# Finding unknown resistance of a resistor using a schematic drawing

• seanmcgowan
In summary, the conversation involved solving a circuit problem to find the value of an unknown resistor. The total resistance of the circuit was given as 21.50 ohms, and the conversation went through different attempts at solving the problem. It was eventually determined that the unknown resistor could be found by creating an equation for the total resistance and solving for the unknown resistor. There was also discussion about identifying parallel and series connections in a circuit.
seanmcgowan

## Homework Statement

What is the value of the unknown resistor in the circuit shown if the equivalent resistance for the circuit is 21.50 ohms.

## Homework Equations

Series= R1+R2+R3...
Parllel= (1/R1)+(1/R2)+(1/R3)...

## The Attempt at a Solution

First off, I'm sorry for the horrendous drawing I did, but its better than nothing i guess.

I have done several of these problems and I am STILL stuck on every single one of them.

What I did was I drew several different possibilities of series and parallel circuits, but none of them came anywhere near the 21.50 ohms.

First I drew a pic, that went like this: (series) 7.5 and 9.1 then parallel 6.81 and ? then series again 9.1 and 3.6. However, just adding the series circuits together I came up with 29.3, and that was before the Parallel circuit.

Then I tried leaving the very first circuits (the first 9.1 and 7.5) and series, and making everything else parallel:
1/9.1= .11
1/3.6= .29
1/6.81= .15
.11+.29+.15= .55
1/.55= 1.82

7.5+9.1= 16.6
16.6+1.82= 18.42

Im fairly certain that since the unknown factor would be prallel you can't just subtract 18.42 from 21.50

How do I find this? And, am I even close to guessing the correct parallel and series combination? I'm clueless, an help is appreciated.

#### Attachments

• Schematic Drawing.bmp
294.7 KB · Views: 1,322
We can't see your attachment until it's been approved. You could upload your picture to a different site and use the IMG][/IMG -code, with "[" added to the start and "]" to finish. That way we could see the circuit right away.

Which website can i use for that? I've never done it before

There's an error in your Relevant equations. For parallel instead of (1/R1) + (1/R2) + (1/R3)... it should be 1/((1/R1) + (1/R2) + (1/R3)...).

I just wrote it down wrong, that's what i menat but wasn't sure how to write it down.

The basic rules of combining resistors are quite simple. If only one end of each resistor are connected, then they are in series. But the connecting wire must be unbranched, otherwise you can't say anything about the connection. If both ends of two resistors are connected they are paralleled. In this circuit there are a total of four series connections and one paralleled. Once you find these connections you can solve the unknown quite easily.

seanmcgowan

You might find the problem easy if you were given the value of all the resistors and asked to find the total resistance. Try taking the problem backwards.

I thought that wa what the 21.50 ohms was about? isn't that the total resistance of all of the resistor's?

seanmcgowan said:
I thought that wa what the 21.50 ohms was about? isn't that the total resistance of all of the resistor's?

Yes it is. But if you have the total resistance, you can plug it in the equation and find the unknown. For this problem you could create the equation for the total resistance of the circuit and use it to solve the unknown resistor, just as Phrak said.

Ok, I am probably going to start sounding real dumb here but I am not quite getting it. WHen figure out all of the parallel and series circuits, I come up with 18.45. If I subtract this from 21.50 i get 3.05. While this sort of fits, the missing resistor is in parallel right? and when I try to figure that out the numbers never even get close, so what's up?

seanmcgowan said:
While this sort of fits, the missing resistor is in parallel right?

Nope.

oh man, so I've had the answer all along! haha, thanks. one last question though, is there an easier way to find out what's parallel and what's series? or does that just come with practice?

Once you've truly understood the basic idea behind series and parallel connection, it all comes quite naturally, of course there could be some more difficult connections.

What did you get for the unknown resistor?

3.05 ohms. Thats right...right?

seanmcgowan said:
3.05 ohms. Thats right...right?
No, it's a bit larger, based on your drawing values.

## 1. How do I identify the unknown resistance value on a schematic drawing?

The resistance value of a resistor can be identified by looking at the resistor symbol on the schematic drawing. The symbol will typically include a number or a color code that represents the resistance value.

## 2. Can I determine the resistance of a resistor by measuring its physical dimensions on a schematic drawing?

No, the physical dimensions of a resistor on a schematic drawing do not directly correspond to its resistance value. The dimensions are used to represent the size and shape of the physical resistor, but the actual resistance value must be determined through other means.

## 3. What are some common methods for finding the resistance of a resistor using a schematic drawing?

Some common methods include using the resistor color code, using a multimeter to measure the resistance, or using a circuit simulator software to analyze the schematic drawing.

## 4. Can I calculate the resistance of a resistor using Ohm's Law and the schematic drawing?

Yes, Ohm's Law (V=IR) can be used to calculate the resistance of a resistor if the voltage (V) and current (I) values are known. These values can be obtained through measurements or simulations based on the schematic drawing.

## 5. Is it possible to have an unknown resistance value in a circuit even with a complete schematic drawing?

Yes, it is possible to have an unknown resistance value in a circuit even with a complete schematic drawing. This could happen if the resistor is damaged or if there are other components in the circuit that affect the resistance value.

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