# Help with 1st year hw



## Mjames (Aug 3, 2017)

Ok I no you guys well probably laugh but having a hard time finding certain resistors in a parallel series. Doesn't help teacher doesn't reall teach. Here's a problem I'm stuck on finding R3 and i3


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## Mjames (Aug 3, 2017)

Need help with questions 7 and 8.


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## 3rd Rail (Jul 9, 2017)

What's so hard about it?
You got IT, and ET. You do subtraction for one, while the other one is division.


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## 3rd Rail (Jul 9, 2017)

I3= it-(i1+i2)
r3= et/i3=r2


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## 3rd Rail (Jul 9, 2017)

BTW this is a regular parallel circuit, not a combination. For the combination one the trick is to follow the current and not assume the resistor/s in series aren't in parallel.


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## Mjames (Aug 3, 2017)

Is IT=15?


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## Mjames (Aug 3, 2017)

So I3=6 I'm still confused on how to get r3


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## backstay (Feb 3, 2011)

How could it be 15? You have three paths, some current must go down all three based on their resistance. You need to backup some. It appears you don't understand the basics of electricity.


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## dronai (Apr 11, 2011)

Ohms law, learn it, and apply. also you need to use your textbook, and start this material from scratch


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## Mjames (Aug 3, 2017)

Well it doesn't help I have a teacher who doesn't teach. I understand ohms law but just confused on how you find r3. 

Not one of you broke it down or anything all I'm looking for is a equation and explanation on how to find r3 and i3


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## dronai (Apr 11, 2011)

http://www.electronics-tutorials.ws/resistor/res_5.html


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## splatz (May 23, 2015)

The answer to #7 is simple based on Kirchhoffs First Law. There's really no work to show other than 15 = 3 + 6 + ? 

Kirchoff's laws are not some obscure theory you only need to know for tests, fundamental useful stuff.


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## hardworkingstiff (Jan 22, 2007)

Mjames said:


> Well it doesn't help I have a teacher who doesn't teach. I understand ohms law but just confused on how you find r3.
> 
> Not one of you broke it down or anything all I'm looking for is a equation and explanation on how to find r3 and i3


The current for all three resistors is 15 amps. You have 3 amps on R1, 6 amps on R2, so R3 has to have how many amps if the total through all three resistors is 15 amps?

The simplest way to find the resistance of R3 is E=IR. You know the voltage is 30 volts (by looking at the resistance and current on R1 and R2), and you will solve for the current in the 1st part of the problem then you can use E=IR to solve for R.


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## splatz (May 23, 2015)

So since the answer to #7 tells you the current through R3 is 6A then you can just notice that the current through R2 is also 6A so they must be the same resistance, 5ohms. 

If you don't notice that, you see the potential across R3 is 30V so if the current through it is 6A 

V = I * R 
30=6 * R3
5 = R3


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## Mjames (Aug 3, 2017)

Thank you guys so much. I have a teacher who doesn't teach us he reads the first few paragraphs then tells us to read it on our own and Do the questions then we just go over the answers he reads the problem number and wants a student to blurt it out. Some answers are right some are wrong but we never really go over them. It sucks. So all the help is appreciated guys.


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## Mjames (Aug 3, 2017)

What about a problem like this where u don't get your current.


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## Mjames (Aug 3, 2017)

I have my It= 10 amps and rt=7.5 for r2 the answer I get don't doesn't match up for my rt

^^^ for the above problem


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## hardworkingstiff (Jan 22, 2007)

Mjames said:


> I have my It= 10 amps and rt=7.5 for r2 the answer I get don't doesn't match up for my rt
> 
> ^^^ for the above problem


What answer did you get and how did you get it?


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## Mjames (Aug 3, 2017)

i got my it=10 which I'm starting to doubt than found i1= 7.5 from e/r1.


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## hardworkingstiff (Jan 22, 2007)

Mjames said:


> i got my it=10 which I'm starting to doubt than found i1= 7.5 from e/r1.


You are given the voltage is 75V and the overall resistance is 7.5ohms.

So, the overall current using E=IR is 10 amps. 75/7.5=10.

The formula for 2 parallel resistors is Rt=(R1*R2)/(R1+R2). 

That equates to 7.5=(10*R2)/(10+R2). I haven't done math like this since 1971 and to be honest, I forgot how to do it. 

Another way to solve it is:

R1 is 10 and the voltage is 75, so that means the current through R1 is ???

So, 10-I1 = I2, what is I2?

Once you know how much current is flowing through R2, it's easy to solve for R2 using E=IR.

Edit: I believe the exercise is really about learning to use the formula for resistance and maybe someone with better math skills will take the time to show how to solve that equation.


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## hardworkingstiff (Jan 22, 2007)

The formula for 2 parallel resistors is Rt=(R1*R2)/(R1+R2). 

RT=7.5, R1=10

7.5=(10*R2)/(10+R2)

7.5(10+R2)=(10*R2)

75+7.5R2=10R2

7.5+.75R2=R2

7.5=R2-.75R2

7.5=.25R2

30=R2


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## Mjames (Aug 3, 2017)

hardworkingstiff said:


> The formula for 2 parallel resistors is Rt=(R1*R2)/(R1+R2).
> 
> RT=7.5, R1=10
> 
> ...


That still doesn't make sense to me. You lost me at the 3rd step 75+7.5r2 why did the 7.5 stay in the equation. Is there a equation for this that I could see exactly how u plugged it in. Thank you for the help


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## hardworkingstiff (Jan 22, 2007)

Mjames said:


> That still doesn't make sense to me. You lost me at the 3rd step 75+7.5r2 why did the 7.5 stay in the equation. Is there a equation for this that I could see exactly how u plugged it in. Thank you for the help


The equation was in the 1st line at the very top (before step 1).

In step 3, both sides of the equation were multiplied by (10+R2) in order to get rid of the denominator on the right side of the equation. 

step 4 shows the simplifying of the equation by (left side) multiplying both numbers inside the () by the number outside the (), (right side) just doing the multiplication and dropping the ().

step 5 is dividing both side by 10 to get R2 by itself on the right side of the equation.

I hope you can follow the rest.

It seems your math skills are a little weak and this might be where your hang up is. If that's the case, I don't know what to tell you to do (other than take a math class). They (school) probably assumes you have the math skills to do this.


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## Mjames (Aug 3, 2017)

hardworkingstiff said:


> Mjames said:
> 
> 
> > That still doesn't make sense to me. You lost me at the 3rd step 75+7.5r2 why did the 7.5 stay in the equation. Is there a equation for this that I could see exactly how u plugged it in. Thank you for the help
> ...


Ok I see what your doing but couldn't you also just find i2 by it-i1=i2

Then once you find i2 divide et/i2=r2????

Is that a simpler way?


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## hardworkingstiff (Jan 22, 2007)

Mjames said:


> Ok I see what your doing but couldn't you also just find i2 by it-i1=i2
> 
> Then once you find i2 divide et/i2=r2????
> 
> Is that a simpler way?


Like I said in post 20, yes. 

It's a matter of what they are trying to get you to learn, and that I don't know.

But, if you don't know the voltage, then you have to use the formula in post 21.


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