Electrostatics
Test Tomorrow
Review Session
Electric Force
● Like charges repel
● Opposite charges attract
Conservation of Charge
● Charge cannot be created or destroyed
● Charge can move from one object to
another, however the total amount of charge
remains the same
Question
When walking across the carpet you acquire a
certain charge from friction. How does your
charge compare to the charge of the floor?
Ions
● An atom or molecule with a different number
of electrons than protons
● Ions can gain electrons making them....?
● Or if they lose electrons they become....?
Charging an Object
● Charge by friction
● Charge by conduction
● Charge by induction
Charge by Conduction
● Contact between two charged objects
For an Insulator:
● Some of the electrons will move
For a conductor:
● Charge will average between the two objects
Question
When three metal spheres are touched what is
the resulting charge on each one?
-2
+5
+3
Charge by Induction
● This effect works with conductors
1. Forces caused by a nearby charged object
will cause polarization of a conductor
2. The far side of the conductor is connected to
ground
3. Some of the access charge leaves the
conductor
4. A net opposite charge is left on the
conductor
Question
When a conductor is charged by induction, how
does the charge on the conductor compare to
the charge on the charged object?
Polarization
Electric forces causing an object to have a
more positive and negative side
The object remains at zero net charge
Question
Why will dust be attracted to a CD or computer
screen which has been wiped with a dry cloth?
Conductor vs. Insulator
● Conductors allow free movement of
electrons
● Insulators do not allow free movement of
electrons
● Conductors can be charged by induction
● Attraction with polarization typically happens
with insulators.
Coulomb's Law
● Determines the force between charges
● Force depends on both charges and
distance
● Follows inverse square law
Question
If two charged objects have a force F between
them. How does the force change if the charge
is doubled on both?
What if the distance between the two is then
doubled?
Q
2Q
Q
d
2d
2Q
Question
Two charged objects have a force F between
them, what if the distance between them
reduced to 1/4th the original?
Electric vs Gravitational Force