Ch 20,21 Notes

1. Electrostatics
Ch 20

2. Electrostatics
 Study of electrical charges that can be collected
and held in one place

3. Electrical Basics
 All atoms have a positively charged
nucleus (made up of protons (+) and
neutrons (0) that are surrounded by
electrons (-))
 When the positive charge equals the
negative charge the atom is neutral
 Electrons can be removed or transferred
by the addition of energy.

4. Conservation of Charge
 Electric charge is never created or
destroyed, just transferred by the
separation of positive and negative charge
 Like charges repel
 Opposite charges attract
 A neutral object will be attracted to a
charged object

5. Electrical Materials
 Conductors – materials through which
charges will move about easily because
they have loosely bound electrons; Ex:
most metals
 Insulators - materials through which
charges will not move about easily
because they have tightly bound
electrons; Ex: glass, wood, plastic

6. Charging a neutral object
 Conduction – charging a neutral object by
touching it with a charged object
 Induction – charging a neutral object
without touching it, but by the separation
of charges

7. Coulomb’s Law
 The electrical force between 2 charged objects
varies directly with the product of the charges
and inversely with the square of the distance
between them
F = kqq’ k = 9EE9 Nm2/C2
d2
 A repulsive force is positive because both
charges are alike
 An attractive force is negative because the
charges are opposites

8. Electric Fields
Ch 21

9. Electric Fields
 A charge creates an electric field in all
directions
 The electric field intensity is the force per
unit charge
E = F/q units are N/C

10. Electric Field Lines
 Provide a model for
the electric field. The
strength of the field is
indicated by the
spacing between
lines. The closer the
lines the stronger the
field. The field always
points away from the
positive towards the
negative charge.

11. Electric Potential Difference
 Change in Potential Energy per unit
charge; this is often called voltage
difference
ΔV = ΔPE/q or ΔV = Work/q
units are J/C = volts
 Also ΔV = Ed

12. Grounding
 Eliminating the
excess charge by
touching a charged
object to the earth
(low voltage). The
charge moves from
higher potential to
lower potential until
they are equal.

13.  All charged objects
are on the surface of
a solid conductor
 Excess charge will
move to the outer
surface of a hollow
conductor

14. Capacitor
 Stores charge; made up
of two oppositely charged
conducting plates
separated by an insulator
 Capacitance – ratio of
charge stored to potential
difference
C = q/V
units are C/V = 1 Farad (F)

15. Millikan Experiment
 He determined the
charge of an electron
to be 1.6 EE -19 C
 Eq = mg