# The Naked Scientists Forum

### Author Topic: Any experts know how to solve this problem?  (Read 3202 times)

#### alternativess

• First timers
• Posts: 4
##### Any experts know how to solve this problem?
« on: 26/08/2005 20:32:24 »

#### David Sparkman

• Sr. Member
• Posts: 234
##### Re: Any experts know how to solve this problem?
« Reply #1 on: 27/08/2005 02:52:11 »
It has been too long and I forgot all the equations, but the above circuit is equalivant to a single 720 ohm resistor. That much I remember.
1/2000 + 1/3000 + 1/1800 = 18/36000 + 12/36000 + 20/36000 = 50/36000. Invert and reduce = 720 ohms total resistance.

David
« Last Edit: 27/08/2005 02:57:41 by David Sparkman »

#### simeonie

• Sr. Member
• Posts: 351
##### Re: Any experts know how to solve this problem?
« Reply #2 on: 27/08/2005 12:03:26 »
OH CRUD! That isn't G.C.S.E work is it? If so I am going to fail because I haven't a clue about all that.

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#### David Sparkman

• Sr. Member
• Posts: 234
##### Re: Any experts know how to solve this problem?
« Reply #3 on: 27/08/2005 12:50:28 »
It seems to be rudimentry electricty that I learned in 7th grade, some in class, some just from a book on electricty. Since V=IR and overall resistance is 720 ohms it follows that -30 Volts =i3*720 ohms or I3 = -30/720. Solve for each part of the circuit, and add the amperage to see if everything matches (within rounding error). Lets see that would have been a tough problem for 7th grade algebra. The only thing I don't remember is the power equation. I think it was I^2 * R.

To explain for those who didn't get the first part, resistance in series is addative. Resistance in parellel follows a simple rule of adding the inverse and reinverting.

Say you have a 2 ohm and a 3 ohm resistor in parellel. Most of the current will go though the 2 ohm resistor, but the effect will be a resistance that is lower than both the 2 and the 3 ohms. Take the inverse of each and you get 1/2 and 1/3 and sum them for 5/6. Reinvert the result for your answer of 6/5 or 1.2 ohms resistance.

David
« Last Edit: 27/08/2005 12:57:34 by David Sparkman »

#### alternativess

• First timers
• Posts: 4
##### Re: Any experts know how to solve this problem?
« Reply #4 on: 27/08/2005 16:51:47 »
Thanks David

#### gsmollin

• Hero Member
• Posts: 749
##### Re: Any experts know how to solve this problem?
« Reply #5 on: 29/08/2005 16:26:30 »
Alternativess, this is your second circuits question. Are you conning us into doing your homework? Really, that won't help you at exam time.

"F = ma, E = mc^2, and you can't push a string."

#### The Naked Scientists Forum

##### Re: Any experts know how to solve this problem?
« Reply #5 on: 29/08/2005 16:26:30 »