1. Charge and Coulomb’s Law
Test 1: Part 1 Chapter 20
Charge and Coulomb’s Law

2. History of Electricity/Magnetism
Ancient Greeks
Observed electric and magnetic phenomena as early as 700 BC
Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers
Magnetic forces were discovered by observing magnetite attracting iron
William Gilbert
Found that electrification was not limited to amber
Charles Coulomb
Confirmed the inverse square relationship of electrical forces

3. History Hans Oersted -1820 Michael Faraday -1831
Compass needle deflects when placed near an electrical current (moving current creates a magnetic field)
Michael Faraday -1831
If a wire is moved near a magnet, an electric current is induced in the wire by that motion (if a wire is moved through a magnetic field, it creates a current)
James Clerk Maxwell –
Formulated the laws of electromagnetism known as Maxwell’s laws
Hertz -1884
Verified Maxwell’s equations and reformulated them to the form we use today

4. Properties of Electric Charges
Two types of charges exist
positive and negative
Like charges repel each other
Unlike charges attract one another
The nucleus contains both protons and neutrons
Protons do not move from one material to another because they are held firmly in the nucleus. They are the particle with positive charge.
Neutrons have a mass very similar to a proton, but they are charge neutral (neither positive nor negative)

5. Properties of Charge
The basic carrier of negative charge is the electron
Gaining or losing electrons (ionization) is how an object becomes charged
Gain electrons – negatively charged
Lose electrons – positively charged
Electric charge is always conserved
Charge is not created, only exchanged
Objects become charged because negative charge is transferred from one object to another

6. Charge is quantized
Quantized means it comes in packets of energy, not just any random quantity.
Charge is always a multiple of a fundamental unit of charge, symbolized by e
e = 1.6 x Coulomb
Electrons have a charge of –e = x Coul
Protons have a charge of + e = x Coul
The SI unit of charge is the Coulomb, C.
How many electrons or protons make up one Coulomb of charge?
1 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 1.6 𝑥 10 −19 𝐶𝑜𝑢𝑙𝑜𝑚𝑏 =6.25 x 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠/𝐶𝑜𝑢𝑙𝑜𝑚𝑏

7. Conductors
Conductors are materials in which the electric charges move freely
Copper, aluminum and silver and other metals are good conductors
When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the conducting material

8. Insulators
Insulators are materials in which electric charges do not move freely
Glass and rubber are examples of insulators
When insulators are charged by rubbing, only the rubbed area becomes charged
There is no tendency for the charge to move into other regions of the material

9. Semiconductors
The characteristics of semiconductors are between those of insulators and conductors
Silicon and germanium are examples of elements that are semiconductors
GaAs and InP are heterojunction semiconductors
Si does not easily conduct electricity by itself but if you dope it with Boron (p-type) or Phosphorus or Arsenic (n-type), those regions now will conduct electricity more readily.

10. Charging by Conduction
A charged object (the rod) is placed in contact with another object (the sphere), which initially had no charge on it.
Some electrons on the rod can move to the sphere.
When the rod is removed, the sphere is left with a charge.
The object being charged is always left with a charge having the same sign as the object doing the charging.
Is this sphere made of a conducting or insulating material? How can you tell?

11. Charging by Induction
In (b), a negatively charged rubber rod is brought near an uncharged sphere as shown in (a).
This attracts the positive charges on the sphere towards the rod, and the negative charges on the sphere are repelled (b).
When an object is connected to a conducting wire or pipe buried in the earth, it is said to be grounded. In (c) a ground wire is connected to the sphere and the negative charge leaks off to (the) ground.
The charges in the sphere are now all positive (d). When the rod is removed (e), the + charges redistribute themselves across the sphere due to repulsion (assumes conducting sphere).
Note: No contact was required between the rod and the sphere to charge by induction. Only contact required is to ground.

12. Polarization
In most neutral atoms or molecules, the center of positive charge coincides with the center of negative charge
In the presence of a charged object, these centers may separate slightly
This results in more positive charge on one side of the molecule than on the other side
This realignment of charge on the surface of an insulator is known as polarization

13. Examples of Polarization
The charged object (on the left) induces charge on the surface of the insulator
A charged comb attracts bits of paper due to polarization of the paper.
3’ video example:

14. Quick Quiz 1
If a suspended object A is attracted to object B, which is charged, we can conclude that (a) object A is uncharged, (b) object A is charged, (c) object B is positively charged, or (d) object A may be either charged or uncharged.

