# Resistors in Electric Circuits (9 of 16) Combination Resistors No. 1

Okay in this video I’m going to go over

a problem involving combination circuits and in this problem we’re going to have

two parallel resistors and those two parallel resistors are going to be in

series with a third resistor. So this is a common circuit using resistors only.

Okay this is what we got here we got R1 R2, R1 R2 as you can see are parallel to

each other and those two resistors R1 and R2 are in series with R3

and we are going to try to do all the following for this circuit, we are going

to get the total voltage gain in the circuit, we are going to get the total

equivalent resistance through the circuit and we’re going to the total current through the

circuit. We’re going to do the voltage the total resistance and

the total current first these are the things I like to call the big three

because once we have those three done, the total, the total, in the total, then we

can get the voltage drop across each of the individual resistors and that we can get

the current through each of the individual resistors. Okay it’s a lot to

do we’re going to go step by step, we’re going to write everything down, we’re

going to think about our thinking units equations checking our answers and we

are going to try to do all of this in 10 minutes or less. The first thing the

total voltage now this might be pretty straightforward but I just like it write

it down so I’m sure that I know what I’m talking about and I kind of check and I

can see that the total voltage is 24 volts because there’s only one voltage

the voltage from the battery over here as 24 volts. Okay the next thing is the

equivalent resistance now we have to find the equivalent distance for all

three resistors and we have resistors in parallel and we have resistors in series

and that’s why we call this a combination circuit and the first thing

that we are going to do now of course we could do this in one big step but this

is going to be broken down to two steps. We’re going to find the equivalent

resistance of R1 and R2 which are in parallel with each other so we’re going

to write down the equation we use for resistors in parallel and that is that

the equivalent of 1 and 2 is equal to R1 plus, no excuse me, one over

R1 plus one over R2 and now we can simply plug the numbers in and you can

punch that into your calculator 1 over 12 plus 1 over 5 equals that the

equivalent resistance of, excuse me, that 1 over the equivalent resistance of 1 and 2 is 0.283 now this is not equal and resistance this is 1 over the

equivalent resistance we got to take the reciprocal of both sides we get that R

1,2 the equivalent resistance of 1 & 2 is simply 3.53 ohms. Now you might

be asking how to get 3.53, all I did is I took the reciprocal at this

side and the reciprocal of this side which basically means that R 1,2 is

equal to 1 divided by 0.283 so on my calculator I go 1 divided by 0.283 and I get that the resistance of 1 & 2, the equivalent resistance is 3.53

ohms and just to remind myself I like to up here draw in my equivalent resistor

and write down its equipment resistance and maybe I even cross these two out not

because I’m getting rid of them but just to remind me that now I have replaced

those two with a single resistor and with its equivalent resistance is to be

0.353 ohms. Alright so now we have a single resistor

with the resistance of 3.53 ohms in series with another resistor and we can

just add those up. The total resistance now is just the equivalent resistance of

1 and 2 plus the resistance of R3 and we can add those up and we get that the

equivalent resistance of the circuit the total resistance of the circuit is

11.53 ohms okay. Two steps find the equivalent resistance

of one and two and then add that resistance to the resistance of number

three because those two are now in series with each other. Okay now we do

the total current we’re going to use Ohm’s law. Ohm’s law is V equals I times R. We find that total current we’re going to put that IT because i total is equal

to VT over RT. Okay I like to write down if it’s the total then I gotta use the

total voltage and the total resistance and remind myself that it’s not one of

these individual resistances or something like that. So the total current

is equal to 24 volts we just found the total resistance or the equivalent

resistance 11.53 we get the total current in that circuit is

2.08 amps okay. So in order to get the current we had to

find the resistance first because this is the resistance that gives us this

okay so now we have the total voltage the total resistance and the total

current and we can go on and do the voltage drops across each resistor. Now

you can see I left in here my equivalent resistance for 1 and 2 and I left

the equivalence up here because maybe we’re going to use that in the future

should I just one habit there and then I put down my total voltage my total

resistance in that total current. Now we want to find the voltage drop across

each resistor which one should we do first well the easiest one to do first

is R3 because we know that we can use Ohm’s law V equals I times R we know the

resistance is obviously 8 ohms and we actually know the current because the

total current is the current that flows through R3. Okay the current obviously

splits here and some of the current goes through R1 some of the current goes

through R2 but then it comes back together right here and that total

current is the current that flows through resistor number 3. So for the

voltage drop across resistor number 3 we have to use a 3 which is

equal to the total and then we can use R3 all right so we just write those down

2.08 times 8 and we get that the voltage drop across resistor

number 3 is 16.64 Volts. All right now we have used 16.64 volts of our energy across resistor number 3 now we have

