# Unknown resistor in circuit - find current through it (tricky)

• Engineering
• Color_of_Cyan
In summary, the homework statement says that there is no solution if the value of the unknown resistor is unknown.
Color_of_Cyan

## Homework Statement

http://imageshack.us/a/img210/3330/homeworkprob15.jpg

Find I0 in the network.

(The middle resistor value was not given)

## Homework Equations

V = IR

Voltage Division:
(Voltage across series resistor) = [(resistance) / total series resistance)](total input V)

Current Division (for 2 parallel resistors):
(current across parallel resistor) = [(other resistor) / (sum of parallel resistors)](total incoming current)]

Parallel resistors = (1/R1 + 1/R2)-1
Series Resistors = R1 + R2

Delta Y conversion and back for resistors

## The Attempt at a Solution

I really do not know where I should start.

Other than all I can do being to simplify the 8 and 4 ohm resistors to 12 ohm. But then there's still that unknown resistor in the middle.

I don't see how it would help to use y to T or the other way around to help.

Last edited by a moderator:
If you redraw this, so that the unknown resistor is "vertical" you can see that it is parallel to some other/s.

Are you sure you aren't given the value of another parameter (e.g., voltage) of the circuit? Otherwise, you can only determine the current as an algebraic expression (in terms of R).

Obviously has no solution if unknown resistor remains unknown.

rude man said:
Obviously has no solution if unknown resistor remains unknown.

...Or if the total current or some other voltage drop is not provided.

Yep, this is exactly all that was given.

Darn... looks like my professor really dropped the ball then (he wrote this problem up himself). Thanks anyway guys.

(By the way, the given answer by him is I knot = 1A)

Last edited:
Well like NascentOxygen says, you could try to work out the algebraic expression for it... All you'd have to do at a later stage would be to insert the numbers...?

gneill said:
...Or if the total current or some other voltage drop is not provided.

I say, good point there, gneill!

## 1. What is a resistor and how does it affect current in a circuit?

A resistor is an electrical component that resists the flow of current through a circuit. It does this by converting electrical energy into heat. The higher the resistance of a resistor, the lower the current that can pass through it.

## 2. How do I calculate the current through an unknown resistor in a circuit?

To calculate the current through an unknown resistor in a circuit, you can use Ohm's Law, which states that current (I) is equal to voltage (V) divided by resistance (R), or I = V/R. Measure the voltage across the resistor and use the known value of the other resistor(s) in the circuit to calculate the current through the unknown resistor.

## 3. Are there any other methods for finding the current through an unknown resistor?

Yes, there are other methods for finding the current through an unknown resistor. One method is to use Kirchhoff's Laws, which state that the sum of currents entering a junction in a circuit is equal to the sum of currents leaving the junction. By applying Kirchhoff's Laws to a circuit with an unknown resistor, you can solve for the current through the resistor.

## 4. Can I use a multimeter to measure the current through an unknown resistor?

Yes, you can use a multimeter to measure the current through an unknown resistor. Set the multimeter to measure current and place it in series with the resistor in the circuit. The multimeter will measure the current passing through the circuit, including the unknown resistor.

## 5. How do I determine the resistance of an unknown resistor in a circuit?

To determine the resistance of an unknown resistor in a circuit, you can use a known voltage source and measure the current passing through the resistor using a multimeter. Then, use Ohm's Law (R = V/I) to calculate the resistance. Alternatively, you can use a circuit simulator tool or manually solve the circuit using Kirchhoff's Laws to determine the resistance of the unknown resistor.

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