# Emf and Terminal Voltage

nautica
Okay here is the question:

a) what is the internal resistance of a voltage source if its terminal voltage drops by 2 V when the current supplied increases by 5 A.

b) Can the emf of the voltage source be found with the information supplied?

I know that E = V - Ir, and I am pretty sure that a simple algebraic equation can be put together. So what formulat am I missing??

Thanks
Nautica

Related Introductory Physics Homework Help News on Phys.org
HallsofIvy
Homework Helper
You know that E = V - Ir so &Delta;E= &Delta;V- &Delta;I r. You are told that &Delta;E= 2volts when &Dekta;I= -5 amps. Of course, &Delta;V= 0 since the voltage source is fixed.
Thatis: 2= -5r. Can you solve for r now? .

The answer to b is NO. In effect V is the "constant of integration" when you "integrate" dE= -rdI. There is no way to find that from the information given.

Last edited by a moderator:
nautica
I am in College Physics, we can not use calculus for an answer. So is there a reason algebraically.

thanks
nautica

If u know the potential diff b/w two terminals, then u can find the emf.

With this information u cannot find

nautica
So ther is no way to find emf without Potential difference???

thanks
nautica

nautica
Okay For a) here is what I got

V=E-Ir

or dV=dE - dIr

2V = -5r

so r = -.4Ohms

BUT, my instructor said this was wrong b/c there I did not no the Rload and there are too many unknowns for this to be solved.

I can see how part B could not be solved but this looked so simple for part a)

He said this was the other equation I should have considered, but I do not see why it was needed.

I = E / (Rload + r)

Thanks
nautica
The text passage contains a physics problem with two parts, both of which are mathematical in nature. The first part asks for the internal resistance of a voltage source given changes in terminal voltage and current, while the second part asks whether the EMF can be determined with the given information. The passage also includes a solution to the first part and confirms that the second part cannot be solved with the given information.
single_question_with_answer
Given a text passage, determine:
    1. If it contains a mathematical question (including word problems or calculations)
    2. If an answer to that mathematical question is provided in the text

First, describe the content structure you found in 1-2 sentences.

Then provide your analysis in the following format:
TYPE: <one of the following values>
- NO_QUESTION
- SINGLE_QUESTION
- SINGLE_QUESTION_WITH_ANSWER
- MULTIPLE_QUESTIONS
- MULTIPLE_QUESTIONS_WITH_ANSWERS
system
# Emf and Terminal Voltage

nautica
Okay here is the question:

a) what is the internal resistance of a voltage source if its terminal voltage drops by 2 V when the current supplied increases by 5 A.

b) Can the emf of the voltage source be found with the information supplied?

I know that E = V - Ir, and I am pretty sure that a simple algebraic equation can be put together. So what formulat am I missing??

Thanks
Nautica

Related Introductory Physics Homework Help News on Phys.org
HallsofIvy
Homework Helper
You know that E = V - Ir so &Delta;E= &Delta;V- &Delta;I r. You are told that &Delta;E= 2volts when &Dekta;I= -5 amps. Of course, &Delta;V= 0 since the voltage source is fixed.
Thatis: 2= -5r. Can you solve for r now? .

The answer to b is NO. In effect V is the "constant of integration" when you "integrate" dE= -rdI. There is no way to find that from the information given.

Last edited by a moderator:
nautica
I am in College Physics, we can not use calculus for an answer. So is there a reason algebraically.

thanks
nautica

If u know the potential diff b/w two terminals, then u can find the emf.

With this information u cannot find

nautica
So ther is no way to find emf without Potential difference???

thanks
nautica

nautica
Okay For a) here is what I got

V=E-Ir

or dV=dE - dIr

2V = -5r

so r = -.4Ohms

BUT, my instructor said this was wrong b/c there I did not no the Rload and there are too many unknowns for this to be solved.

I can see how part B could not be solved but this looked so simple for part a)

He said this was the other equation I should have considered, but I do not see why it was needed.

I = E / (Rload + r)

Thanks
nautica
user
The text passage contains a physics problem with two parts, both of which are mathematical in nature. The first part asks for the internal resistance of a voltage source given changes in terminal voltage and current, while the second part asks whether the EMF can be determined with the given information. The passage also includes a solution to the first part and confirms that the second part cannot be solved with the given information.

TYPE: SINGLE_QUESTION_WITH_ANSWER