# Voltage Across a resistor

1. Jan 29, 2006

### the_d

I have a 12V power source in a circuit and 4 resistors in a line and 1 on the side how do i find the voltage across each resistor?

2. Jan 29, 2006

### Averagesupernova

You haven't given us resistor values and your description is vague. Oh yeah by the way, this sounds like homework.

3. Jan 29, 2006

### Maxwell

Do you have a diagram?

From your description it seems like all of the resistors are in series. Do you know what to do when resistors are in series?

After you find that out, use Ohm's Law.

4. Jan 30, 2006

### david90

Try to simply the circuit by combining the resistors to get an equivalent circuit. If ur resistors is in series, add the resistance. If in parallel do 1/r+1/r2=1/Requvilant.

5. Feb 1, 2006

### Link_pad

Well, asking the current (I) with no resistance is like asking how long is a piece of string to find the total Resistance in a parralel circuit then =
R1 X R2 / R1 + R2

And the current can be worked out by I = V/R

6. Nov 5, 2009

### syedadeel2

I have a 12V power source in a circuit and 1 resistors in a line of 1k , how do i find the voltage across resistor?

please help me?

i want some basic formula so i can calculate the voltage across each resistor..

thank you

7. Nov 5, 2009

### vk6kro

If you have 12 volts across a string of resistors in series. the current flowing is 12 / total resistance.
Say the total resistance is 5430 ohms. The current will be 12 volts / 5430 ohms = 2.21 mA

Now, this current flows through all the resistors in series, so the voltage across any one of them = I * R.

If R= 337 ohms, then the voltage across it will be 0.00221 amps * 337 ohms = 0.744 volts.

You can get the same result by doing it like this: 12 volts * 337 ohms / 5430 ohms = 0.744 volts.

So, if you know the total resistance and the voltage across it, you can work out the voltage across any resistor in the series string.

8. Nov 6, 2009

### syedadeel2

Hello,

Thanks for reply. i have read somewhere that to find out the voltage across resistor do this

Calculate the amount of voltage drop or reduction in an electrical circuit that has a power source of "100" volts at "0.1" amperes and a resistor of "100" ohms. Using the formula "V=I*R" the equation would result "V= 0.1*100". The answer would be "10" volts. This means that a voltage reduction of "10" volts would occur across the "100"-ohm resistor and the circuit would have only "90" volts of power past the resistor placed in the circuit.

is that right ?

but when i do myself i am not getting correct values.

i have 0.02 mA and 1kohms but when i multiple that i write this way :

0.02 * 1000 = 20v

this is not right i have 12v main power and after doing this i am getting 20v how it will subtract 20 from 12.

then i found 1 more simple formule to figure it out what resistor i need for any object,

e.g.

r = es - eb / i

12v - 5v / 0.02 = 350 ohms

is this also right ?

please help me...

9. Nov 6, 2009

### vk6kro

The Ohms Law calculations have to be done with Volts, Amps and Ohms.

So, if you have 0.02 mA you need to convert this to 0.00002 Amps before doing the calculation.

So this one:
12v - 5v / 0.02 = 350 ohms
should be
7 / 0.00002 = 350 K

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