142 Quiz 1 Flashcards
What are the two forms of electric charges?
proton and electron
Are electric charges created or destroyed?
no, we just move them around
electric charges
Does anything truly have no charge?
no, a neutral object just has the same number of positive and negative charges
Conductor
at least some of the charges are free to move about within the material
Examples of conductors
ions in water and metals
Insulator
charges do not move freely / charges are stuck to molecules
Examples of insulators
plain water, wood, plastic, glass
Charging by induction
start with electrically neutral material and bring a charged material near it (not touching)
the like-charges repel away and can connect another wire to the ground
like-charges want to get far away from the charge so will move into the ground
disconnecting the ground wire will make the neutral object have the opposite charge of the charged object near it
Why do you have to disconnect ground wire first in charging by induction?
if instead you would have removed the charged object, the charges would have just migrated from the ground into the object to return to neutral
Charging by conduction
touch a charged object to a neutral object
charges migrate to the neutral object and the neutral object becomes charged
How do you determine the direction of the electric force using Coloumb’s law?
Don’t actually use Coloumb’s law, instead you use attract/repel with a coordinate system
principle of superposition
the net force acting on a charge is the vector sum of the forces acting on it
Why are neutral objects attracted to charged objects?
the charged object pulls the opposite charges of the neutral object towards it and pushes the like charges away from it
overall, the opposite charges interact at a shorter distance (since going towards it) so they have a stronger force
this makes the attractive forces overcome the repulsive forces and the charged object is attracted to the neutral object
Do test charges influence electric field?
No! The charge at a point in space does not influence the electric field exactly at that point
What is a field?
a quantity that has a value everywhere in space
can be a scalar or a vector field
What direction do electric fields point for positive charges?
Away from positive charges
What direction do electric fields point for negative charges?
Towards negative charges
How do positive charges feel forces from electric fields?
In the same direction as the electric fields
How do negative charges feel forces from electric fields?
Feel a force in the opposite direction of the electric field
What are the units of an electric field?
N/C
What represents the strength of an electric field?
the density of lines
What represents the direction of an electric field?
direction is tangent to the lines of E
Electric fields in conductors
electric field will create areas in the conducting sphere of more negative charge and areas with more positive charge
the area of more negative charge will create its own electric field in the opposite direction
over time, the field from charge separation will cancel out the original field and there will be no field in the conducting sphere
In general, what is the electric field in a conductor?
0
Electrostatics
case where nothing is changing over time
an example is inside a conducting sphere
What happens inside a conducting sphere?
1) E=0
2) any excess charge on sphere can’t be inside the sphere, excess charge is found on the surface
3) E at surface of sphere is always perpendicular to surface
4) Charge accumulates at sharp points on an irregular surface
What happens when a neutral object is brought near a Vandegraff generator?
the neutral object is initially attracted to the negatively charged generator
then, the generator transfers charge to the neutral object when close enough
the neutral object is now likely charged and will be repelled from the generator
Flux
measure of how much of the electric field vectors penetrate through a given surface
Gauss’s law
a technique for calculating the avg electric field on a closed surface
What angle do we use for Gauss’s law?
angle between the Normal Surface and the Electric Field line
Where do we place the Normal Surface?
kind of at the tail of the electric field line
makes more sense with a picture
normal surface is perpendicular to the material’s surface
For a closed surface through which net flux is zero which statement must be true?
the net charge inside the surface is 0
electric field lines entering the surface matches the electric field lines leaving the surface
E*A = Qinside/episolon not
