Electricity Experiment (Internal Resistance)

[leviathanX777]

1. Hi I'm going to a physics Summer camp but they asked me to carry out my own experiment. This is because my dad's a physics teacher and he has a lot of equipment readily available to him. I have to find the internal resistance of a dc power supply with the following equipment given:
Power Supply
Ammeter
Voltmeter
Variable Resistor



2. V=IR



3. I don't know if I need more equations but would love to hear some feedback on how to do this!

Thanks.

 

[rl.bhat]

Connect DC power supply, ammeter and variable resistor in series. Connect volt meter parallel to the variable resistor.
Unplug a resistance R1 and find the current I1 and voltage V1.
I1 = V1/R1 = E/(R1 + r). repeat the same thing for another resistor. Solve the two equations to get the internal resistance of power supply.

 

[leviathanX777]

So I'm guessing r is the internal resistance and E is the emf(voltage)? Why do I ahve to unplg the resistor in the first place to find the current and voltage?

 

[rl.bhat]

What is the use of variable resistor?

 

[leviathanX777]

To find different resistances, thus find different currents and voltages to get the average? Is little r internal resistance? So can you give me a step by step guideline for the experiment?

 

[rl.bhat]

Well. connect DC power supply, ammeter and variable resistance in series. Set the variable at one position. Note down the ammeter reading. Note down the voltage V1 across variable resistor using volt meter. Let I1 be the current, . Repeat the same thing for another position of the variable resistor.
You can also write E = V1 + I1r = V2 + I2r, and find r