Transcript Electrostatics, Part 2

Physics 102-002
Announcements
• WebAssign –
– Chapter 22 due next Wednesday
• Exam #2 corrections still not graded
• Exam #3 is on April 9
Class Schedule
3/19
Chapter 10
Projectile and Satellite Motion (Pg 184-191)
3/21
Chapter 11
Atomic Nature of Matter (Pg 211-223)
3/26
Chapter 22
Electrostatics, Part 1 (Pg 410-419)
3/28
Chapter 22
Electrostatics, Part 2 (Pg 419-430)
4/2
Chapter 23
Electric Current, Part 1 (Pg 436-443)
4/4
Chapter 23
Electric Current, Part 2 (Pg 444-452)
4/9
Midterm Exam #3
Chapter 22
Electrostatics, Part 2
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Charge Polarization
Electric Field
Electric Potential
Electric Charge Storage
Charge Polarization
A charge can be induced in a conductor by bringing another charged object
close by:
+
This is because charges are free
to move around in a conductor.
+
+++++++
Charged rod
+
Metal sphere
In an insulator, charges aren’t free to move around, so when a charged
object is brought near, the charges within the insulator’s atoms and
molecules get redistributed:
+++++++
Insulator atom
Examples: A comb and paper
A balloon and the wall.
-
+
More accurately, the centers of
charge are moved so that one
side of the atom is more
positive, the other side is more
negative. The atom becomes
electrically polarized.
Electric Field
We know that a force acts between charges – the “Coulomb Force”
This force acts even though the objects aren’t in contact, and extends
through all space … just gets weaker as you move away from the charged
object.
The force field due to a charged object exists in the space around it
regardless of whether another object is there to “feel” it (just like gravity) ..
And it’s called the “Electric Field”.
The field acts in the direction that a positive “test” charge would be pushed.
+
Field due to a
positive charge
-
F
E
q
Field due to a
negative charge
See text pg 423 for pictures of some example electric field diagrams.
Electric Shielding
The electric field on the inside of any conductor is zero, regardless of how
large the field is on the outside.
The field strengths due to all the charges on the outside of the conductor
cancel each other on the inside, making the field inside completely zero.
Shape and size of the conductor don’t matter.
Electric Potential
Recall that gravitational potential energy depended on position of a
mass in a gravitational field.
Similarly, the potential energy of a charge depends on its position in an
electric field. We call this its Electric Potential Energy.
The electric potential energy per unit charge is the Electric Potential.
Electric Potential (Volts) 
Electric Potential Energy
Charge
1Volt  1
Joule
Coulomb
The voltage in your wall plug is an indication of the energy that each
electron has as it goes through your house.
The “120V” electric potential of your outlets means there is that much
potential difference between the sides of the plug.
Question 1
How much energy is give to 1 Coulomb of
charge flowing through a 1.5 Volt
battery?
A.
B.
C.
D.
400 Joules
25 Joules
1.5 Joules
1.9 billion Joules
Electric Energy Storage
Electric energy can be stored in a device called a Capacitor
conductors
+
+
+
+
-
Air Or
Other insulator
The plates of the conductors are charged, then the wires are
disconnected, leaving the charges on the plates. The electric energy is
stored in the field between the charged plates.
A Van de Graaff Generator is another way to store electric energy.