# Value of a Resistor

1. Mar 20, 2009

### Phoenixtears

SOLVED

1. The problem statement, all variables and given/known data

What is the value of resistor R in Figure Ex31.21, in which ΔV = 5.0 V and I = 62 mA?

(Image Attached)

2. Relevant equations

RI=V

3. The attempt at a solution

My attempt at the solution is literally solely the equation above. I can't see any other way to angle it. I've done it a few different ways (all using the above equation) and keep getting the answer 80.6. Can anyone help me on this one?

~Phoenix

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Last edited: Mar 20, 2009
2. Mar 20, 2009

### tiny-tim

Hi Phoenix!

erm … did you remember to subtract the 10Ω and the 15Ω?

3. Mar 20, 2009

### ImAnEngineer

R = V / I = 5 / 0.062 gives you the total resistance

4. Mar 20, 2009

### Phoenixtears

*Sigh* Thank you so much, both of you! Figures I'd forget something so simple.

Now I'll never forget that.... hopefully.

Thank you!

5. Mar 20, 2009

### ImAnEngineer

Physics is not about remembering ''tricks'' for certain situations. You can forget this solution, but what you should have learned from this is that it's important to analyze the problem. Understanding the problem is half the solution. If you would have asked yourself ''what resistance is it I'm calculating if I plug V and I into Ohm's law?'', then you would've been able to solve the problem yourself!

6. Mar 20, 2009

### Phoenixtears

Actually, this is the first time I've really used resistance in this manner at all. I didn't know about a 'total resistance' and the fact that each small section holds a separate resistance that adds up to the total resistance. And I'm not in physics to remember tricks. I want to understand the material. My comment about not forgetting had nothing to do with the solution, but about the idea of total resistance.

Thanks for helping me, but please don't assume that I'm only in physics to pass.

7. Mar 20, 2009

### ImAnEngineer

That's very well then!

I didn't assume you only do physics to pass. I assumed you knew about multiple (serial) resistances would add up to a total resistance and tried to remember all the ''tricks'' to solve questions regarding resistance. That appears not to be the case, no big deal then.

You're welcome.