1. Electric Forces

2. Goal of the class
To understand electric charges and the electric force
Question of the day: How does static electricity affect the flow of water and why?

3. Electric Charge
Discovered by the early Greeks when pieces amber could attract small objects when rubbed.
Later it was discovered by Benjamin Franklin that there were two types of charge, positive (+), and negative (–).

4. Electric Charge Electrons me = 9.11x10-31 kg
Protons mp = 1.673x10-27 kg
Neutrons mn = 1.675x10-27 kg
All materials are made from atoms. Inside the atom there are 3 diff types of particles.
E = -1charge (-e) = -1.6x10-19 C it takes a LOT of electrons to make 1 C
P = +1 charge
N= 0 charge
All e / p / n all have intrinsic property
Normally objects are neutral. Show Carbon atom
An object with excess electrons will be negatively charged

5. Electric Force
If I take this balloon and rub it against my hair there is a transfer of electrons.
Law of charges states that like charges repel and opposite charges attract.
Rubber rod rubbed with fur  rod –ve, fur +ve. Triboelectric charging
Glass rid rubbed with silk  glass +ve, silk –ve.
If we hang them on string, they repel or attract based on config.
G+g = repel
R+r = repel
G+r = attract

6. Magic!

7. Conductor and Insulators
Some materials are good at conducting electricity where others are not.
Give me an example of a material that’s good at conducting and insulating electricity
Metal like cu wire for conductor or air for insulator.
Mention semiconductors
Why are some good an others are not? (Pick on AP Chemistry people)
We know the structure of atoms have electrons in shells. In the presence of other atoms some of the electrons are free to roam. This delocalisation is the basis of metallic bonding. The electrons are no longer bound to the atoms.
In some solutions it doesn’t need to be electrons, and it could be positive ions.
In insulators there is no such thing. Each atom has the electrons tightly bound and can’t wonder from the parent atom.
If I place an object with surplus negative charge on one end of a rod and an object with too few on the other. Some will flow if it’s a conductor, where they will not if it’s an insulator.

8. Charging a Metal Object
By contact
By Induction

9. Coulomb’s Law
Draw 2 point charges labelled q1 and q2 with a distance between them r.
Explain that the force between these charges is given by
F=kq1q2/r2. With q in Coulombs [C] and r [m] and F [N]
Then k is about 9x109 Nm2/C2 =1/(4πε0) where ε0 = permittivity of free space
This is the magnitude of the force. What about the direction?
It can be attractive or repulsive. (Law of charges)

10. Charges on the Vertices of a Triangle
For simplicity let’s use a 30,60,90 right angled triangle.
Suppose we put a charge on each corner as shown:
90: q1 = 4μC; 60: q2 = -5μC; 30: q3 = -2μC - opposite 30 degree = 0.3m
Note: WE use μC as the C is a huge value. If you get an answer that has a large number of Coulombs it may be wrong.
Let’s say we want to find the force felt on the -5μC charge.
Draw modulus bars and show that
F1 = k q1 q2/r2 = 2 N
F2 = 0.25 N
We can add the vectors and find the net force. How do we do this?
Component form x and y.
F1x = 0 F1y = -2 N
F2x = F2 cos 150 = - root 3/8 N F2y = F2 sin 150 = 1/8 N
F = F1 + F2 = {Fx = …. Fy = ….}

11. Three Charges on a Line
2 charges, +1μC and -4μC are on the x-axis. Where should a third charge be placed so the net force is zero?
+1μC on the left, -4μC on the right.
It can’t be to the right as the attraction from -4μC will be much greater than the repulsion from +1μC
It can’t be in between as +1μC will repel it and -4μC will attract it and they are in the same direction.
So it has to be on the left side.
If the distance between q and +1μC is d then
F1 = k (1x10-6)(q)/d2
F2 = k (4x10-6)(q)/(d+1)2
1/d2 = 4/(d+1)2
D=1m as d must be +ve. Here the net force is zero.

12. Two Small Spheres Attached to Strings
Find the charge on 2 small spheres attached to strings
Sphere: mass m, length of string = L
Given: m,L,theta
Show q2 = 4L2mg tan(theta) sin2(theta)

13. SHM of a Point Charge
Draw 2 charges in the y axis both +Q at +L and -L
And a thrid charge offset in the x axis –q
Equate forces when released and show SHM
F=-alpha x

14. Homework
Please hand in 22nd Jan
See homework sheet number 1