142 Final

Question 1

Conductors

Answer 1

at least some of the charges are free to move around within the material

ex: metals, ions in water

Question 2

Insulators

Answer 2

charges do not move freely

ex: plain water, wood, plastic, glass

Question 3

Charging by conduction

Answer 3

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

Question 4

Charging by induction

Answer 4

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

Question 5

What happens if you move the insulator away from neutral object before disconnecting ground wire?

Answer 5

The ground wire will push charges back into the object to make it neutral

Question 6

Why is net charge inside conductors 0?

Answer 6

charges move around and adjust to make sure the net electric field is 0

Question 7

If you release a particle from rest in what direction will it move?

Answer 7

towards decreasing potential energy

Question 8

For potential energy graphs, what trick helps draw them?

Answer 8

use the charges when calculating potential energy

tells you +y or -y

Question 9

What happens to electric potential when distance decreases for a positive charge?

Answer 9

when r decreases, V increases

Question 10

What happens to electric potential when distance decreases for a negative charge?

Answer 10

when r decreases, V decreases

V becomes more negative

Question 11

What direction does a + charge move in a battery?

Answer 11

From - to +

Against the E field, which is unnatural

Does this through a chemical reaction

*only applies to batteries not capacitors

Question 12

Why can we normally ignore internal resistance of a battery? And what can we do when we do?

Answer 12

Can normally ignore because r«

Question 13

Why is voltage difference across battery actually lower than EMF?

Answer 13

internal resistance

∆V=E(1-r/r+R)

Question 14

What happens to current across a capacitor as voltage increases?

Answer 14

current decreases

Question 15

What can not change instantly on a capacitor?

Answer 15

charge / voltage

Q=cV

Question 16

What can not change instantly on an inductor?

Answer 16

current

voltage can (rate of change)

Question 17

If you wait a long time after closing a capacitor what always happens to CURRENT?

Answer 17

current goes to 0

*check rules but this should work

Question 18

How can you treat a capacitor if it has no charge the moment when you open / close a switch?

Answer 18

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

Question 19

What would happen to an inductor if you changed the current through a circuit?

Answer 19

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)

Question 20

How can you treat the inductor after a long time?

Answer 20

the current is no longer changing, so there is no voltage across the inductor

can treat as a wire

Question 21

What is the magnetic force between two parallel wires with current in the same direction?

Answer 21

attractive

Question 22

What is the magnetic force between two parallel wires with current in opposite directions?

Answer 22

repulsive

Question 23

What do you have to make sure to do for AC circuits?

Answer 23

make sure calculator is in radians when using sin functio

Question 24

AC circuits: relationship between current and voltage across resistors

Answer 24

current and voltage are in phase

V=IR

when current increases, so does voltage

Question 25

What is rms?

Answer 25

a type of average

Question 26

AC circuits: relationship between current and voltage in capacitors

Answer 26

voltage can't change instantly the current leads the voltage

Question 27

AC circuits: relationship between current and voltage in inductors

Answer 27

current can't change instantly the current lags the voltage

Question 28

What does Xc stand for?

Answer 28

capacitive reactance has units of olmns

Question 29

Transformers general premise

Answer 29

when current changes in one wire, the other wire gets an induced current

Question 30

Why do transformers not work in DC circuits?

Answer 30

the current is not changing to create an induced current

Question 31

When just have Fb acting on a velocity, not completely perpendicular, what is the motion of the particle?

Answer 31

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

Question 32

When have Fb and Fe acting on a particle what shape do you get?

Answer 32

spiral with a non-constant pitch

Question 33

Back current

Answer 33

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

Question 34

What happens to back current over time?

Answer 34

no back current since nothing is changing to induce current

Question 35

What is the angle used for flux?

Answer 35

the angle between the "A" vector (perpendicular to the surface) and B

Question 36

How to find the direction of the "A" vector for torque problems? For example, between into and out of the page?

Answer 36

Have to use the direction of the current and the right hand rule

Question 37

How to find the direction of torque?

Answer 37

use the RHR with the "A" vector and B field

Question 38

What is something to be careful of when using wavelength?

Answer 38

often need to convert nanometers to meters in problems

Question 39

What direction do + charges feel an electric force?

Answer 39

feel a force in same direction as E field

Question 40

What direction do - charges feel an electric force?

Answer 40

feel a force in opposite direction as E field

Question 41

When light is perfectly polarized from Brewster's angle what direction is the E field?

Answer 41

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

Question 42

What is strategy for doing circuit problems?

Answer 42

take picture on iPad highlight and look to see if anything is in parallel or series simplify the circuit

Question 43

What is strategy for inductor problems?

Answer 43

always see what the current is through the inductor before a change happenns!

Question 44

When the question says magnitude what should you give?

Answer 44

a positive answer

Question 45

When can you use a screen to see an image? Why can't you see the image without the screen?

Answer 45

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

Question 46

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?

Answer 46

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

Question 47

Positive work

Answer 47

put energy into the system

Question 48

negative work

Answer 48

take energy out of the system

Question 49

How can you tell if a capacitor is at negative or positive V based on motion of particle released close to a capacitor?

Answer 49

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

Question 50

How do you find voltage at a point between the two plates of the capacitor?

Answer 50

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