142 Final Flashcards
Conductors
at least some of the charges are free to move around within the material
ex: metals, ions in water
Insulators
charges do not move freely
ex: plain water, wood, plastic, glass
Charging by conduction
actually touch the charged object to neutral object
charge is free to move from the conductor to the object
neutral object will have the same charge as the charged conductor brought near
Charging by induction
bring charged material towards a neutral object
like charges in neutral object will repel
attach a wire to ground, like charges will leave
neutral object becomes charged the opposite of the charged insulator brought near
What happens if you move the insulator away from neutral object before disconnecting ground wire?
The ground wire will push charges back into the object to make it neutral
Why is net charge inside conductors 0?
charges move around and adjust to make sure the net electric field is 0
If you release a particle from rest in what direction will it move?
towards decreasing potential energy
For potential energy graphs, what trick helps draw them?
use the charges when calculating potential energy
tells you +y or -y
What happens to electric potential when distance decreases for a positive charge?
when r decreases, V increases
What happens to electric potential when distance decreases for a negative charge?
when r decreases, V decreases
V becomes more negative
What direction does a + charge move in a battery?
From - to +
Against the E field, which is unnatural
Does this through a chemical reaction
*only applies to batteries not capacitors
Why can we normally ignore internal resistance of a battery? And what can we do when we do?
Can normally ignore because r«
Why is voltage difference across battery actually lower than EMF?
internal resistance
∆V=E(1-r/r+R)
What happens to current across a capacitor as voltage increases?
current decreases
What can not change instantly on a capacitor?
charge / voltage
Q=cV
What can not change instantly on an inductor?
current
voltage can (rate of change)
If you wait a long time after closing a capacitor what always happens to CURRENT?
current goes to 0
*check rules but this should work
How can you treat a capacitor if it has no charge the moment when you open / close a switch?
can treat the capacitor as a wire and find the current through it
since V=0 (can’t change), the capacitor is basically a current carrying wire
What would happen to an inductor if you changed the current through a circuit?
you could produce an induced emf in the inductor (like a solenoid)
emf would be in the direction that opposes the created B-field (if increasing B magnitude)
How can you treat the inductor after a long time?
the current is no longer changing, so there is no voltage across the inductor
can treat as a wire
What is the magnetic force between two parallel wires with current in the same direction?
attractive
What is the magnetic force between two parallel wires with current in opposite directions?
repulsive
What do you have to make sure to do for AC circuits?
make sure calculator is in radians when using sin functio
AC circuits: relationship between current and voltage across resistors
current and voltage are in phase
V=IR
when current increases, so does voltage
What is rms?
a type of average
AC circuits: relationship between current and voltage in capacitors
voltage can’t change instantly
the current leads the voltage
AC circuits: relationship between current and voltage in inductors
current can’t change instantly
the current lags the voltage
What does Xc stand for?
capacitive reactance
has units of olmns
Transformers general premise
when current changes in one wire, the other wire gets an induced current
Why do transformers not work in DC circuits?
the current is not changing to create an induced current
When just have Fb acting on a velocity, not completely perpendicular, what is the motion of the particle?
only the perpendicular component gets acted on
the parallel component does’t do anything and just moves at a constant speed
gives us a spiral with a constant pitch
When have Fb and Fe acting on a particle what shape do you get?
spiral with a non-constant pitch
Back current
current change by flipping switch creates its own B (from current), thus has a ∆I and an EMF
want to oppose the current’s B field, so create an induced current in the opposite direction
What happens to back current over time?
no back current since nothing is changing to induce current
What is the angle used for flux?
the angle between the “A” vector (perpendicular to the surface) and B
How to find the direction of the “A” vector for torque problems? For example, between into and out of the page?
Have to use the direction of the current and the right hand rule
How to find the direction of torque?
use the RHR with the “A” vector and B field
What is something to be careful of when using wavelength?
often need to convert nanometers to meters in problems
What direction do + charges feel an electric force?
feel a force in same direction as E field
What direction do - charges feel an electric force?
feel a force in opposite direction as E field
When light is perfectly polarized from Brewster’s angle what direction is the E field?
the E field goes in and out of the page (perpendicular to the v)
is parallel to the surface that the light reflected off of
What is strategy for doing circuit problems?
take picture on iPad
highlight and look to see if anything is in parallel or series
simplify the circuit
What is strategy for inductor problems?
always see what the current is through the inductor before a change happenns!
When the question says magnitude what should you give?
a positive answer
When can you use a screen to see an image? Why can’t you see the image without the screen?
use a screen to see an image if the virtual image is real (where the light is)
can’t see the image without the screen because the light rays that make the image are going away from you
If a negative charge moves from 0V to +V do you have to put energy into the system to slow the particle or take energy out of the system to slow the particle?
as the negative particle moves to a higher V, PE is decreasing
this is because higher V is more negative, so smaller PE
then, as PE decreases KE increases. So, to slow the particle you would have to take KE out of the system
Positive work
put energy into the system
negative work
take energy out of the system
How can you tell if a capacitor is at negative or positive V based on motion of particle released close to a capacitor?
particle will move to a lower PE
if a positive charge moves away, it is moving towards 0V which means capacitor was at + V
if a positive charge moves towards, it is moving towards - V from 0 V to have a decreasing PE
How do you find voltage at a point between the two plates of the capacitor?
find the voltage drop across the capacitor and the starting / final voltages
at the certain point, the particle has performed a ratio of the voltage drop
Vpoint = Vstarting - (ratio traveled / distance between plates)Vdrop
