1. Electric Forces & Fields
Chapter 18

2. The Origins of Electricity
In the mid 18th century Ben Franklin created the idea on positive and negative electric charge.
It wasn’t until 150 years later the electron was discovered.
Franklin described an electric “fluid” that would flow depending on electric pressure.

3. Electric Charge
In 1909 Robert Millikan discovered charge was “quantized.”
This means there is a smallest amount.
Thing about it like this, In order to have a car all the pieces must be there. If you try to take the engine out, it’s no longer a car.
Electric charge is much the same.
The electron has a set charge, take some away and it’s no longer an electron.

4. The Millikan Experiment
Click here to recreate the Millikan oil-drop experiment

5. Charged Particles
Protons (+e): Mass = x kg, Charge = 1.60 x C
Neutron: Mass = x kg, Charge = 0
Electron (-e): Mass = 9.11 x kg, Charge = x C
e = 1.60 x C

6. Neutral Objects
If the number of electrons equals the number of protons the object is said to be electrically neutral.
In general q (charge) = Ne, where N is an integer.
Since proton are much more difficult to remove, most objects are charged by removing or adding electrons.

7. Charged Objects
When two dissimilar materials are rubbed together electron usually go from one to the other.
Look on the triboelectric scale to see which way they go, positive or negative
Also, charge is conserved.
The net charge of an isolated system is constant

8. Like and Unlike charges
Like charges repel each other
Unlike (opposite) charges attract

9. Conductors & Insulators
Materials that have lose valence electrons are conductors
Materials with tightly held valence electrons are insulators
Can you think of some?

10. Charging by Induction (Conductors)
Click on the picture to open an applet

11. Polarization (Insulators)

12. Coulomb’s Law F = 1/(4peo) q1q2 / r2 1/(4peo) = k F = Force (N)
eo = 8.85 x (electric permittivity of a vacuum)
q = charge (Coulombs)
r = distance between charges
1/(4peo) = k

13. Point Charges
When more than two charges are acting on each other we sum the forces.
Treat each pair independently, then add the forces. r1 r2 q1 q2 q3

14. Point Charges in 2D
When more than two charges are acting on each other in 2D, sum the forces for x and y dimensions.
Again, treat each pair independently, then add the forces. q3 r2 q1 q2 r1

15. Electric Field
Just like mass create gravitational fields, charges create electric fields
With gravity the field strength is measure as Newton per kilogram
What do you think Electric fields are measured in?
Newton's per Coulomb

16. Measuring the Electric Field
If the unit is Newton's per Coulomb, what is the equation?
E = F / qo
Simple enough, right.
E = Electric Field
F = Force
qo = charge producing field

17. Summing electric Fields
It is the surrounding charges that create an electric field at a given point in space.
Look at Example 8

18. Parallel Plate Capacitors
To store charge a capacitor is used.
The most common type is a parallel plate capacitor.
Charge is spread uniformly, not really, through out the plate
E = q /eoA = s / eo
s = charge Density

19. Picturing the Electric Field
Click on the picture to open an applet

20. Electric Field Rules
Fields start at positive and end at negative, or start or end at infinity.
This is by convention. The field is said to predict the movement of a positive charge.
The density of lines should represent the strength of the field.
A positive charge will have a velocity tangent to a field line.
Field lines do not actually exist since the are an infinite number of paths a test charge can take.

21. Electric Fields Inside Conductors
Excess electric charge moves to the surface of a conductor
At equilibrium the electric field inside a conductor is zero
This comes from the fact that free electrons will not move inside the conductor.
So the electric field lines don’t penetrate the conductor
The electric field outside a conductor is perpendicular to the surface
Click here for more info

22. Triboelectric Scale
Human hands (usually too moist, though) (Very positive)
Rabbit Fur
Glass
Human hair
Nylon
Wool
Fur
Lead
Silk
Aluminum
Paper
Cotton
Steel (Neutral)
Wood
Amber
Hard rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Polyester
Styrene (Styrofoam)
Saran Wrap
Polyurethane
Polyethylene (like Scotch Tape)
Polypropylene
Vinyl (PVC)
Silicon
Teflon (Very negative )