Analyzing a resistor circuit with two batteries | Electrical engineering | Khan Academy

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An example of simplifying a seemingly complicated resistor circuit. Created by Willy McAllister.
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Electrical engineering on Khan Academy: A summary of the math and science preparation that will help you have the best experience with electrical engineering taught on Khan Academy. Become familiar with engineering numbers and notation, and learn about the two most important electrical quantities: current and voltage.
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[@Madsy9]

Creating that "short-circuit" based on the fact that the two batteries had the same voltage is really clever. I've never thought about that!

[@jenniferschober3027]

thats not a short

[@axelmeramas976]

@@jenniferschober3027 yes you are right , it's not a short it a connection between two poles of both batteries. is smart trick though

[@sazokuotsutsuki8848]

@@axelmeramas976 ye, thinking about bridging in such a problem is clever

[@bigworldmemes]

(up-vote so people can see the right solution) Solving this using Kirchhoff's law : basically identify a point where the current splits like c/d then do an equation : input current = output current (at a point) so for c : i1 + i2 = i ===> (1) now you have an equation with 3 variables means you need 2 more equations to solve i now we use the second law which is basically at any closed circuit : sum of batteries voltage = sum of voltages generated by resistances (or sum of ixR) but with respect to the direction of the circuit that you choosed this will give you 2 more equations so you can select any closed circuit like (ADCBA,EFCDF,FBAEF,...) for ABCDA : 11(Battery in AB) - 11 (Because the battery in CD pushes current at the opposite direction i choosed) = 1.4 x i1 - 1.4 x i2 which means : i1 = i2 ===> (2) for CFEDC : 11 (Battery in CD) = 1.4 x i2 + 2.7 i ===> (3) now to solve 3 equations : (an advice : make each equation has a variable separate from the others) ====================== i = i1 + i2 (1) i1 = i2 (2) i2 = (11 - 2.7 x i) / 1.4 (3) ========================== you want [i] right ? so the master equation is (1) OR (3) AND from (2,3) i = i2 x 2 i = ((11 - 2.7 x i) / 1.4 ) x 2 i = (22 - 4.8 i) / 1.4 1.4 x i = 22 - 4.8 x i 6.2 i = 22 i = 3.5483870967741935483870967741935 which is almost 3.5 and here you go

[@rahul558]

+Crossfire Memer There is a small calculation mistake. 2.7 x 2 = 5.4. You have written it as 4.8, which led to the wrong answer. i = i2 x 2 i = ((11 - 2.7 x i) / 1.4 ) x 2 i =  (22 - 5.4 i) / 1.4 1.4 x i = 22 - 5.4 x i 6.8 i = 22 I = 22/6.8 which is the same answer he got.Still yours is a very nice explanation to solve this problem using KVL.

[@bigworldmemes]

+rahul558 yea I figured it out after I had posted and couldn't edit thanks to youtube

[@Flame082]

There's a typo/mistake, it should be 5.4 i not 4.8 i. If you fix that you get the correct answer 22 / (5.4 + 1.4) = 3.23 A

[@mcconkeyb]

It would be far more interesting if the voltage sources were different. Then you would have to use KCL and do a mesh analysis.

[@aselim20.]

I watched it twice and ı took notes.

[@munamahlasela3665]

why is there not 22v?

[@raisulislamremu7416]

What if the voltages are different :/

[@RO-zk3ni]

is not permissible because any 2 thing in parallel must have same voltage

[@RO-zk3ni]

so do any 2 thing in series must have same current

[@henrique7612]

of course it is

[@luongmaihunggia]

raisul islam you can't put different voltages in parallel, that would damage the circuit and the batteries will try to charge each other until they both have the same voltage.

[@raghumalhotra3490]

Can anybody explain in detail what Redstone said

[@tamimgopalggonj6281]

Why did we use a 11v battery in spite of two 11v battery? Any body please tell me.

[@gurujay3850]

Can you please make a video on energy calculations ( W kwh J )

[@ImEliteBR]

Do some kirchoff please.

