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Where did the idea of a field result form?
How one object can have an effect on another object over a distance
What is a shorter definition of field?
action at a distance
What is an electric field?
Generated by any object which has an electric charge
How many characteristics do electric fields have?
3
What are electric fields produced by?
positive or negative objects
When do electric fields decrease in strength?
with increased distance
What type of fields are electric fields?
vector fields
So what are the 3 characteristics of electric fields?
Electric fields can be produced by either positive or negative charged objects
Electric fields decrease in strength with increased distance (so inverse)
Electric fields are vector fields
What happens when a small positive test charge is placed in an electric field produced by a larger point charge?
it will experience a force
SIDEBAR: What is a test charge?
Always positive smaller charge that is acted on
SIDEBAR: What is a point charge
Positive/Negative larger charge that produces the field
What is electric field strength
Ratio of the magnitude of electric force between the charges (Fe) to the magnitude of the test charge (q)
Is electric field strength a vector quantity or a scalar quantity?
vector (in the vector field)
What is the equation for the electric field strength (force dependent)?
Fe/q = E
What does the equation for electric field strength (force dependent) calculate?
magnitude
How do you find the direction of an electriec field?
Direction in which a small imaginary positive test charge will move in the field produced
What do we imagine about the test charge in thei field?
So small it responds to the electric field without changing the filed
What will a positive test charge be repelled by?
a positive point charge
WHat will a positive test charge be attracted by?
a negative point charge
What is the equation for electric field strength (Force Independent)?
E = kQ/r^2
SIDEBAR: What is Q?
the charge of the object producing the field (point charge)
What is an electiric field generated by?
Q a point charge
The larger the charge....
the greater the strength of the field arouind it
the farther we are from the charge...
the weaker hte electric field becomes
What is a non uniform electric field?
Field with strength vectors that point all directions away from/towards the source
When two charges are involved....
the resulting electric field is the vector sum of the fields from each of the charges
SIDEBAR: What charge is the force dependent on?
The test charge
What are electric field diagrams useful for?
visualize the direction and strength of an electric field at various points in space as a map
what did Micheal Faraday propose?
electric field lines/lines of force
What is divergence?
Electric field lines change in strength as they go out and don't come back to the origin
What happens when 2 or more point charges are creating the field?
Vector sum of all charge
Where are electric field lines directed?
Away from positive charges and toward negative charges
What do the number of field lines indicate?
the strength of the field by that point
do the electric field lines ever cross?
no
what are the field lines perpendicular to?
the point charge
how do electrons move in a conductor?
freely
how do electrons in a conductor reach a state of static equilibrium?
follow the electric field strength
what does static equilibrium mean?
means all charges are at rest and experience no net force
What does a charged flat surface do to the electrons?
use electrostatic forces of repulsion to cause them to spread and distribute evenly along the outer surface
What part of an irregularly shaped conductor has increased field strength? Why?
Convex as the perpendicular lines fan out more causing more charges to be able to accumulate
What causes the charges to accumulate at sharp points?
net electrostatic forces
why will charges do in an irregularly shaped concave conductive object?
to prevent bunching so it is weaker here
So where is charge density and electric field density greates?
With the sharpest curve
What causes charges within a conductor to redistribute?
mutual repulsion
How long does it take charges in conductors to redistribute?
fraction of a second
What does the time depend on?
he physical details of a conductor
because of this redistribution, what is the electrostatic field inside a conductor?
0
How did michael faraday prove that the electrostatic field inside a conductor is 0?
