Chapter 17 Atoms, Molecules and Atomic Processes
Let this thought roam around in your mind for a time: If a golf ball was expanded to be the size of the earth, an atom in the "golf ball" would be the a little bit bigger than the size of the original golf ball.
*TRUE or FALSE It is now possible to detect and manipulate the positions of individual atoms.
You should have a general knowledge of the development of the concept of the atom: Dalton’s periodic table 1808, 1800’s evidence for building blocks of matter in the formation of chemical compounds according to the law of proportions, 1895 discovery of the electron, a subatomic particle, 1912 discovery of the nucleus, 1913 Bohr’s classical model of the hydrogen atom, 1920’s development of quantum mechanics that describes the behavior of matter at small sizes and energies.
I will award one bonus point to the first person who sends me e-mail with the correct answer to the following question: In what year was the first good estimate of the size of an atom made and what process was studied to make the estimate?
page 17-2 Molecules Interesting reading, but we don’t have time to explore this topic.
page 17-4 Atomic Processes
States of Matter: solid, liquid, gas, plasma (ionized atoms)
density, ρ = mass/volume
Why does the density decrease by a factor of about 1000 when the distance between molecules increases by about a factor of 10?
page 17-6 Thermal Motion
When we discussed the speed of sound in air I mentioned that the air molecules are in motion and that the speed is related to the temperature of the air. More precisely, the average kinetic energy of the center of mass of an air molecule is proportional to the temperature. This will be discussed in more detail in a few pages.
*What did the botanist Robert Brown observe in 1827? Give more details than the phrase "Brownian Motion."
You should view the movie on Brownian Motion that is on your CDROM.
It is interesting and important that the average kinetic energy of the smoke particles is equal to the average kinetic energy of the air molecules. Can you think of a reason why this might be true?
page 17-8 Take the speed for Helium to be a given. Verify that the speed listed for Nitrogen molecules is correct. Calculate the speed for an Oxygen molecule.
Thermal Equlibrium
Imagine the good old days, the hot days of August of this year. Imagine that you have been outside in the hot sun working but have been given a cold glass of water that you set on a table nearby. Which has the higher average kinetic energy: air molecules, water molecules? Imagine that you laid down in a hammock and slept for 2 hours. Describe the temperature of the water (we will assume it did not all evaporate). Why does this happen?
page 17-9 Temperature
In what ways is temperature important in your life?
What kinds of thermometers are you familiar with?
Why don’t thermometers give an instant reading of the temperature of an object as the thermometer comes into contact with the object?
Speculate how you could design a thermometer that would give a correct temperature reading of an object in a very short amount of time.
Absolute Zero
What is the coldest outdoor temperature you have experienced?
Do colder temperatures than this exist?
Imagine that you put one hand into a glass of cold water that does not have ice in the glass.
When do we describe something as "cold?"
Imagine that you now put this same hand into a glass of colder water that has ice in the glass and leave your hand there for a few minutes.
Can you tell that the water is colder?
Imagine that you now put your hand into the first glass. How would you describe the temperature of the first glass?
How does your body sense temperature?
Is your body a good thermometer?
What are the characteristics of a good thermometer?
It is true that the average kinetic energy of a system of molecules is proportional to the temperature of the molecules. What problem exists if we use the Fahrenheit or Celsius temperature scales with this concept?
Describe the temperature if the average kinetic energy is zero. (This is a Classical Physics discussion.)
This hypothetical state is assigned a temperature called Absolute Zero. The temperature scale that has a value of 0 at this state is the Kelvin temperature scale.
Quantum Mechanics more accurately describes the way the universe works than Classical Physics. Quantum Mechanics predicts that it is not possible for a molecule or atom to have zero kinetic energy for its center of mass. The minimum energy for an object is called the zero point energy. This energy value is not zero Joules.
*Do physicists believe it is possible for an object to have a temperature of 0 Kelvin?
page 17-10 Temperature Scales
What is the freezing point of water on the Fahrenheit scale? boiling point?
What is the freezing point of water on the Celsius (Centigrade) scale? boiling point?
These points form the calibration points for these scales. (We will skip some technical details for now.)
Which is the larger physical change in temperature: one degree Fahrenheit or one degree Celsius?
Subtract the calibration point temperatures for both oF and oC.
Divide the values. This becomes a conversion factor when temperature changes are discussed.
What problem do we encounter if we want to change one temperature value from one scale into the other scale? e.g. 32 oF into oC
Can you invent an algorithm to accomplish the conversion?
You may well ask: "Why are the freezing points and boiling points of water used?", "Why not use the freezing points and boiling points for chocolate?"
The oF and oC scales are arbitrary as to the location of zero on the scale. The Kelvin scale has a physically significant "zero" point, absolute zero. The size of one unit of Kelvin temperature is the same size as one unit of Celsius temperature. i.e. The change in temperature from the freezing point of water to the boiling point of water is 100 Kelvin.
The freezing point of ice is about 273 Kelvin. Develop an algorithm that coverts oC into a Kelvin value.
With a non-arbitrary temperature scale defined we can now make a definite connection between the average KE of the center of mass and the Kelvin temperature:
average KE = (3/2)kT
Guess what k represents. It is not the force constant of a spring. It is not the spacial frequency. It is Boltzmann’s Constant, 1.38 x 10-23 Joules/Kelvin.
