# AP Physics 2: 1.3 Systems Interactions and Changes

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

AP | AP Physics 2 |

AP Physics 2 | Systems Interactions and Changes |

Language | English Language |

Science Practice 1 | Using representations and models |

Test Prep | AP Physics 2 |

### Transcript

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ohm all right now we're getting somewhere

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Luis another cupcake