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AP Physics 2 Videos 59 videos

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AP Physics 2: 1.1 Properties of Objects and Systems 190 Views


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AP Physics 2: 1.1 Properties of Objects and Systems. What is the magnitude and direction of the conventional current in this wire?

Language:
English Language

Transcript

00:00

Hurry And here's your smoke too Sure brought to you

00:05

by day dreams We don't daydream We maintain focus at

00:09

all times and never let our imaginations run away Because

00:11

we're always dedicated Tio What was that Sorry Drifted away

00:16

there from all right Tear the following passage and image

00:19

Here we go jakes learning about electric current in class

00:22

But the monotonous voice of the teacher sends jake into

00:24

a daydream Imagine shrinking down to the size of an

00:29

electron In exploring the wire jake abruptly wakes up when

00:33

his teacher asked him are you paying attention Howto electrons

00:36

behave Jake replies Well they're pretty cool They go with

00:39

the flow in his dream when a potential difference is

00:43

set up across the ends of the wire jake and

00:45

the electrons all started bumping along in the same direction

00:50

The current the wires calculated with the equation i equals

00:53

nada que where N is the number of charged particles

00:57

per unit Volume is the cross sectional area of the

01:01

wire is the drift velocity of the particles and cue

01:05

is the charge on each particle brought to you by

01:08

american express Yeah All right The cylindrical wire above has

01:12

a radius of zero point five millimeters The electrons have

01:15

a speed of be equals one millimeters per second and

01:18

a density of unequal six times ten to the twenty

01:20

eight meters to the negative third what's the magnitude and

01:24

direction of the conventional current in this wire and hear

01:28

the potential answers all right Interesting Well when we daydream

01:32

were usually in a spaceship headed to mars not wearing

01:35

clothes or we're making the winning shot at the buzzer

01:37

Yeah like that We don't dream about being an electron

01:40

and a wire but we don't judge jake here he's

01:42

going to solve the world's problems and we're grateful were

01:46

given most of what we need here were given number

01:48

of particles see were given the radius of the wire

01:52

And with that we were able to find the area

01:54

of the cross section with a little old school gym

01:56

called pi r squared remember that didi Yeah area and

02:01

because we're dealing with some crazy big numbers will make

02:04

it a little easier on ourselves and round pie down

02:06

to three Oh and will express the radius in meters

02:10

rather than millimeters keep everything in the same firm we're

02:14

given The drift velocity which is one millimeter per second

02:18

or one meter times ten to the negative third per

02:20

second We're not given the charge of each particle but

02:23

we know that we're dealing with electrons Electrons are teeny

02:26

tiny little particles with a tv tiny electrical charge Their

02:30

charges so tiny in fact that they produce the smallest

02:33

possible unit of charge called the elementary charge or just

02:37

eat elementary charge is produced by protons and electrons The

02:41

proton is all smiles with a positive elementary charged electrons

02:46

so they don't go in for all that positive attitude

02:48

stuff they're always negative Negative negative twenty four seven Yeah

02:52

like that real downer Sad mood The elementary charge of

02:55

one electron is one point six two one seven six

02:58

five seven times ten to the negative nineteen who longs

03:03

Yeah but like electricity we try to take the path

03:06

of least resistance So we'll round that up to times

03:09

ten to the negative nineteenth Cool Okay we've got our

03:12

numbers and we can plug him into our equations Quick

03:14

reminder Current equals the number of charged particles per unit

03:18

of volume times the cross sectional area of the wire

03:21

times The drift velocity the particles times the charge of

03:24

each particle Got that Now if you think and we

03:27

have our density six times ten to the twenty eighth

03:29

meters to the negative third we have our area three

03:32

times point five times ten to the negative Thirty meters

03:34

squared You have our velocity one meter times ten to

03:37

the negative Third per second And we have our charge

03:40

two times ten to the negative nineteenth cool arms putting

03:42

in the numbers just looks like this and it's pretty

03:45

ugly And we can clean that up a little by

03:47

taking care of the square in the middle making the

03:49

area equal three times point two five times ten to

03:52

the negative six meters square And when we do the

03:55

math or have our fancy pants calculated to the mat

03:58

for us we come up with an answer of a

04:00

charge of nine films per second A nine amps Look

04:04

at us making progress All right One last part of

04:07

the puzzle is knowing which direction the current is going

04:09

in this wire We're dealing in terms of conventional charge

04:13

which assumes the charge is positive Well positive charge moves

04:16

from higher potential towards lower potential Those since a has

04:20

a higher potential than be the conventional current is making

04:23

a beeline towards b b line and say we did

04:26

there get it pretty good All right now if we're

04:29

talking in terms of electron flow it'd be the opposite

04:31

But we're not talking about electron flow so don't sweat

04:34

that right now after all of this were able to

04:36

definitively state that the answer is there's A current of

04:39

nine amps headed towards you shouldn't worry if we daydream

04:43

little it's actually an important part of the scientific process

04:45

It also prepare us for a nobel prize speech Yeah 00:04:48.867 --> [endTime] now that's some great daydreaming

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