Michael Faraday proved this by sitting in a charged cage with an electroscope in hand, and no field was detected inside even while sparks were flying outside
If a positive test charge is moved toward a positive point charge what is done on the test charge?
work must be done against the electric field
when work is done against the electric field what happens to the electric potential energy?
it increases
If a positive test charge is moved toward a negative point charge what is done on the test charge?
work is done by the electric field (working together)
when work is done with the electric field what happens to the electric potential energy?
it decreases
when a positive test charge is moved in a direction that has no correlation with the electric field, what happens to the electric potential energy
it does not change
what is electric potential hinged on?
the change in the position of a charge within an electric field
what is the unit ans symbol for electric potential
V and Volt
is it possible to measure the absolute electric potential in an electric field?
no but we measure the electric potential difference
so what is electric potential energy?
change in electric potential energy per unit of charge
is electric potential a scalar of a vector quantity?
scalar
what are equipotential lines?
have the same potential energy at all points on the line
where do equipotential lines exist?
parallel to the plates, perpendicular to the electric field lines
where do equipotential linnes exist?
in a uniform field (so parallel planes)
when is voltage equal?
when measure from equal distance along the equipotential liens
what happens when 2 metallic plates are set a distance apart and are then hooked up to a potential difference?
one plate will have a negative charge while the other will have a positive charge
what will happen to the charges in each plate?
they will distribute evenly due to the electrostatic repulsion of like charges
what will happen to the electrostatic forces between the plates?
it will cause the charges to accumulate on the inner surfaces due to the attraction
in which direction will the electric field be?
positive to negative
Is the electric field between two parallel plates uniform or non uniform?
uniform
Where is it nonuniform in a parallel plate system?
at the ends of plates
WHat is the equation for parallel plates?
E = V/d
does potential difference work with non-charged particles?
nope
when can we ignore gravity with parallel plates?
when the gravitational force is less than 100 times the electrical force
when must we facotr in gravity?
when the particle is suspended, moving vertically, or accelerating upward or downward
when did JJ Thomson measure the charge-to-mass ratio of an electron?
1897
when did RA milkian determine the charge of an Electron?
1906 - 1913
What did the basic design of the oil drop experiment have?
2 parallel plates a distance apart, hooked up to a distance apart, atomizer with oil inside
What caused the oil to have a static charge?
Friction between the droplets and the atomizer plastic nozzle
What did Milikan deduce form this experiment?
When the droplet is motionless, the electric and gravitational forces are balanced
How many educated assumptions did Milkian make?
3
What did he deduce about the identity of electrons?
All electrons are identical - the same amount of charge
What did he deduce about the electron mass?
The mass of each electron is so small that the addition or subtraction of a few will not significantly change the mass of the oil droplet
What did he deduce about the ratio of the charge of the oil trop and the elementary charge
The amount of charge on the oil droplet will be a whole number multiple of the charge of the electron
what is quantization?
physical quantity can only exist in discrete fixed values
so all charge is a multiple of the elementary charge (whole number)
what was the elementary charge Millikan discovered?
1.69E-19 C
who designed the first electrochemical cell?
alessandro volta
what is an electrochemical cell?
One in which a spontaneous chemical reaction produces electrical energy
what was the electrochemical cell the first to produce?
a steady flow of electrical current
where is the charge built up?
electrodes
what does the electrochemical cell create?
potential differences between the electrodes
what is the external cathode inside the cell?
internal anode
why is it an internal anode?
because it accumulates electrons
what is the external anode inside?c
athode inside the cell
why is it an internal cathode?
electron holes travel to the cathode where they receive these holes
what is required to complete the circuit
a wire
what did ampere use the volta cell for?
to study the flow of charge through a conductor
what is current?
When a Coulomb (C) of charge flows past a given point in a conductor over a one-second time interval, a current of one Amp exists
what is a conductor?
Material through which electric charges spread easily
what is an insulator?
Any material that retards or restricts the flow of electric charge
when the potential difference across a conductor is large, what does an insulator do?
it conducts
what factors affect resistance?
Type of material
Directly proportional to the length of the conductor
Inversely proportional to the cross-sectional area of the conductor
Resistance is generally directly proportional to the absolute temperature of the conductor
what are the limitations of ohms law?
This applies only to solid conductors
Since resistance varies with temp, Ohm's law varies with temperature
This applies to direct current and instantaneous alternating current
Combos of materials conduct charge better in one direction than the other (Ohm's law does not account for this)
