Transcript Static Electricity: Electric Charge & Electric Field

Static Electricity: Electric
Charge & Electric Field
Ch 16 in your book
Electric Charge Ideas




2 types of charge: + and –
All of “electricity” is a result of charges and their
interactions
Most materials have equal numbers of positive and
negative charges, so we don’t notice “electric
affects”
Objects become charged by friction, conduction
(touching a charged object) or induction (we did this
in a lab……)
Electric charge



Charge is quantized, it exists only in discrete
amounts
The smallest amount of charge is one
electron = one proton = 1.6 x 10-19 C
Conductors (metals typically) allow charges
(electrons) to move about easily, insulators
(plastic, glass, waffles) don't
Electric charge on conductors

If a conductor is charged (say has a bunch of
excess electrons) where are the charges
located



A) evenly distributed?
B) on the outside?
C) anywhere they want?
Electric Charge & Force







Coulomb's Law: 2 charged objects
experience an electric force.
The electric force can attract (opposite
charges) or repel (like charges)
F = kQ1Q2/r2
k = koulomb constant = 9 x 109
Q = charge in coulombs
r = distance between charged objects
Notice electric Force looks a lot like the
Universal Gravitation equation (coincidence?)
Electric Force


Electric force is a vector with direction. If
there are multiple charges interacting, you
can determine the total force by summing the
force from each charged object.
Principle of Superposition
Electric Field



Every charged object has an "electric field"
around it. This means that a charge in that
field will feel an electric force.
E = F/q the electric field E equals the
electric force F divided the charge of the
object in the field (called the test charge).
By dividing by q, the electric field is
essentially the amount of Force per charge. E
does not depend on the size of the test
charge.
Electric Field


E = F/q but F = kQq/r2
So E = kQ/r2 , this is the amount of the
electric field around a charge Q at a distance
r away from Q. This is the electric field from a
point source (as opposed to an electric field
between 2 parallel plates which we'll talk
about soon).
Electric Field Lines


Electric field lines point in the direction that a
Positive (+) test charge would move.
Electric field lines point away from positive
(+) Q and towards a negative (-) Q.