to now get the voltage drop across R 1 and R2 we use 16.64 Volts

well where is the rest of that energy going to be used because we got to use

all 24. It’s going to use across R1 and R2. Now this is the important rule you

have to remember and you got to think about because they have different

resistances but they are in parallel R1 and R2 are in parallel and the rule

concerning the voltage drop across parallel elements, in this case parallel

resistors in a circuit is that the voltage drop is the same, a voltage drop

across any elements in this case resistors in parallel is

the same. So we know the voltage drop across 1 is going to be equal to the

voltage drop across 2. Well that has to be the remaining voltage. Okay we used 16.64 volts there’s only one other place to use the remainder of the

voltage the remainder of the energy and that’s across R1 and R2. So we can simply find the voltage drop across R1 and R2 by taking the total voltage which is the

voltage from the battery and subtracting from it what we’ve already used which is

the voltage across resistor number 3. So we can take 16 we can take 24 volts

minus 16.64 volts. We get that the voltage drop across

resistor 1 and resistor 2 has to be 7.36 volts okay. Now we

can also do that and check that using Ohm’s law because V equals I times R. The voltage drop across 1 and 2 we know like we just talked about because they’re in

parallel the voltage has to be the same. Well therefore we use the current

through 1 and 2. Well that’s current through 1 and 2 is equal to the

total, some goes through 1 and some goes through 2, but the current through 1

and 2 is 2.08 A and the resistance of 1 and 2 we have to use

the equivalent resistance in this case not the resistance of 1 or the resitance 2, but we use the equivalent which is 3.53. So we

can say that 2.08 Amps times 3.53 ohms equals 7.4

volts. Now this number is close enough to this number that they’re basically the

same number probably due to rounding somewhere so we know we got the same

answer twice and therefore we’re pretty sure that that’s the right answer.

You can use this method or you can use this method if you come up with the same

answer you know you probably did it correctly. Use either one. I also want

to point out that this problem would be the same if

R3 was on the other side of these two parallel resistors it doesn’t matter.

Okay so now we know the voltage drop across each resistor now we can get the

current through each resistor. Okay well which one is the easiest one to do well

we already said earlier the preview slide that the current through resistor

number 3 is equal to the total current which is 2.08 amps right once

again some current goes through R1 some current goes through R2 they come back

together and that total current has to be flowing through R3. Okay so that is

2.08 amps, now we can use Ohm’s law, V equals I times R. We’re going to

solve it for I which means that I will be divided by R, we want to find the

current through resistor number 1. So I1, this is current through number 1,

if we’re going to find the current through resistor number 1 we have to

use the voltage across number 1 and the resistance of number 1. Okay this

is where you got to keep all this straight I 1 V 1 R 1 well that means

that I 1 is equal to the voltage across number 1 well we found out the voltage

across number 1 which is equal to the voltage across number 2 is 7.36 volts we divide that by the resistance of number 1 and we get 0.61

amps. Alright now we do the same thing for R2, we can get I2 we’re going to

use the voltage across 2 and the resistance of 2 and that means that

the current through number 2 is the voltage which is 7.36,

remember the voltage across one and the voltage across number 2 is the same,

divided by its resistance which is 5 and we get 1.47 amps. Okay

so now did we do those right, well let’s see we have some current through 1 and

some current for 2 well those two currents have to add up to 2.08, if you can see this 0.6 through 1, this 1.47 went

through resistor number 2 those two currents have to add up to 2.08, because they do so we have a high degree of confidence that we probably

did this correctly. Alright and that’s the whole problem. You can see we started

off with the total voltage, then we did the total resistance, then we did the

total current, then we were able to find the voltage drops across each resistor,

then we will find able to find the current through each resistor. Okay we

wrote everything down, for example we wrote all the equations down. We labeled

everything V1, R1 I1 want I2, V2, R2, to keep everything straight

so we use the correct voltages the correct currents the correct resistances

when we needed to all right so I think if you do that really careful and go

step by step that you can solve those problems also. So thank you very much for

watching if you found that video helpful you can give me a thumbs up or comment

in the comment section below thank you very much we will see you in the next

video

Thank you!!!

Is a combination circuit the same as a compound circuit?

