1. Electrostatics

2. Static Electricity
Static Electricity – charges build up on an object and are “at rest”.
Fundamental Rule of Charge
Opposite charges attract
Like charges repel
3 methods of charging : friction, conduction, & induction

3. Electric Charges Electrons have a negative charge
Protons have a positive charge
Charge is measured in “coulombs”
“qo” or “q” in an equation is used for charge
Elementary charge = 1.6 x C
One electron has a charge of -1.6 x C
One proton has a charge of x C

4. Conductors & Insulators
- electrons flow easily
- charge spreads over entire surface
- examples: copper, gold, silver, aluminum, metals
Insulators
- electrons do not flow easily
- charge stays where it was charged
- examples: plastic, glass, rubber, wood, styrofoam

5. Methods of Charging
Charging by friction – two neutral objects are rubbed together and become oppositely charged ( the object that gains electrons becomes negatively charged and the one that loses electrons becomes positively charged)
Charging by conduction – a charged object touches a neutral object so charges are transferred between objects until they reach charge equilibrium, ie. have equal charge (the neutral object gets the same charge)
Charging by induction – a charged object is brought near but not touching a neutral object ( the neutral object gets a temporary charge separation – gets opposite charge near the charged object)

6. Induction and Grounding
If grounding is used along with induction, the neutral object will become permanently charged. It will have a charge opposite that of the charging object.
This is done by grounding the neutral object on the side opposite the charging object.
Grounding is when a path is provided for the excess charge to flow into or out of the ground.

7. Electric Forces Coulomb’s Law:
Given 2 charges of magnitude q1 & q2 separated by a distance d, the magnitude of the force that each of the charges exerts on the other is calculated using Coulomb’s Law. The forces can be attractive or repulsive and must be equal and opposite … Newton’s 3rd Law! q1 q2 d + -
Coulomb’s Law: F F + q1 q2 F
FE = electrostatic force, Newtons (N) k = electric or Coulomb’s constant = 9 x 109 Nm2/C q1 = charge of the first object, C q2 = charge of the second object, C d = distance between the two charges (center to center), m

8. Direction of electric force
If there are 2 like charges, the force will be repulsive. (Fe will be positive)
If there are 2 unlike charges, the force will be repulsive. (Fe will be negative)
Note: When finding net force for 3 or more charges, it is best to decide whether forces are attractive or repulsive and draw the force vectors rather than use the signs of Fe.

9. Electroscopes Devices that detect that an object is charged
Pith-ball electroscope
- a foil covered styrofoam
ball hanging by a thread
Metal leaf electroscope
- 2 metal leaves on the end
of a metal rod enclosed in
glass

10. Electroscopes – detecting charge
The pith ball is attracted
to the negative rod due
to charge separation (induction)
The metal leaves both get a
negative charge (conduction) so
they repel each other (spread apart) - - - - + - - - - - - - -

11. Electric Fields
Electric fields are the “energy fields” that surround any charged particle”
Any charge placed in this field will experience a force.
Electric fields are vectors (magnitude & direction).
The direction of an electric field is defined by the direction of the force on a tiny “+” test charge placed in that field. Thus electric fields are always in a direction that is away from “+”
and toward “-”.
Electric Field strength is the force per unit charge and is measured in units of N/C (Newtons per Coulomb)
E = Electric Field Strength, N/C
F = Electrostatic Force, N
qo = Charge placed in the electric field, C

12. Electric Field around a Charge
Field around a Field around a
Negative charge Positive charge - +

13. Electric field around two opposite charges

14. Electric Field around two like (positive) charges

15. Electric Field Strength at a point near a charge. q E + d
E= electric field strength or intensity, N/C
k = electric or Coulomb’s constant (= 9 x 109)
q = charge causing the field, C
d = distance from the charge to where the field
strength is being measured, m

16. q = charge of the object causing the electric field, C
Note - In the formulas on the previous slides, q0 and q are described as follows:
q = charge of the object causing the electric field, C
qo = test charge or the charge of the object placed in the field, C

17. Capacitor – a device used to store charge
Capacitor – a device used to store charge. Example of a common simple capacitor:
2 oppositely charged conductors
To get more stored charge:
increase the size of the plates
decrease the plate separation
increase the voltage of the battery - + E
Note: This is an example of a uniform electric field. + -
battery
Insulator such as air (keeps the charges separated)

18. Old Formulas and Constants:

19. New Formulas
Constant: k = 9 x 109 Nm2/C2