# Calculating the missing resistance when only given voltage

• jacob1995
In summary, the student was asked to calculate the missing resistance (R5) in a circuit when only given the voltage drop of 14v across it. They attempted to use the current and divider rule, as well as Ohm's law, but were unsure of how to proceed since they did not know the total resistance. The expert suggested thinking of R3, R4, and R5 as a single equivalent resistance (Req) and using the voltage divider rule to find its value. The student was able to use this method to find a value of 10 for R5.
jacob1995

## Homework Statement

I am asked to calculate the missing resistance(R5) when only given the voltage drop of of 14v across it

## Homework Equations

current and divider rule
ohms law

## The Attempt at a Solution

[/B]
i tried finding the current of the 6+10 ohm series resistor that is parallel to R5, which would mean they have the same voltage drop. Not sure where i would go from here since i don't know the total resistance, i can't find the total current that is being split between R5 and 6+10 resistor.

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jacob1995 said:

## Homework Statement

I am asked to calculate the missing resistance(R5) when only given the voltage drop of of 14v across it

## Homework Equations

current and divider rule
ohms law

## The Attempt at a Solution

[/B]
i tried finding the current of the 6+10 ohm series resistor that is parallel to R5, which would mean they have the same voltage drop. Not sure where i would go from here since i don't know the total resistance, i can't find the total current that is being split between R5 and 6+10 resistor.
Hello,

We would need the actual question with all the information provided to help you. Can you provide more details?

nrqed said:
Hello,

We would need the actual question with all the information provided to help you. Can you provide more details?

jacob1995 said:
Ok, thank you.

As a first step, think of the resistors R3, R4 and R5 as a single equivalent resistance, let's call it Req. The voltage across this Req will be 14 volts too. Now, just from the information given, can you determine what must be the value of this Req?

nrqed said:
Ok, thank you.

As a first step, think of the resistors R3, R4 and R5 as a single equivalent resistance, let's call it Req. The voltage across this Req will be 14 volts too. Now, just from the information given, can you determine what must be the value of this Req?

3 and 4 are in series, which is in parallel to R5? would i not require current to find the value of Req, since i am missing the value of R5

jacob1995 said:
3 and 4 are in series, which is in parallel to R5? would i not require current to find the value of Req, since i am missing the value of R5
What I suggest is to do the problem in two steps. As a first step, it is better to forget completely about R3, R4 and R5. Just call the combination of these three Req. Can you figure out the value of Req? Once you will know that, we will figure out R5.

jacob1995 said:
3 and 4 are in series, which is in parallel to R5? would i not require current to find the value of Req, since i am missing the value of R5
Hint: you know the potential of the battery (28 volts), you know the other resistors in the circuit an you know that there is 14 volts across Req. This is enough to find Req.

nrqed said:
What I suggest is to do the problem in two steps. As a first step, it is better to forget completely about R3, R4 and R5. Just call the combination of these three Req. Can you figure out the value of Req? Once you will know that, we will figure out R5.

voltage divider rule?

jacob1995 said:
voltage divider rule?
Yes, that's one way to do it. Here the numbers given make the calculation very easy.

nrqed said:
Yes, that's one way to do it. Here the numbers given make the calculation very easy.
so i got 14 = 28 x Req/ R(1+2+2)
= 2.5?

jacob1995 said:
so i got 14 = 28 x Req/ R(1+2+2)
= 2.5?
I am not sure what R(1+2+2) means.

The correct equation is ##14 = \frac{28 R_{eq}}{1+2+2+R_{eq}}##

nrqed said:
I am not sure what R(1+2+2) means.

The correct equation is ##14 = \frac{28 R_{eq}}{1+2+2+R_{eq}}##
forgot to add Req back, got a vlaue of 10 for R5

jacob1995 said:
forgot to add Req back, got a vlaue of 10 for R5
Good job!

nrqed said:
Good job!
thank you for the help

## 1. How do I calculate the missing resistance when only given voltage?

The formula for calculating resistance, R, when only given voltage, V, is R = V/I, where I is the current in the circuit. This means that the resistance is equal to the voltage divided by the current. Make sure to use the same units for voltage and current (e.g. volts and amps) to get an accurate result.

## 2. Can I use Ohm's Law to calculate the missing resistance?

Yes, Ohm's Law states that the voltage in a circuit is equal to the current multiplied by the resistance (V = IR). So, if you are given the voltage and current, you can rearrange the formula to solve for resistance (R = V/I).

## 3. Is there a specific unit for resistance?

Yes, the unit for resistance is ohms (Ω). This unit is named after the German physicist Georg Ohm, who first discovered the relationship between voltage, current, and resistance.

## 4. What if I am given the power instead of the voltage?

If you are given the power, P, in watts, you can use the formula P = VI to calculate the voltage. Then, you can use Ohm's Law (R = V/I) to calculate the resistance.

## 5. Can I use the same formula to calculate the resistance in both series and parallel circuits?

No, the formula for calculating resistance in a series circuit is R = R1 + R2 + R3 + ... where R1, R2, R3, etc. are the individual resistances in the circuit. In a parallel circuit, the formula is 1/R = 1/R1 + 1/R2 + 1/R3 + ..., and then you can take the reciprocal to get the total resistance (R). However, if you are only given the voltage, you can still use R = V/I to solve for the total resistance in both types of circuits.

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