# 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

## 69 comments on “Resistors in Electric Circuits (9 of 16) Combination Resistors No. 1”

1. JJ Abraham says:

Thank you!!!

2. pipefitters1 says:

Is a combination circuit the same as a compound circuit?

3. Kevin Celentano says:

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

4. spalding47 says:

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

5. Julia H says:

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

6. Jalyn Massey says:

Thanks so much!!

7. Suresh Balmacoon says:

Beautiful…

8. Monish Singh says:

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

9. Extra Bass says:

hi

10. Sheisashe Kelsworth says:

very Good

11. Tania Ghasriani says:

thank ya! it really helped me

12. Seshagiri Rao says:

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

13. Emil Jacobo says:

brilliant! thanks

14. Dominic Gambrill-Abotomey says:

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).

15. 2014 Student CHEN YIHENG says:

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

16. Aj Gonzales says:

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

17. derekeves1 says:

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

18. ROSE PADILLA says:

19. Belos haido says:

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

20. Mr.Mister says:

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.

21. David Jackson says:

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

22. Abdullah Taufiq says:

thank u so much sir

23. Manuel Flores says:

Thank you very much

24. Saabir Yaxye says:

thanks

25. Tony Drizy says:

You are a total pro professional teacher

26. Nothing says:

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.

27. justin cambpell says:

awesome job thank u

28. Jacob A says:

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

29. Itsuki Yuuto says:

You are better than my teacher

30. XxTrickyRickyxX says:

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

31. Itsuki Yuuto says:

I totally get it now thanks a lot!!

32. abdullahi abdi says:

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

33. Scott Smith says:

34. Mak Song says:

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!

35. Daniel says:

Thank you very much!

36. Jason Hosein says:

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

37. Alisonxox says:

omggg thank you sm

38. Jackson Bartle says:

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

39. Nellisha Johnson says:

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

40. narra prathin says:

Thanks

41. Production Mash says:

you're 10 times better than my teacher.

42. Aamna Jawed says:

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

43. Fahim Al-Huq says:

44. Thaer Qasem says:

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

45. David Ramirez says:

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

46. Robert Matthews says:

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

47. Paige Thomson says:

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

48. Nader Nad says:

You aptly great. Professor …greating to you from Bahrain

49. Flor Mcdaniel says:

This the best step by step

50. katarinas butt cheek says:

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.

51. avocado toast says:

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

52. Dean Chamberlain says:

Excellent Video Very Clear to Understand

53. venice quines says:

This video is my savior, tysm

54. Jabba Knight says:

Thank you.

55. Barney Dinosaur says:

Thankyou Sir! hope to see more future videos from you

56. Abdulrahman Almuaiqil says:

I love u ❤️

57. Hi, it's me Shanella says:

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

58. w.f says:

Thank you 🌼🌼🙏

59. MandagMiss says:

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

60. Elviz Hernandez says:

Labeling is key

61. CranJo Games says:

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

62. Adelaide hula hoopers says:

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

63. Yashwant Sharma says:

suberb video…i love it.

64. WritingWally says:

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

65. Rk tutorial says:

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

Current is following +ve to -ve

66. Nomad says:

Very nice video …Easy to follow : )

67. Tim & Shai says:

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

68. Ndifelani Nemaridili says:

69. Bea Cruz says:
70. KayLow says: