1. Electrostatics #1 Introduction to Electrostatics
Textbook Readings:
Ch. 20, Static Electricity
Turn in HW Stamp Sheet.

2. Definition: Static electricity means . . . . . . .
I. Introduction: The types of forces studied so far have involved some type of contact or were attracted to one another by gravity. List some everyday experiences that do not fall into the above interactions:
When clothing is removed from the drying machine, many times the clothing seems to stick together. There is a lot of “snap, crackle, and pop” when the articles of clothing are pulled apart. Sometimes, a small shock accompanies the pulling apart of the clothing.
Definition: Static electricity means
Static electricity refers to the cases where there are charges distributed on different objects, and these distributions are not changing over time (static). Systems where charge will be changing with time will be studied in the next unit, electrodynamics.

3. II. Historical Development:
When certain materials are rubbed together, something is exchanged between the materials. This something is referred to as ________________ .
charge

4. Originally, charge was seen as a fluid that could be transferred from one substance to another. When one substance accepts this fluid, it becomes positively charged, while the other that gave up the fluid becomes negatively charged. The idea of positive and negative charge is credited to
_________ ___________ .
Benjamin
Franklin

5. Franklin deduced two types of opposite charges based on the forces of attraction and repulsion that form when two charges are brought near one another.
Like charges ___________________ .
repel
attract
Opposite charges ___________________ .

6. Ex. #1: Based on this information, determine the directions
of the unmarked forces in the diagram below.
A & B have opposite charges.
A & D have like charges.
A & C have opposite charges.
Thus D & C have opposite charges and attract.
Also D & B have opposite charges and attract.

7. III. Chemistry Review:
A. Structure of matter, types of charge:
All matter is made of atoms. All positive and negative charge is distributed in the atom.
B. What are atoms composed of?
All atoms are made of positively charged protons, electrically neutral neutrons, and negatively charged electrons.
C. Distribution in atom?
The protons and neutrons are found inside the nucleus, which is a small and very dense body at the center of the atom. The electrons are found in orbitals outside the nucleus.
Size of atom ≈ size of electron cloud
Size of nucleus

8. D. Atoms with the number of ___________ and ____________ equal are said to be neutral.
protons
electrons
E. How are atoms charged?
Atoms are charged by transferring electrons from one atom to another.
Charge can be transferred from on object to another through direct contact (conduction) or can be separated to opposite ends within an object (induction). {More later…}
electrons
F. The ease with which ____________ transfer or move depends on how tightly they are held.
Conducting materials allow electrons to flow very easily across its own body or from one material to another.
Insulators hold the electrons tightly in chemical bonds, so electrons are not able to flow freely, as in the conductor.

9. H. If the # of electrons > # of protons, atom (or object) has a ____________ charge.
negative
If the # of electrons < # of protons, atom (or object) has a ____________ charge.
positive
Electrons are bound to the atom with much less energy as compared to the protons bound to the nucleus. All charge is ultimately caused by moving electrons.
Early scientists studying electricity assumed that positive charge moves during the charging process, a standard we keep today. In reality, electrons are negative and would move in the opposite direction for any charging process.

10. IV. Units of Charge:
The basic unit of charge is called the ______________, and it is represented by the letter ___. This unit is its own Fundamental unit. From chemistry, another useful charge is that of the electron and proton. This charge, known as the Fundamental charge, is:
coulomb C
proton charge = + 1e, electron charge = – 1e

11. V. Conductors & Insulators
A. A conducting material allows for the easy transport of
___________ throughout the medium. Electrons may also be lost
from the ___________ of the medium.
B. An insulating material keeps its electrons __________
bound, even in the presence of a net charge on the material.
electrons
surface
tightly
Electric charge on a conducting material will always migrate to the surface of the conductor, as shown at right.
No net electric charge remains on the interior of the conductor (Faraday cage)

12. Faraday’ Cage refers to any enclosed metal box that holds charge only on the surface of the box. Occupants inside will not feel any form of electrical shock.

13. VI. Transfer or Motion of Charges:
A. Conduction: If a charged object comes in direct contact with an uncharged object, the charge will ______________ itself throughout both objects. When the objects are separated, the bodies will ____________ their charges. The new charge is:
distribute
retain
Q1 = original charge on object #1
Q2 = original charge on object #2
Qeach body = new charge on each object
This is a form of conservation of charge. The total amount of charge cannot change. Charge may only be moved from one object to another.

15. Ex. #2: A charge of +14. 0 mC comes in contact with a charge of –25
Ex. #2: A charge of mC comes in contact with a charge of –25.0 mC. What is the charge on each object after conduction of charge occurs?

16. B. Induction: If a charged object is brought near, but not touching, an uncharged object, the uncharged object will experience a net charge separation. When the charged object is removed, the object with the net charge separation will ____________ its charge separation.
lose

17. Charging by Induction and Conduction:
Induction was discovered in 1753 by an English physicist, John Canton.
Here is one example of charging by induction. Two metal spheres, A and B, are placed in contact and a negatively charged rod C is brought near but not touching sphere A.
According to Franklin’s view, positive charge is attracted to the side of A nearer C, thereby leaving an equal negative charge on the opposite side of B.
With C still present, A and B are separated. This leaves A with an excess positive charge and B with an equal negative charge. The charged rod C is now removed, and the charge states of the spheres remain the same.
Note the charge is equal and opposite!

