Calculating Internal Resistance and EMF of a Voltage Source

In summary, the conversation discussed a problem involving the internal resistance of a voltage source and its relationship to voltage and current. The participants attempted to use equations to solve the problem, but struggled due to a lack of information. Eventually, they were able to simplify the problem and solve for the internal resistance, which was found to be 2/5 ohms.
  • #1
BDR
13
0
Hello everyone!

I have a question and any help would be appreciated!

(a) What is the internal resistance of a voltage source if its terminal voltage drops by 2.00 V when the current supplied increases by 5.00 A?

(b) Can the emf of the voltage source be found with the information supplied?
 
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  • #2
BDR said:
Hello everyone!

I have a question and any help would be appreciated!

(a) What is the internal resistance of a voltage source if its terminal voltage drops by 2.00 V when the current supplied increases by 5.00 A?

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

What do you know about the relationship between voltage current and resistance? Show us some attempt to solve this problem, and then you can get help.

https://www.physicsforums.com/showthread.php?t=4825
 
  • #3
I know that the internal resistance of a voltage source affects the output voltage when a current flows. I was thinking that I would use the equation V = E - Ir...but I seem to get a different answer each time I use it. I also know that V = IR.

Im my view of this problem I do not have enough information to work the problem with the above equations. Am Ii close?
 
  • #4
BDR said:
I know that the internal resistance of a voltage source affects the output voltage when a current flows. I was thinking that I would use the equation V = E - Ir...but I seem to get a different answer each time I use it. I also know that V = IR.

Im my view of this problem I do not have enough information to work the problem with the above equations. Am Ii close?
From V = E - Ir you know what a graph of V vs I should look like. E is the emf (unknown, but constant) and r is the internal resistance (also an unknown constant). The information tells you how much V changes for a change in I. Relate that information to the graph of V vs I and you will have the answer to part a). If you think about trying to draw the graph using the information given, you will also know the answer to part b).
 
  • #5
BDR said:
I know that the internal resistance of a voltage source affects the output voltage when a current flows. I was thinking that I would use the equation V = E - Ir...but I seem to get a different answer each time I use it. I also know that V = IR.

Im my view of this problem I do not have enough information to work the problem with the above equations. Am Ii close?
also write what you know in equation form:

[tex] V_{1} \ = \ E - I_{1}r [/tex]
[tex] V_{2} \ = \ E - I_{2}r [/tex]

[tex] V_{2} - V_{1} \ = \ (E - I_{2}r) - (E - I_{1}r) [/tex]

[tex] V_{2} \, - \, V_{1} \ = \ -2 \ \mbox{Volts \ \ WHEN} \ \ \ I_{2} - I_{1} \ = \ \mbox{+5 amps} [/tex]

can you now solve for internal resistance r?
 
  • #6
I'm sorry, but I guess I am not understanding about how I get the information from the graph. Since E and r are unknown constants; does that mean that I could leave them out of the equation and just have V = I? Or am i making it harder than it really is?

Is this close? 2V = E - (5 A)r
 
  • #7
BDR said:
I'm sorry, but I guess I am not understanding about how I get the information from the graph. Since E and r are unknown constants; does that mean that I could leave them out of the equation and just have V = I? Or am i making it harder than it really is?

Is this close? 2V = E - (5 A)r
you didn't simplify the following equation correctly:
[tex] V_{2} - V_{1} \ = \ (E - I_{2}r) - (E - I_{1}r) [/tex]
what happens to E?
 
  • #8
The E's would cancel each other out? But it seems to me there are more variables than numbers for this problem.
 
  • #9
BDR said:
The E's would cancel each other out? But it seems to me there are more variables than numbers for this problem.
write the full (simplified) equation:
[tex] V_{2} - V_{1} \ = \ (E - I_{2}r) - (E - I_{1}r) \ = \ I_{1}r \, - \, I_{2}r \ = \ (I_{1} \, - \, I_{2})r[/tex]
then use what you know:
[tex] V_{2} \, - \, V_{1} \ = \ -2 \ \mbox{Volts \ \ WHEN} \ \ \ I_{2} \, - \, I_{1} \ = \ \mbox{+5 amps} [/tex]
 
  • #10
Lets see if i understand: -2 V = 5r...r = -2/5??
 
  • #11
BDR said:
Lets see if i understand: -2 V = 5r...r = -2/5??
almost!
what is the sign of (I1 - I2) in the following equation?
[tex] V_{2} - V_{1} \ = \ (E - I_{2}r) - (E - I_{1}r) \ = \ I_{1}r \, - \, I_{2}r \ = \ (I_{1} \, - \, I_{2})r[/tex]
look carefully. it's a little tricky. remember:
[tex] V_{2} \, - \, V_{1} \ = \ -2 \ \mbox{Volts \ \ WHEN} \ \ \ \color{red}I_{2} \, - \, I_{1} \ = \ \mbox{+5 amps} [/tex]
 
  • #12
Is this what you mean...I1 - I2 would be negative, causing a +2/5...right?
 
  • #13
BDR said:
Is this what you mean...I1 - I2 would be negative, causing a +2/5...right?
congratulations!
r = 2/5 ohms
 
  • #14
Thanks for the help, its greatly appreciated!
 

1. What is EMF?

EMF stands for electromotive force, which is the energy that drives an electric current. It is measured in volts and can be produced by a variety of sources, such as batteries, generators, and solar cells.

2. How is EMF related to terminal voltage?

Terminal voltage is the voltage measured at the terminals of a power source, such as a battery. It is equal to the EMF minus the voltage drop due to internal resistance. In other words, terminal voltage is the amount of EMF that is available to do work in a circuit.

3. What is the effect of EMF on circuit performance?

EMF is essential for the operation of electrical circuits. It provides the driving force for electric charges to flow and powers devices connected to the circuit. Without sufficient EMF, a circuit may not function properly or may not work at all.

4. How does distance affect EMF and terminal voltage?

Distance can affect EMF and terminal voltage in several ways. For example, in a circuit with a long wire, the resistance due to the wire's length can cause a voltage drop, leading to a lower terminal voltage. Additionally, EMF can decrease with distance due to energy losses, such as heat, as the current flows through the circuit.

5. What are some common sources of EMF and terminal voltage?

Some common sources of EMF and terminal voltage include batteries, power plants, solar panels, and generators. These sources convert various forms of energy, such as chemical, mechanical, or light energy, into electrical energy, which can then be used to power devices and equipment.

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