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AP Physics 2 Videos 66 videos

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AP Physics 2: 1.3 Systems Interactions and Changes 1 Views


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AP Physics 2: 1.3 Systems Interactions and Changes. When the circuit is completed, what current flows through the 750 ohm resistor?

Language:
English Language

Transcript

00:04

And here's your shmoop du jour brought to you by cupcakes because we

00:07

love bite-sized desserts whether they're supposed to be bite-sized or not let it [Woman swallows cupcake]

00:13

go Louis said his mother let it go no way

00:16

Louise would not rest until justice was served minutes earlier he had opened the

00:22

cupboard to find that his cupcake was missing Louise began his investigation [Luis opens cupboard]

00:26

he lined up the usual suspects dad and Fido no one was talking or barking his

00:33

case was colder than a penguin at the Winter Olympic then Luis remembered [Penguin skiing]

00:37

something that might blow this case wide open the cops on that TV show had used a

00:42

lie detector so Luis built one for himself [Luis with a lie detector]

00:45

he hooked the suspects up one by one phyto just wagged his tail [Fido performing lie detector test]

00:50

dad denied the crime at first but for the questioning broke him he mumbled and

00:54

hesitated a single bead of sweat running down his face the buzzer sounded so we [Dad sweating while doing lie detector test]

00:59

could barely believe it cupcake crossed by his own father okay take a look at

01:04

the following diagram yeah this lie-detector a circuit took advantage of [Lie detector circuit diagram appears]

01:09

a suspect physical inability to lie a moist finger from a sweaty skinned liar

01:15

would complete the circuit and ring the buzzer when the circuit is completed

01:20

what current flows through the 750 ohm resistor assume that the buzzards

01:26

resistance can be ignored all right here is potential answer.....

01:31

we appreciate Luis quest for justice here but he probably could have baked a [Luis in a classroom]

01:36

dozen cupcakes in the time it took him to rig this thing up however it's the

01:39

principle of the thing that matters plus it's pretty sweet to get your dad

01:43

red-handed that's assuming it was a red velvet [Dad with red hand and red velvet cake appears]

01:45

cupcake of course in this circuit we have 1.5 volt batteries with a total

01:50

potential difference of three volts let's figure out the resistance we're

01:55

dealing with first we have four resistors here two of them are in series

01:59

so we can add those up to get a total resistance of 1500 ohms in that section

02:04

of the circuit now we have two resistors in parallel to calculate total [Finger points to resistors on diagram]

02:10

resistance in parallel resistors we need add the reciprocals of the resistances

02:13

then find the reciprocal of that sum and it looks like this now if we only have [Parallel resistance formula appears]

02:18

two parallel resistors we can also find the answer by multiplying your

02:21

resistances then dividing that by their sum no matter how we do the math the

02:26

answer is the same which is good because that's kind of how math is supposed to

02:29

work we have a total resistance of 500 ohms for this part of the circuit so now [Finger points to circuit]

02:34

we can look at the circuit as having two resistors in series one of 500 ohms and

02:39

one of a thousand ohm and we can add that to find a total resistance of 1500

02:45

ohm all right now we're getting somewhere

02:47

we can find the current using Ohm's law which states that the current equals [Ohm's law appears]

02:51

volts divided by resistance well in this case we have three volts and fifteen

02:56

hundred ohms giving us a current of two milliamps now to be frank at this point

03:01

we've pretty much forgotten what's question was in the first place but [Man scratches head]

03:04

looking back we're supposed to find the amount of current that's flowing through

03:07

the 750 ohm resistor there we go since one part of this Junction has a

03:11

resistance of 1500 ohms and the other has half of that 750 ohms [Arrow points to resistances]

03:16

yeah that's paths we know that twice as much current flows through the 750 ohm

03:21

resistance well we can look at it like this the total current equals 2I plus

03:27

I giving a total of three I that equals the current two milliamps

03:32

well one third of the current goes through the 1500 ohm resistance and we

03:36

double that to find the portion of current that flows through the 750 ohm

03:40

resistor which means the correct answer is d and if this situation ever happens

03:45

again well let's save everyone a whole lot of trouble and just chip in by [Dad gives Luis a cupcake]

03:49

Luis another cupcake

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