Chapter 19 The Electric Interaction
During the first semester we mentioned that there are four known forces active in the universe. We studied gravity in some detail. This semester starts with the study of the electrical force. We will then study magnetism (which is actually a manifestation of electrical force). Later in the semester we will spend a brief time discussing the strong and weak nuclear forces.
page 2 The model of the atom shown at the bottom of the page is OK for the level of this discussion. Later in the course we will discuss a better model of the atom in which the electrons do not have definite orbits like planets around the sun. Why do you think the electric interaction is more important in the atom than for the case of the earth orbiting the sun?
Later in this chapter we will perform some calculations that will verify statements made on this page.
page 3 Atomic Structure
What objects are found in most atoms?
Have you ever heard the phrase "cathode ray tube?" Experiments on cathode rays from 1894 – 1897 led to the use of the term "electrons." Experiments gave evidence for the proton from 1898 through 1919 when the term "proton" was accepted. The neutron was experimentally isolated in 1932.
In drawings of atoms, why are the electrons the only objects represented as having motion in the atom?
Try to make a scale drawing of an atom. The approximate diameter of the nucleus is 10-15 meters. The approximate diameter of an atom is 10-10 meters.
What is in between the electron and the nucleus?
Are you mostly solid or mostly empty space?
Why can’t you push your hand slowly through a table?
Gravity only attracts objects. We will see a demo of static electricity that shows that the electrical force may attract or may repel.
What circumstances lead to an attractive electrical force?
What circumstances lead to a repelling electrical force?
A hydrogen atom has one proton and one electron. Is this atom electrically neutral? Why?
Carbon has 6 protons in its nucleus. How many electrons does a neutral carbon atom have?
What is an ion?
How can you create an ion of carbon?
Does any other element in the periodic table, other than carbon, have 6 protons in its nucleus?
For elements that are found on the earth, not man made, the largest atom is uranium, which has 92 protons. The elements with more than 92 protons in Table 1 can be man made in laboratories. It has no net charge. What is your observation about the number of neutrons compared to the number of protons in a particular element as you scan Table 1 from hydrogen up to uranium? This fact will be explained towards the end of the course.
Is there another way to present the elements than as a list such as occurs in Table 1?
What is "periodic" in the periodic table?
Why do certain chemical properties reoccur as one examines elements throughout the list in Table 1?
What is the primary factor that governs the chemical properties of an element?
The periodic table started to take on a form as we know it after 1860. It was not until the late 1920’s that a full explanation of the periodic table was developed using Quantum Mechanics (Chapter 40).
page 6 Isotopes
The strong nuclear force is stronger than the electrical force and causes protons and neutrons to remain close to each other in the nucleus. The strong nuclear force is much different than the gravitational and electrical forces in that it is a short range force. The strong nuclear force is only important (strong!) when the objects are within about 10-9 meters of each other. The strong nuclear force does not affect electrons.
Think about carbon with its 6 protons in the nucleus. Explain why the existence of a small nucleus leads one to conclude that the strong nuclear force has a greater strength than the electrical force.
There are three isotopes of hydrogen. Normal hydrogen has 1 proton and 0 neutrons. Deuterium has 1 proton and 1 neutron. Tritium has 1 proton and 2 neutrons. Why are all of these properly called hydrogen?
While there are 92 naturally occurring elements, there are over 1000 known isotopes. About 280 isotopes are stable. Over 800 isotopes are radioactive (they change into some other isotope after a period of time). We will talk about radioactivity near the end of the course.
page 7 The Electric Force Law
The electric force came under serious and productive scientific investigation in the late 1700’s. At this time is was verified that the strength of the electric force decreases as the distance between the charged objects increases. The rate of decrease is the same as for gravity, 1/r2 . The objects in Figure 3 are all the same distance apart in each line. What do you observe about the length of the vector arrows in each sketch?
The characteristic of matter called charge leads to electric force. What do you speculate is true regarding the amount of charge on a proton and the amount of charge on the electron?
page 8 Strength of the Electric Interaction
The magnitude of the electrical force can be calculated. F = (k q1 q2 )/ r2
In what ways is this calculation similar to the calculation for the gravitational force?
Do you know why it is similar?
Are the electrical and gravitational forces just variations of some more fundamental force law?
When we want to calculate force in units of Newtons, k has a value of 9 x 109 , q represents the charge (in Coulombs) on each object and r is the distance (in meters) between the centers of the two object.
We will not use the CGS system of units.
The charge on the electron was determined in 1909 - 1913 by Millikan. Where did Millikan attend high school? (Hint: This is my wife’s home town.)
The charge on the electron is 1.6 X 10-19 Coulombs. Calculate the magnitude of the gravitational force between two electrons that are separated by 2 cm. Calculate the magnitude of the electrical force between two electrons that are separated by 2 cm. Calculate the ratio of the two forces.
page 10 Positive and Negative Charge
When did Benjamin Franklin perform experiments in electricity?
Franklin’s view of electricity followed the view of "caloric" in thermodynamics. In that (erroneous) view an object has a higher temperature if it obtains more "caloric." In Franklin’s view of electricity an object has a positive charge if it has a surplus of electrical fluid. An object that has a deficiency of electrical fluid is said to have a negative charge.
How has our view of electrical charge changed since the time of Franklin?
By convention (tradition) a rubber rod acquires a net negative charge after it is rubbed on rabbit fur.
By convention (tradition) a glass rod acquires a net positive charge after it is rubbed on silk.
When the rod is moved across a material why does it obtain a net charge? Would it be best to talk about the protons moving from object to another or the electrons moving from one object to another? Why?
Addition of Charge
page 11 Work exercise 2.
A certain object has 6 protons and 4 electrons. Calculate the net charge.
Calculate the number of electrons in a penny coin.
Suppose that all of the electrons are removed from two pennies and the pennies are located 2 meters away from each other. Calculate the net electrical force on one of the pennies due to the charge located on the other penny.
page 13 Conservation of Charge
In all processes there is no change in the net charge of the universe.
There are no known violations of this statement in any experiment (chemical or nuclear).
In 1932 the list of known subatomic particles was: proton, neutron, electron. These were called the "elementary" particles as they seemed to be the building blocks of all matter. (Some may add the photon, "particle" of light, to this list.) Nuclear physics developed rapidly after 1930 and the list of subatomic particles is long. The text only gives a few examples on page 13. What is your observation regarding the conservation of charge as you inspect the decays listed on page 13?
We may discuss antimatter later in the course. This term is not science fiction, it is science fact.
page 14 Stability of Matter
The proton and electron have not been observed to decay. There are experiments underway searching for evidence of proton decay. Neutrinos probably change form as they travel. This is too technical for our discussions. Photons (light) can change into matter (electron and positron) under the right conditions.
In the last few years, evidence has been produced that hints that neutrinos have a small, non-zero, mass.
Quantization of Electric Charge
An important result of Millikan’s experiments on electrons was that net charge is only observed in certain values, namely some integer multiplied by the value of the charge on the electron ( - ) or proton (+). In the 1960’s the quark theory of matter was developed. In this theory there are objects (quarks) that have a fraction of the value of charge found on electrons and protons. Isolated (single) quarks have not been observed but there is good evidence that they, or something like them, exist as more fundamental building blocks of matter. This is beyond the scope of this course.
We will skip the discussion of molecules.
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