Joules. It is the energy transferred because of a
difference in temperature. If Q is negative that means it lost heat
and if Q is positive that means it absorbed energy.
Thermal Energy is transferred in 3 ways
1. Conduction
2. Convection
3. Radiation
Conduction – it involves the transfer of kinetic energy when the
particles of an object collide. It is most common in solids. It is
the principle on which household thermometers work.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 12.1 Notes
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Convection – heat transfer by means of motion of fluid. Convection
currents in the atmosphere are responsible for much of earth’s
weather.
Both Conduction and Convection depend on the presence of Matter.
Radiation – electromagnetic waves that carry energy. It does not
depend on the presence of matter. Thermal energy can be transferred
through space in the form of electromagnetic waves, such as solar
energy transmitted to the Earth.
SPECIFIC HEAT
Read Section.
Specific Heat – the amount of energy that must be added to raise the
temperature of a unit mass one temperature unit. The symbol for
specific heat is C. It is measured in Joules per kg Kelvin (J /
kg*K).
Table 12.1 p. 318 gives a list of Common Specific Heats
The Heat Gained or Lost by an object as its temperature changes
depends on the mass, the change in temperature and the specific heat
of the substance.
Heat Transfer – is equal to the mass of an object times the specific
heat of the object times the difference between the final and
initial temperatures.
To find the heat gained or lost by an object we use the equation
Q = mCT
Heat Gained or lost (Q) = the mass (m) times by the specific heat
(C) times by the change in temperature (T).
We can calculate ΔT in Kelvins or in °C.
Do Example 1 p. 318
Q = mCΔT
Q = (5.1)(450)(450 – 295)
Q = (5.1)(450)(155)
Q = 355,725 Joules
Do Practice Problems p. 319 # 3-5
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 12.1 Notes
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CALORIMETRY: MEASURING SPECIFIC HEAT
Read Section.
Calorimeter – device that isolates objects to measure temperature
changes due to heat flow. It is a device used to measure changes in
thermal energy. It depends on the conservation of energy in a
closed, isolated system.
As a result of the isolation, if the energy of one part of the
system increases, the energy of another part must decrease by the
same amount.
Conservation of Energy – in a Closed, Isolated System, the thermal
energy of object A plus the thermal energy of object B is Constant.
EA + EB = constant
EA + EB = 0
If the thermal energy change is positive the temperature of that
block rises. If the thermal energy change is negative the
temperature of that block falls.
Heat will flow from the hotter object to the colder object until the
objects reach thermal equilibrium (Have same temperature).
The change in thermal energy is equal to the heat transferred:
E = Q = mCT
Increase of thermal energy of Block A is equal to the decrease in
thermal energy of Block B, thus we have
mACATA + mBCBTB = 0
The final Temperature of the 2 blocks are equal thus the equation
for the Transfer of Energy is
mACA(TF – TAI) + mBCB(TF - TBI) = 0
Do Example Problem 2 p. 251 321
mACA(TF – TAI) + mBCB(TF - TBI) = 0
.04(388)(TF – 115) + .5(4180)(TF – 15) = 0
15.52 TF – 1,784.8 + 2,090 TF – 31,350 = 0
2,105.52 TF = 33,134.8
TF = 15.74 C
Do Practice Problems p. 321 # 6-9
Do 12. 1 Section Review p. 322 # 10-18
Physics Principals and Problems © 2005 Started 2006-2007 School Year