[@lyslayer]

Hey, Why Volt not 22V instead 11V ?

[@m8oneguy913]

Nice to see that you teach better than my teacher in primary school.

[@Ghexus]

Millman's Theorem comes to mind as well when we first look at the circuit.

[@nikhilwardrobe]

More on circuits please !

[@jayviz]

thank you for these excellent videos. I am always confused by the ohms law equation because it seems relationship based, yet frequently resistance is treated as some sort of constant. But if voltage is increased, doesn't that affect resistance? My brain gets tangled in knots because I can never figure out which part of the equation is the dominant factor.

[@haaey1197]

This was clever

[@josephbangura5382]

Can you please do similar problem with with two different voltage values. Nice job

[@bosslovely27]

Good job

[@BoilerUp3030]

There were two batteries originally. I get that two DC sources in parallel have the same voltage at their end points, but shouldn't there be an increase in current? You seemed to calculate the current through R3 after eliminating one of the two current SOURCES. For the sake of argument, let's say each 11V battery is putting out 3A each. Then the current into R3 would be the sum of those two currents and would actually be 6A, wouldn't it?

[@curtpiazza1688]

Great problem!

[@JamesKlafehn]

Going back to the original circuit. The current measured over R1 should be exactly half of the current measured over R3. Correct? Current at R1 + current at R2 = current at R3. Is that right?

[@tahjcole2595]

try this one plzzzzzzzzz ..... a battery have an e.m.f of 20v and an internal resistance of 4.5homs calculate the current being supplied by the battery . So find the potential difference across the battery terminals.

[@arundhati3923]

What would happen if the batteries had different potential differences? What would be the value of the master battery?

[@RapiBurrito]

Then you would solve this using some other methods, nodal analisys would be the easiest. Shorts between ideal voltage sources should be handled with care unless you want infinite numbers.

[@jeypav1961]

anyone understand why he shorted the two batteries at 3:45?

[@avinashegala5799]

Consider your water purifier and assume it had two motors when it pump the water into a sand filter does it make a difference in the amount of water went into filter when you send it through two pipes or a single pipe .

[@DekuStickGamer]

I can hear the saliva make clicking, sloshing sounds. It's really tingly in my ears. Then drowned out by the emergence of your soft booming voice. It felts as if my ears were about to climax. Great Job.

[@thesickbeat]

+DekuStickGamer Check out ASMR.

[@louiskelly7277]

Please do more like this

[@DamjanPavlica]

Great explanation!

[@tw3ist]

Why doesn't the battery become 22 V???????

[@ImEliteBR]

Because they are in paralell not in series. When in paralell the voltage remains unaltered and the amperage increases.

[@jasonwalker4610]

Because they are in parallel, that just allows them to supply more current. If they are in series then the voltages would add up to 22volts

[@ImEliteBR]

If they were in series then the voltage would be 22V and the amperage would be unaltered

[@jasonwalker4610]

ImElite the voltage remains unaltered only because they are the same. You should clarify that if the voltages are different then the voltage is that avg, like a 10v in parallel with a 5v would produce 7.5v

[@ImEliteBR]

+Jason Walker yes. However generators with distinct voltages in pararell rarely appear in pre-university level questions.

[@nayan7398]

Why the potential be still 11 and not 22?

[@lyslayer]

1+

[@ivettechatman9126]

What effect would it have on the top resistor if the circuit were broken between the middle battery and 1.4 ohm resistor

[@rospotrebpozor3873]

IT WOULD BE FUN IF THERE'D BE 11 AND 10 VOLTAGES

[@virusinsfecting8321]

i agree. i am wondering if we use different voltage battery in this circuit.

[@MALR7]

virus insfecting no we cant if you do this the battery with higher voltage will discharge at a very high rate into the lower voltage , possibly ruining it

[@rospotrebpozor3873]

I like that word - "possibly"

[@AR-hk8qt]

What if the battery directions were different?.. What could have been the current? And if the current 'i' is zero in that case, than what could have been the current in the circuit?

[@bdasaw]

Omg... i'm a headphone user and your voice makes me want to clear my throat!!

