ch 20 & 21 review electrostatics
TRANSCRIPT
ElectrostaticsReview Glencoe Ch 20 & 21
Monday, May 9, 2011
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
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
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
Coulomb’s Law Problem
What is the net force acting on charge C below?
C2 µC
A-3 µC
B2 µC
20o 30o
5 cm
7.5 cm
Monday, May 9, 2011
Coulomb’s Law Problem
What is the net force acting on charge C below?
C2 µC
A-3 µC
B2 µC
20o 30o
5 cm
7.5 cm
Answer: 58.6 N @ -19 deg
Monday, May 9, 2011
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
CapacitorsCapacitors store charge.
C = Q / V (Unit = Farad)
C = κεo A / d (εo = permittivity constant = 8.8 E -12 F / m)
Monday, May 9, 2011
Millikan’s Oil Drop Experiment
http://www.youtube.com/watch?v=XMfYHag7Liw
Monday, May 9, 2011
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
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-17NW = 2E-17N * 0.043m = 8.6E-19 J
Monday, May 9, 2011