# This has to be an easy answer basic series calc.

Discussion in 'Homework Help' started by destrey, Apr 21, 2009.

1. ### destrey Thread Starter New Member

Apr 21, 2009
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Im trying to really understand basic theory . My question is how could i determine the resistance of a resistor if given only the suplly voltage and the voltage drop across the resistor, and is that enough info?

2. ### StayatHomeElectronics AAC Fanatic!

Sep 25, 2008
1,020
71
Since we don't really know what the circuit looks like, it is not enough information. The basic theory is V = i * R, or Voltage = current * Resistance. Two must be know in order to find the other.

If you have a complete circuit, you may be able to find the current from using other components and voltages. But, the given information is not enough.

3. ### destrey Thread Starter New Member

Apr 21, 2009
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sorry what if you know its a 480v power supply. say four resistors with voltage drops of 120v each and its a series ckt. I was trying to make up my own problems to solve as practice for series ckts. I know how to find amps given supply voltage and ohms . I can find voltage drop given amps and ckt resistance. why cant i find ohms given voltage drop and supply voltage.

Mar 26, 2009
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5. ### StayatHomeElectronics AAC Fanatic!

Sep 25, 2008
1,020
71
You can't find ohms from a given voltage drop and supply voltage because you only have one of the three variables in the equation V = i * R.

In this example you know

480 volts = i * (R + R + R + R)

which when simplified is

120 volts = i * R.

The two pieces of information that are given are not independent of each other. You are still left with two unknown variables.

All of the other problems that you CAN solve are when you are given two of the variables, "finding amps given supply voltage and ohm." You know V and R, the only variable is i. "find voltage drop given amps and ckt resistance," you know i and R, the only variable is V.

Now if you came up with a problem that you knew V and i you would be able to figure out the R. V = i * R.

Try looking at the tutorial listed in the above comment. It might provide a slightly different explanation that may help as well.

6. ### mattc82 Member

Mar 13, 2009
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Think of it in terms of simple algebra, if you have (480=x*y) ,x could be 120 and y could be 4 or x could be 240 and y could be 2......... x depends on y or in this case R depends on i. You can't solve one equation with two unknowns and come up with one unique solution.