[@morgengabe1]

Do some statistical mechanics videos!

[@ameymorshed4744]

morgengabe1 OMG HOW DID U GET UR ACCOUNT PICTURE UPSIDE

[@17joren]

Wouldn’t the voltage at the negative terminals of the batteries be zero? Why are you saying it’s 11v between the batteries and R1/R2?

[@antoniovillarealgutierrez6236]

I've been serries both 2 batteries which has 5volt each then the voltage become 10volt but how about their ampere which has 1.2 amp did they become 2.4 amp

[@kelvinadimas8851]

Why the resistors have to be summed?? I thought we only looking for i of the r3?

[@looijiahao2359]

can someone explain to me the voltage part ?

[@raghumalhotra3490]

Love you bro😘😘😘

[@Mahmood-]

that's why we learn kirchoff

[@muneebabbas2424]

Does anyone know if there's a video about Norton and Thevenin Circuits? I'm trying to get a bit ahead of my course and preview some more things on electrical principles but there isnt a clear explanation of it online, does anyone else know what it is or if there's a site/video that explains it clearly? Got to make the most out of reading week!

[@KanekiKen-vm7zq]

By organic chem tutor

[@KanekiKen-vm7zq]

U can check it out

[@NikhilKumar-uq7is]

what would be happen if the second battery is 10 v ?????

[@hadofhfo3369]

What is the software for the blackboard thingy? Will leave like if you tell me, it would be cool to know, have been wanting to for ages!!

[@Theraot]

+CasualGaming Just Microsoft Paint. Open Paint, fill the image with a dark color^1, select pen^2 (not pencil) and set it to the minimun line width, choose a color to draw (different front the background) and there you go. ^1: I took an screen grab to check the color, it is RGB(17, 16, 34) - you can choose "Edit Colors" in paint and input those numbers for Red, Green and Blue. ^2: Once you set it to pen you will notice the same mouse pointer that appears on the videos. You will need to record with something else tho... Cam Studio may do the trick (Or fraps or camtasia or whatever). --- I beleive they are not using a mouse for input, but some pen instead - I don't know for sure.

[@Theraot]

+CasualGaming Ok, I knew it started with Paint doing what I said because they said so (in a TEDTalk iirc) - but I decided to look into what hardware they have and found that they don't use Paint anymore... It is currently (as far as I can tell) done with a Wacom Bamboo Tablet and the software is Smooth Draw. Edit: You can download Smooth Draw for no fee from smoothdraw (dot) com

[@jameswood7207]

That resistor rule isn't very good because it doesn't work with more than two resistors. A better way is (1/total resistance)=(1/R1)+(1/R2)+(1/R3)........

[@RamRam-217]

Are the batteries in series?🤔

[@pablogriswold421]

Ram Ram nope, they are parallel, not end to end.

[@jamshid_ochilov]

Where is Sal??

[@SHAYDAV1]

Shoulden't you work this one out with kirchhoff's laws?

[@Drivabletree]

i got exaclty double the answer he got ? why would that be ? i used kirchhoffs

[@SilBoydens]

+Drivabletree the right answer is 11V/3,4ohm while typing this he is still drawing his shematic EDIT: his first shematic

[@jameshandysam]

Correct answer would be 3.2A as we are only given values to 1 significant place in the question.

[@AllAreHere]

are you therock ???

[@mugheerahkundi5387]

lol

[@mental_suicide]

so this is complex? lol

[@TheGarrbear3]

i do not get why you drew nodes that look random to me. they were not in original, I'm sorry, I do not follow

[@logandihel]

it's always frustrating when text books assume obsurd values for batteries and resistors. I always scoff at these schematics and just imagine all of the smoke pouring these poor through hole resistors. wouldn't it be more practical for students to be able to build the circuit (say a 9V) battery and use 1.4K resistors to power some LEDs and then use a multimeter to confirm their findings mathematically? either that or I need to get me some 50W 1.4 ohm resistors

[@sabiqhasan5573]

hey am i the only kid here?

[@rini3367]

What did he explain