18. Induction can be used to charge a single conducting object:
A positively charged glass rod is brought near to but not touching an uncharged metal sphere. The Franklin positive charges on the sphere are repelled by the rod, leaving the side nearer the rod with an equal number of negative charges (figure a).
A ground wire† is now brought into contact with the side of the sphere opposite the rod, thereby leading the positive charges to ground. (figure b)
With the charged rod still in place, the ground wire is disconnected. (figure c) Finally, when the rod is removed, the sphere remains with a net negative charge which becomes uniformly distributed over the sphere. (figure d)

22. Electrostatics #2 Introduction to Electrostatics
HW #1, last page of handout

23. Forces and The Inverse Square Law
As with any force, at least ___________ objects must be present: one object ______________ a force on the other object, and by Newton's _____________ law, that other object exerts an ______________ but _______________ force on the first.
two
exerts
3rd
equal
opposite

24. I. Electrostatic Force Between Two Charges:
The force of attraction or repulsion between two charges behaves the same as the gravitational force. The force will be proportional to the charges, q1 and q2, and inversely proportional to the distance between the centers of the objects (r).
The charge q of any object is measured in Coulombs, the basic unit of charge. Like mass, distance, and time, charge is a fundamental unit. The magnitude of the charge on a proton or an electron is x 10 – 19 C.
The magnitude of the force is given by the following:
Q or q represents the charges. The absolute value is used only to determine the size (or magnitude) of the force.

25. The constant k has the value 8. 988 x 109 Nm2/C2
The constant k has the value x 109 Nm2/C2 . The direction of the force is based on the actual value of the charges: if both charges are either positive or negative, the force is ______________ and directed away from the other charge; if, on the other hand, the two charges have opposite signs, then it is an _____________ force.
repulsive
attractive

26. Example #1: Two charges are placed on the x-axis as follows: A charge of mC is placed at the origin and a charge of –34.2 mC is placed at the 14.0 cm mark. a. What is the force between the two charges, given in magnitude and direction?
opposite charges attract

27. b. The two charges are briefly connected by a conducting wire
b. The two charges are briefly connected by a conducting wire. What will happen to the force now?
like charges repel

28. Example #2: Two identical charges are placed 16
Example #2: Two identical charges are placed 16.0 cm apart and exert a force of N on one another. What is the amount of charge on each object?
identical charges
Either both positive or both negative

29. Example #3: For the hydrogen atom, the proton and the electron are approximately 0.50 Å apart. Compare the size of the electric force between the charges and the gravitational force between the masses. Which is more important to the structure of the hydrogen atom?

30. substitute values
The force of gravity is extremely weak compared to the coulomb force for this case. All of chemistry is based on electric interactions, not gravitational.

31. II. Electric Force Between Multiple Charges:
When calculating the force on a charge, q, by several other charges, Qi, compute the force on the charge q pair wise for each other charge. The total force is the vector sum of all the forces from the other charges. Below is a sample image showing three interacting charges. The diagram shows the forces acting on the charge Q3 at the upper left corner. Describe the results of the image:

32. Example #4: Three charges are placed on the x-axis: Q1 = +3
Example #4: Three charges are placed on the x-axis: Q1 = mC is placed at x = 0, Q2 = mC is placed at x = 3 cm, and Q3 = –8.40 mC is placed at x = 7.00 cm. (a) Find the total force on the charge Q2.
Add the magnitudes of the two vectors, they point in the same direction!
like charges repel
opposite charges attract

33. total force directed towards the right

34. b. If the charge Q2 has a mass of 275 grams, what would be its acceleration at that moment?
c. Determine the magnitude and direction of the net force acting on the charge Q1. {on your own}
points left
points right
points left

35. Example #5: Two charges are placed on the x-axis: Q1 = +16
Example #5: Two charges are placed on the x-axis: Q1 = mC is placed at x = 0 and Q2 = +144 mC is placed at x = 9.00 cm. Where should a charge, q, be placed so that the total force on q is zero?
Assume q > 0. Both forces on q are repulsive!
The only place to put q so that the forces point in opposite directions is somewhere between the two given charges.
The forces must lie on the same line, have equal magnitude, and must point in opposite directions to make the total force zero!

36. Assume at the distance x the two forces have the same magnitude:

37. square root both sides:
Keep only the + solution, the position of q must be between the other two charges.

38. Electrostatics #2 Introduction to Electrostatics {day #2}
HW #1, last page of handout

39. Example #6: Three charges are placed on the x,y coordinate system
Example #6: Three charges are placed on the x,y coordinate system. A charge Q1 = mC sits at (x,y) of (0,+3.00 m), a charge Q2 = mC sits at the origin, and a charge Q3 = –23.0 mC sits at (4.00 m,0). What is the force on the charge Q2?

40. Find the magnitude of each force:
force points towards the – y direction.
force points towards the + x direction.

41. +y –x +x –y

42. Example #7: In a simple model of the hydrogen atom, the electron revolves in a circular orbit around the proton with a speed of 1.1 x 106 m/s. Determine the radius of the electron’s orbit.
Set the centripetal force equal to the electric force:

43. Example #8: Determine the value of the quantity Q if each mass below is 24.0 grams and the system is in equilibrium.
and

44. Divide the two force equations:

45. Ex. #9: Two charges are placed 80. 0 cm apart
Ex. #9: Two charges are placed 80.0 cm apart. The sum of the two charges is 100 mC and the two charges exert a repulsive force of 32.0 N on one another. What is the size of each charge?
two equations, two unknowns
repulsive force, like charges
positive sum, both charges positive

46. Solve the simultaneous equations…
If Q = mC, then q = mC and vice versa.

47. Ex. #10: Solve the net force on the charge Q3.
Add the two forces together using components,
F32 only has a y – component, F13 has x & y – components:

48. This can be turned back into magnitude and direction:
ccw from +x – axis.