Calculating voltmeter resistance in DC circuit

• ztalira
In summary, the problem involves finding the resistance of a voltmeter connected to two resistors in series. By using Ohm's law and Kirchhoff's laws, the equivalent resistance and total current can be calculated, allowing for the determination of the voltmeter resistance.
ztalira

Homework Statement

initial problem: A 228 −Ω resistor and a 586 −Ω resistor are connected in series across a 90.0−V line.
A voltmeter connected across the 228 −Ω resistor reads 24.0 V . Find the voltmeter resistance.
For Req, i got Req=228R/(2+228), R being the resistance of the voltmeter.
But after that, I'm stuck. How do I find the necessary I(current) in order to calculate the R (resistance of voltmeter).?

V=IR
1/Req=1/R1 +1/R2

The Attempt at a Solution

For Req, i got Req=228R/(2+228), R being the resistance of the voltmeter.
But after that, I'm stuck. How do I find the necessary I(current) in order to calculate the R (resistance of voltmeter).?

The total current is going to be given by V = IR

They give you the total voltage and they give you the resistors. To find the total resistance you have to find the equivalent resistance. The resistors are connected in series, not in parallel, so what does that tell you about how to calculate the equivalent resistance?

Once you solve for the equivalent resistance and find the total current, you can use Ohm's law again and find the resistance with the voltmeter. The resistance of the voltmeter itself will then be the difference between the value of the resistance with it and the value without it.

ztalira said:
For Req, i got Req=228R/(2+228), R being the resistance of the voltmeter.
But after that, I'm stuck. How do I find the necessary I(current) in order to calculate the R (resistance of voltmeter).?

normally a voltmeter does not disturb a circuit parameters - but suppose it draws a current say( Iv) then the current in 228 ohm resistance will be reduced by Iv so you have two loops one smaller one through voltmeterand the 228 ohm. a larger loop is current flowing through the two resistances- one can apply Kirchhoff's loop equations to relatecurrent and voltage and resistances. that should solve your problem

https://www.physicsforums.com/attachments/96995

The first calculation is simply a voltage divider where you can determine the resulting resistance after puting the voltmeter across R1 => Rx.
Then take Rx and calculate back using the formula you use when you have two resistors in parallel and want to know the total resistance.
you don't need a current to find Rv.
except you have to calculate them. then just use the total resistance without the voltmeter and then with the voltmeter...where R1 changes to Rx.

br Chris

1. What is a voltmeter?

A voltmeter is an instrument used to measure the voltage or potential difference between two points in an electrical circuit. It is typically connected in parallel with the circuit component being measured.

2. What is the importance of calculating voltmeter resistance in a DC circuit?

The resistance of a voltmeter is important in order to ensure accurate measurements. If the voltmeter has too high of a resistance, it can alter the voltage being measured, leading to inaccurate results.

3. How do you calculate the voltmeter resistance in a DC circuit?

The voltmeter resistance can be calculated by dividing the voltage range of the voltmeter by the current range. This value is then multiplied by the resistance of the voltmeter's internal circuitry.

4. What are the units of voltmeter resistance?

The units of voltmeter resistance are ohms (Ω), which is the standard unit of measurement for electrical resistance.

5. How does the voltmeter resistance affect the measured voltage in a circuit?

The voltmeter resistance creates a small amount of resistance in the circuit, which can alter the measured voltage. This is why it is important to use a voltmeter with a high enough resistance to minimize this effect.

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