Finding Potential Difference between two points homework

R1 = 120ohms
R2 = 820ohms
R3 = 2200ohms
E1 = 15v
E2 = 9v

What is the potential difference between points "B" and "C"?

(I've attached the circuit diagram)

The Attempt at a Solution

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ehild
Homework Helper
What have you learnt about electric current and electric potential so far?

ehild

Femme_physics
Gold Member
Hmm. I'm not a hmework helper and not entirely sure myself, but I believe E2 won't play a factor since its current leads nowhere. Is that right, helpers?

I know the basic electronic theories, Ohms law, Kirchhoff's Theorem and etc,

ehild
Homework Helper
I know the basic electronic theories, Ohms law, Kirchhoff's Theorem and etc,

You should write them under the title "Relevant equations".
And then show some attempt to solve the problem.
As a first hint: Does any current flow through resistor R2?

ehild

I'm sorry this will not happen again, I'm new to this forum :)
ok I think there is no current flowing through the R2

ehild
Homework Helper
Hmm. I'm not a hmework helper and not entirely sure myself, but I believe E2 won't play a factor since its current leads nowhere. Is that right, helpers?

No, it is wrong, it influences the potential difference between C and B.
And "its current" has no sense. It is a voltage source, characterized by its emf (electromotive force).

ehild

ehild
Homework Helper
ok I think there is no current flowing through the R2
Very well, so what do you know about the potential drop across R2?

ehild

before that I think we should calculate the current through the whole circuit

ya it's wrong, we need E2 when we calculate potential difference of point "B"

ehild
Homework Helper
No, you just said that zero current flows through R2. And you know Ohm's law. What does it say?

ehild

according to the method I've studied in school, we should follow that step, if i'm wrong please show me how to get the potential difference of point B and C

ehild
Homework Helper
ya it's wrong, we need E2 when we calculate potential difference of point "B"

A point has a potential, and potential difference is established between two points. So it is "potential at B, UB" and "potential difference between B and C, UBC =UC-UB".
The zero of the potential can be assigned to any point of a circuit. So potential of B means potential with respect to the zero point.

ehild

Thanks that is another problem i had on my head :),it's mean we can assign zero point to any point, thanks for telling me that before i ask, so you still didn't tell me how to fine 'Uc" or "Ub"

I like Serena
Homework Helper
Hi harshakantha! Welcome to PF!

Ehild is helping you.
See the "Homework Helper" medal?
That means ehild know this stuff quite well and knows how to help people!
As far as I'm concerned ehild has already shown remarkable patience...

Cheers!

ya Serena, he knows how to help people very well :-)

ehild
Homework Helper
What does Ohm's law state?

What is the potential difference between A and B? We will assign the zero of potential to point A.

ehild

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there is substantially constant ratio between the applied potential difference and the current flowing through a resistor, this relation ship known as Ohm's law.
can u give me a hint to get potential of point B :)

ehild now I'm going to leave computer, I hope you will leave a better answer for my question when next time i log on to The PF, till then bye.... :)

ehild
Homework Helper
As others might be interested in this thread, I continue.

According to Ohm's law, zero current means zero potential difference across a resistor. So the potential at B is the same as at the positive terminal of the battery E2.

There is current in the closed loop at the LHS of the drawing: KVL can be applied to get it. The potential of B with respect to A is obtained from Ohm's law again. If you know the potential both at B and C the potential difference is just a subtraction.

ehild

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Hi ehild, I solve the question,

Total Resistance = R1+R3 = (120+2200)ohms
consider current through Resister R1 and R3 is "I"
then I=E1/(R1+R3) = 15/(120+2200) = 0.0065A

At point "C" let's assume there to be zero potential

1) by going over R3 we lose -14.3V (0.0065*R3)
2) The E2 power supply adds 9V, leaving us with -5.3V (-14.3V + 9V)
3) no current flows through R2
4) therefor the potential different between point "B" and "C" is 5.3V

is this correct??

ehild
Homework Helper
Yes, but add for clarity that B is negative with respect to C.

ehild

you mean to the final answer?? can you show me how to write the final answer :)

ehild
Homework Helper
you mean to the final answer?? can you show me how to write the final answer :)

The answer depends how your teacher defined the potential difference between two points, say B and C. It might be Ub-Uc or Uc-Ub. But it is always correct if you give the absolute value and indicate which point is more positive or more negative than the other one.

ehild