Calculate the average kinetic energy in Joules for an Oxygen molecule at room temperature.
Calculate the speed of an Oxygen molecule that has this value for KE.
An Oxygen gas has about 32 grams of mass for one mole.
page 17-12 Molecular Forces This is interesting reading but we will skip this material.
page 17-14 Evaporation
Why does the temperature of water decrease when evaporation takes place?
Why do you sweat?
page 17-16 Pressure
When you blow air into a balloon does the balloon material stretch?
The rubber material of the balloon is similar to a spring. When the stretch occurs there is a restoring force. What is the direction of the restoring force of the balloon material?
What does this force act on?
How does Newton’s Second Law relate to a balloon?
F = Δp / Δt What does p represent?
Pressure, P, is defined as P = Force/Area
The metric unit for pressure is the Pascal. 1 Pascal = 1 Newton/1 m2
Place a not-too-sharp pencil between two fingers. Press in a little but don’t puncture your skin. Which end of the pencil creates more feeling in your fingers? Why?
Calculate the pressure I create by standing with two feet on the floor.
90 kg rectangular shoes 8cm x 29cm
Calculate the pressure for a 50 kg female who is rocking back on the heels of two high heeled shoes. Let the heel of each shoe be 1.2 cm x 1.2 cm.
Assuming other important quantities are constant, why does pressure increase as the temperature of the gas increases?
Do you regularly check the air pressure in the tires of your car?
What condition are the tires in at the recommended time to check the air pressure?
page 17-17 Stellar Evolution (Changes)
Calculate the average density of the sun.
*Which of these three best describes the sun: gas, liquid, solid.
Why does the sun have the radius it has today?
Is the sun expanding or contracting noticeably today?
What force is trying to collapse the sun? What balances this force?
When do stars expand? When do stars contract?
You can earn one bonus point by being the first student to send me the correct answer via e-mail to this question: The white dwarfs do not have a source of internal energy, why don’t they collapse? The paragraph in the book is not specific enough in its description of what prevents the collapse. Hint: The answer is related to the fact that there is a periodic table.
page 17-18 The Ideal Gas Law
An ideal gas has two important properties: 1) the particles have zero size, 2) the particles do not exert any long range forces on their neighbors (force only occurs when a collision occurs). The gas in our classroom is very close to an ideal gas.
* Does the author allow gas molecules to exist to the right of the piston in Figure 20 ?
Why is a force on the piston, directed to the left, required in order that the piston remains stationary?
Equation 4 Why is the change in momentum equal to 2 times the momentum of the incoming molecule?
page 17-19 What questions do you have on equations 5,6 and 7?
In the last paragraph of the first column on page 17-19 you need to imagine a three dimensional box. Let N molecules be placed in the box. Suppose that N is equal to 30. How many of these molecules are basically moving in the left to right (back and forth) direction? If you don’t think it is 10 be sure to ask me "Why it is 10?" when we discuss this material in class.
What questions do you have about equation 8?
What questions do you have about equations 9 and 10?
Why is equation 11 true?
IDEAL GAS LAW P V = N k T
You should write down on this page what you think each symbol represents in this formula.
Calculate the pressure in Pascals for a container 20 cm x 40 cm x 10 cm, with 3 x 1023 particles in the container at a temperature of 68 oF.
page 17-20 Ideal Gas Thermometer
Read through this section.
* Describe the temperature at which the volume of the ideal gas thermometer is expected to be zero.
* Why do ideal gas thermometers fail to work at temperatures below 4 Kelvins?
page 17-22 The Mercury Barometer and Pressure Measurements
*In the construction of a mercury barometer as described in the book how many ends of the tube are open to the atmosphere?
What questions do you have on equations 18, 19, and 20? I will give one bonus point to the first person who sends me e-mail with a correct description of the error in one of these equations.
1 atmosphere of pressure = 1.01 x 105 Pascals or 76 cm of mercury or 14.7 lb/in2
P = ρ g h
Calculate h if water is used in the barometer instead of mercury.
page 17-24 Avogadro’s Law
We will use 6.02 x 1023 objects for 1 mole. This is Avogadro’s Number.
How many grams of N2 are present in 1 mole of N2?
You should read through the reformulation of the Ideal Gas Law on these pages in the text.
Another version of the Ideal Gas Law is P V = n R T R is the universal gas constant 8.315 J/mole/K
*Write Avogadro’s Law in your own words.
page 17-26 Heat Capacity
By now you should have come to accept that there is a relationship between the average kinetic energy in an object and the temperature of the object.
*TRUE or FALSE "Heat" is a substance contained in objects such that when the object has more of the substance the temperature of the object is higher.
The caloric theory has been proven to be false. The caloric theory had an influence on the beginning theories of electricity.
1 calorie = 4.186 Joules This calorie is a physics calorie, not a food calorie. A food calorie = 1000 physics calories. 1 Calorie is required to change the temperature of 1 gram of water from 14.5 oC to 15.5 oC.
Q = m c ∆T Q is the heat value, m is the mass, c is the Specific Heat, ∆T is the temperature change for the system. Some values for c are (in units of calories/gram/ oC ): water = 1, aluminum = 0.22, copper = 0.093, lead = 0.031, ice = 0.5, steam = 0.48, human body = 0.83.
We will skip the rest of this section.
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