Calculating Resistance in a Circuit with Unknown Components

[ravsterphysics]

Homework Statement

I understand part a, but not part B.

Homework Equations

The Attempt at a Solution

For part b) the PD across the resistor is 2V, but we don't know the current so we can't calculate using ohm's law eqn.

And because the new resistor's resistance is unkown, how do we actually calculate it?

 

[Jonathan Scott]

You've not actually said what the question is for part b, but if it's to determine the resistance R that isn't difficult. You can calculate the currents through meter V₁ and through V₂, so the current through the unknown resistance will be the difference between those.

 

[ravsterphysics]

You've not actually said what the question is for part b, but if it's to determine the resistance R that isn't difficult. You can calculate the currents through meter V₁ and through V₂, so the current through the unknown resistance will be the difference between those.

shoot.

the question is: Calculate the value of R.

 

[cnh1995]

shoot.

the question is: Calculate the value of R.

For part b), what can you say about the current flowing through the circuit?

 

[ravsterphysics]

For part b), what can you say about the current flowing through the circuit?

okay if V1 has voltage of 4v and its resistance is 1,000,000 ohms, then it's current must be 0.000004.

And this current must also flow through the resistor. Since total voltage is 6v, then this component must have voltage of 2V, so its resistance must be (2/0.000004) = 500,000 ohms, right?

 

[cnh1995]

okay if V1 has voltage of 4v and its resistance is 1,000,000 ohms, then it's current must be 0.000004.

And this current must also flow through the resistor. Since total voltage is 6v, then this component must have voltage of 2V, so its resistance must be (2/0.000004) = 500,000 ohms, right?

Right.
But that's not the final answer. You need to find R.
Edit: Resistance of V1 is 10MΩ. You have taken it as 1MΩ..

 

[ravsterphysics]

Right.
But that's not the final answer. You need to find R.

okay the mistake I just realized is that the Mohms = 10,000,000 ohms so the current is I = 0.0000004 so resistance R is (2/0.0000006) = 5,000,000

but you're saying that's not the value of R?? But isn't the new component R??

 

[cnh1995]

okay the mistake I just realized is that the Mohms = 10,000,000 ohms so the current is I = 0.0000004 so resistance R is (2/0.0000006) = 5,000,000

Yes I had edited my post immediately after posting..

Right.
But that's not the final answer. You need to find R.
Edit: Resistance of V1 is 10MΩ. You have taken it as 1MΩ..

but you're saying that's not the value of R?? But isn't the new component R??

R is the resistance used in place of the 10 ohm resistor. What about the resistance of voltmeter V2?

 

[CWatters]

okay if V1 has voltage of 4v and its resistance is 1,000,000 ohms, then it's current must be 0.000004.

And this current must also flow through the resistor.

Is V₂ removed for part b) ?

 

[ravsterphysics]

Yes I had edited my post immediately after posting..R is the resistance used in place of the 10 ohm resistor. What about the resistance of voltmeter V2?

Is V₂ removed for part b) ?

I'm still confused about what v2 has to do with the resistance of the component R? (and no, v2 is not removed)

 

[cnh1995]

I'm still confused about what v2 has to do with the resistance of the component R? (and no, v2 is not removed)

What is the current flowing through the component resistance R? Is it same as that flowing through V1?

 

[Jonathan Scott]

I'm still confused about what v2 has to do with the resistance of the component R? (and no, v2 is not removed)

You need to consider the current flow. From the information available, you can calculate the current through V₁ and the current through V₂. The total of the currents through R and V₂ must be equal to the current through V₁, so the current through R is given by the difference of the two calculated currents, as I mentioned above. You then know both the potential across R and the current through it so you can calculate R.