2. Electric Charge, Q
Electric charge is conserved. When charging occurs, it is
because charges are separated, not created.
Charges can be positive or negative.
Opposites attract and like charges repel.
The fundamental charge was measured by Millikan and is the
charge of an electron (or proton) and is 1.6E-19 Coulomb.
The unit of charge is the Coulomb, C.
Monday, May 9, 2011
3. Inductors & Conductors
Charges sit on the surface of inductors (e.g. plastics) and flow
through conductors (e.g. metals). YOU are a conductor to
ground (an infinite sink for electrons).
When a charge is INDUCED on a surface, that means like
charges were driven away and opposite charges were brought to
surface:
Monday, May 9, 2011
4. Coulomb’s Law
The force of attraction between two charges varies directly with
the product of their charges and inversely with the square of the
distance between them: Fe = k Q1Q2 / r2
k = 9E9 N m2 / C2
Monday, May 9, 2011
5. Coulomb’s Law Problem
What is the net force acting on charge C below?
C
2 µC
5 cm
B 30o A
20o
2 µC -3 µC
7.5 cm
Monday, May 9, 2011
6. Coulomb’s Law Problem
What is the net force acting on charge C below?
C
2 µC
5 cm
B 30o A
20o
2 µC -3 µC
7.5 cm
Answer: 58.6 N @ -19 deg
Monday, May 9, 2011
7. Electric Field
Any charge creates an electric field.
E = Fe / q (Units N/C)
Draw field lines from + => -
It takes work to set up an electric field. W = Fe * d = Ue.
Electric potential difference is V = Ue / q or V = E * d.
Monday, May 9, 2011
8. Capacitors
Capacitors store charge.
C = Q / V (Unit = Farad)
C = κεo A / d (εo = permittivity constant = 8.8 E -12 F / m)
Monday, May 9, 2011
9. Millikan’s Oil Drop Experiment
http://www.youtube.com/watch?v=XMfYHag7Liw
Monday, May 9, 2011
10. Ch 21 Sample Problems
How much work is done on a proton to move it from a negative
plate to a positive plate 4.3 cm away if the field is 125 N/C?
Monday, May 9, 2011
11. Ch 21 Sample Problems
How much work is done on a proton to move it from a negative
plate to a positive plate 4.3 cm away if the field is 125 N/C?
Answer:
W = Fed
Fe = Eq = 125 N/C * 1.6E-19C = 2 E-17N
W = 2E-17N * 0.043m = 8.6E-19 J
Monday, May 9, 2011