Thank you so so so much! you have saved my physics grade!

u just saved my life my teacher can't teach shit

now i won't fail my physics test lol. really helpful vid

Thanks so much!!

Beautiful…

Thanks sir .. With the help of these videos , i am able to do all the exercises related to my topics.

hi

very Good

thank ya! it really helped me

Now I am able to solve any problem thank you sir…

brilliant! thanks

thanks. I found this video extremely helpful. I positively had no idea how to correctly carry out these processes (before I came across this video).

Hi, if one the the resistance in the parallel circuit is removed what will be the new overall resistance

YOU JUST MADE ME PASS MY EXAM THANK U VERY MUCH SIR

Thanks Great help , I had a bad teacher teach me this , your explanation was spot on and easy to follow!

thank you so much your video is really helpful!

I have been yo school for quite a while…and you explained this stuff in 10 min.. Consider teaching bro LOL

Great video. I am not sure about one part though. When you got the voltage drop across V1,2 and worked it out to be 7.36V. To find the voltage dropped across R1 and R2 do we just divide 7.36V by 2? so 3.68V dropped across R1 and 3.68V dropped across R2? Thanks.

Thanks for taking the time in explaining your theory by using step by step method. Easy to follow.

thank u so much sir

Thank you very much

thanks

You are a total pro professional teacher

This video was instrumental in me finally figuring this out. Thanks for posting it. Some other explanation I found before had you "stretch" the circuit out into a straight line, and for some reason start with series resistors and all kinds of convoluted junk. Think was just straight to the point, and I especially loved that you didn't assume any knowledge. Even taking the time to briefly explain the thing about the reciprocal, but without beating a dead horse for people who already knew what it was.

awesome job thank u

what if you had two resistors and wanted to find the voltage in between? is that possible?

You are better than my teacher

Thank you so much for this! I felt very lost after being taught this in class today, and this video has really helped me understand what we are learning!

I think I can actually do my homework now <3

I totally get it now thanks a lot!!

In 7.03 why are you taking 24 as the voltage total and not the voltage drop in Resistor 1 and 2

Your great, thanks.

Thank you so much for this video! I'm currently taking physics right now, and your video helped to clarify the step-by-step process to solving this combination problem!

Thank you very much!

thanks so much, what if the resistors are all same numbers, eg 20 ohms

omggg thank you sm

thanks dude, got a test tomorrow and really needed someone to explain this to me

This was very helpful. I now understand this topic better. I have a test tomorrow.Thank you 😊

Thanks

you're 10 times better than my teacher.

To find equivalent resistance we can also use the formula

R(1,2)= products of R1 and R2 divided by sum of R1 and R2

THANKS A LOT SIR.PLEASE KEEP UPLOADING THESE HELPFUL VIDEOS.

you made this way simpler then my professor ever could in 10 lessons thanks!

Thank you very much sir. I really learned with you. You're a good teacher and made my homework much easier.

Is it not possible to find the voltage drop across R1 because it is in parallel with R2?

OMG I understand I this helped me get it through my think skull thank you!!!!!

You aptly great. Professor …greating to you from Bahrain

This the best step by step

what if the problem is the same like this but the difference is that it has a another box of 2 parallel circuit? so it has a 4 parallel circuit in total. Pls help im having a hard time for this my teacher is not that effective in teaching i hope youll notice my comment and do a video about it.

i think he was wrong in the voltage drop part cuz u aren’t supposed to subtract it

Excellent Video Very Clear to Understand

This video is my savior, tysm

Thank you.

Thankyou Sir! hope to see more future videos from you

I love u ❤️

I don't get how you get 0.283. How did you find the LCD?

Thank you 🌼🌼🙏

Thank you thank you thank you my IB Physics exam is in 2 weeks and you just gave me some confidence!!!

Labeling is key

hi, is it possible to replace a missing resistor in my project with 2 combined resistor to get 3.9k R? & what is the right way to do it, parallel or serie?

its for hearing aid project that i find here: https://www.youtube.com/watch?v=lPKGX8ys0wk&t=64s, at 0:51 you will find the shematics for it.

thanks

Well explained. I've spent about 5 hours thrashing this all out, so thank you! Wish I'd found you first.

suberb video…i love it.

That was better than my entire physics class, also, nice voice.

Equivalent resistance is 3.53 ओह्म. Why we can't soloving parallel equation 1st then R 3.

Current is following +ve to -ve

Very nice video …Easy to follow : )

you dont know how to explain shit you talk to fast you fuck

This was helpful, thanks

u r a life saver

so helpful, thanks