I have a question which is:

Three Resistors are connected in parallel across a d.c supply. The resistor values are 12 ohms and 18 ohms. The 18 ohms resistor has a current of 18 amperes flowing through it and total current drawn from the supply is 81 amperes

Questions

• calculate the supply voltage
• value of the current through each resistor
• the value of the third resistor

 

Since they're parallel, the voltage across each of the resistors is the same. Calculate the voltage (18 Ohm, 18 Ampere) using Ohm's law.

Then figure out the current across the 12 Ohm resistor using the voltage you got in step one.

Finally subtract the currents through both resistors from the supply current and apply Ohm's law to the resulting current and your voltage.

 

Please post your best attempt at the problem so that others can advise on how to proceed.

 

Hello,

Try to draw a circuit with the three resistors and the voltage source.
Add the known values in the drawing.
Perhaps that will visualize the problem.

Bertus

 

Here is what I have:

• R1 12 ohms I 27a V 324V
• R2 18 ohms I 18a V 324V
• R3

Supply Current is 81A so then if I add up the two currents from each resistor I get 45A. I then take that away to get 36A.

 

Here is what I have:

• R1 12 ohms I 27a V 324V
• R2 18 ohms I 18a V 324V
• R3

Supply Current is 81A so then if I add up the two currents from each resistor I get 45A. I then take that away to get 36A.

Good job. Now use Ohm's law and find R3 resistor value.

And remember that In parallel circuit the voltage is the same but the current is splitting between the branches.
So we have a current divider
http://forum.allaboutcircuits.com/attachments/40_1211642965-jpg.40686/

 

I have

Would the supply voltage be 324V

Current I1 is 27A I2 is 18A and I3 is 36A

The third resistor is 9 ohms.