15. Quick Quiz answer
(d). Object A could possess a net charge whose sign is opposite that of the excess charge on B. If object A is neutral, B would also attract it by creating an induced charge on the surface of A (B is rod here, A is the sphere)

16. Pith Ball and a Charged Rod
The Mysterious Shuttling Ball Physics Experiment (Investigate! p. 180 in Oxford SL book)

17. Coulomb’s Law
Coulomb showed that an electrical force has the following properties:
It is inversely proportional to the square of the separation between the two particles and is along the line joining them.
It is proportional to the product of the magnitudes of the charges q1 and q2 on the two particles.
It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs.
The force vector goes in the direction that a positive test charge would go (more on this later).

18. Coulomb’s Law, continued
Mathematically, 𝐹= 𝑘 𝑒 𝑞 1 | 𝑞 2 | 𝑟 2
ke is called the Coulomb Constant
ke = 8.99 x 109 N m2/C2 (we will use this one a lot)
ke = 1 / 4πε0 where ε0 is the permittivity of free space.
ε0 = 8.85 x C2/N m2 (use this more in AP Physics)
If it’s not free space, but a dielectric then use ε = κε0 where κ is the dielectric constant
Typical charges, q1 or q2 can be in the µC range (10-6 C)
Force is a vector quantity – there is an attraction or a repulsion, so direction matters!

19. Vector Nature of Electric Forces
Two of the same point charges ( + + or - - ) are separated by a distance r
The like charges produce a repulsive force between them
The force on q1 is equal in magnitude and opposite in direction to the force on q2

20. Vector Nature of Forces, cont.
Two opposite point charges ( + - or - + ) are separated by a distance r
The unlike charges produce an attractive force between them
The force on q1 is equal in magnitude and opposite in direction to the force on q2

21. Similarities between Coulomb’s Law and Universal Gravitation

22. Electrical Forces are Field Forces

23. Electrical Force Compared to Gravitational Force

24. Quick Quiz 2
Object A has a charge of +2 µC, and object B has a charge of +6 µC. Which statement is true: (a) FAB = –3FBA (b) FAB = –FBA (c) 3FAB = –FBA

25. Quick Quiz 2 answer
(b). By Newton’s third law, the two objects will exert forces having equal magnitudes but opposite directions on each other.

26. Coulomb’s Law in 2 Dimensions

27. The Superposition Principle
The resultant force on any one charge equals the vector sum of the forces exerted by the other individual charges that are present.
Remember to add the forces as vectors
Find all x and y components
Add all x components to get Fx total
Add all y components to get Fy total
Use Pythagorean equation to find Ftotal overall
Use tan-1(Fy/Fx) to find the angle of the net force

28. Example Three Charges on a Line
Determine the magnitude and direction of the net force on q1.

30. Superposition Principle Example
The force exerted by q1 on q3 is F13
F13 has x and y components
The force exerted by q2 on q3 is F23
F23 has only an x component in the -x direction
The total force exerted on q3 is the vector sum of F13 and F23

31. Demonstrations – Scotch Tape
Scotch tape: Take 2 pieces of tape about 3” long.
Fold over about ½” on one end of each to make a handle.
Stick both pieces of tape down on the desk.
Quickly pull both pieces off the desk and bring them near each other.
Do they attract or repel each other?
Now take the 2 pieces of tape and tape the first one down on the desk, then tape the second one down on top of it.
Now pull both pieces off the desk as one piece and rub them with your fingers until they are no longer attracted to your hand, removing the charge.
Now quickly pull the strips apart. Now do they attract or repel each other?

32. Triboelectric Series
The items at the top of this list tend to acquire a positive charge. The items at the bottom tend to acquire a negative charge.
So if I rub a glass rod with silk, since the glass is closer to the top than the silk, the glass, the glass ends up + and the silk ends up – charged.
If I use rabbit hair to rub a rubber or plastic rod, the rabbit hair ends up + charged, and the plastic rod ends up – charged.
9 minutes into video demonstration starts