Determining EMF and Internal Resistance

In summary, the problem involves finding the EMF and internal resistance of a battery based on the given information of current and resistance. To solve this, two equations are written using Ohm's Law and the values given in the problem. The unknowns can be eliminated by equating the two equations and solving for the internal resistance.
  • #1
joseppi
3
0
Basically I have been given this question:

A battery of unknown EMF and internal resistance is connected in series with an ammeter and a resistance box. The current was 2.0A when R= 4.0 Ohms and 1.5A when R= 6.0 Ohms. Calculate the EMF and internal resistance.

I feel you may have to arrange this somehow although I am probably wrong:

Internal resistance = v1-v2/I2-I1


I have had no attempt at a solution as I am utterly confused by this question.

Thank you for reading.
x
 
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  • #2
Hi joseppi, welcome to PF.
You have to write two equations for two resistances.
Use the equation
I = E/(R + r). Here r is the internal resistance.
Then solve for r.
 
  • #3
Hi, sorry for me not having understood your instruction but how can I use the below equation for the answer if I have neither the: r or the E value?

I = E/(R + r)

x
 
  • #4
E in this case is the emf, which you can work out with Ohm's Law.
 
  • #5
You can write two equations for two resistances. With two equations you can find two unknowns.
I1 = E/(R1 + r)-------(1)
I2 = E/(R2 + r)-------(2)
Substitute the values given in the problem. Try to eliminate one unknown to find the other unknown.
 
  • #6
2A = ?E/ (4.0 Ohms + ?r)

1.5A = ?E/ (6.0 Ohms + ?r)

I'll be honest I am quite bad with Maths even though I take AS level physics. I have no clue how to eliminate the r or E unknown or even really how to move it around ;/ You couldn't explain how and why you do this? It would be a great help, thank you.
x
 
  • #7
From the first equation
E = 2A(4.0 Ohm + r)----(1)
Similarly from the second equation write E = ...---(2)
Equate eq 1 and 2, and solve for r.
 

1. What is EMF and how is it determined?

EMF stands for electromotive force, which is the potential difference or voltage across a circuit. It is determined by measuring the voltage output of a power source, such as a battery, using a voltmeter.

2. How do you calculate the internal resistance of a power source?

The internal resistance of a power source can be calculated by measuring the current and voltage output of the source and using the equation R = (E - V)/I, where R is the internal resistance, E is the EMF, V is the voltage output, and I is the current.

3. Why is it important to determine the internal resistance of a power source?

Determining the internal resistance of a power source is important because it affects the overall performance of the source. A high internal resistance can lead to a decrease in voltage output and efficiency, while a low internal resistance can result in a more stable and efficient power source.

4. What factors can affect the accuracy of measuring EMF and internal resistance?

The accuracy of measuring EMF and internal resistance can be affected by factors such as the quality and calibration of the measuring equipment, the temperature and age of the power source, and the presence of any external resistances in the circuit.

5. How can the internal resistance of a power source be reduced?

The internal resistance of a power source can be reduced by using a larger or more efficient power source, using thicker or shorter wires to minimize resistance, and keeping the power source at a consistent temperature. Additionally, using a parallel circuit instead of a series circuit can also decrease the overall internal resistance.

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