To find out the internal resistance and EMF of a given power supply

Authors Avatar

Internal Resistance and EMF (ELC)

Physics Portfolio By Clement Ng 12.6

Aim: To find out the internal resistance and EMF of a given power supply.


  • Arrange apparatus as shown in the below circuit diagram
  • Start off by recording the corresponding voltage (terminal pd) and current by figuring out the values available in the voltmeter and ammeter.
  • Repeat previous procedures while the external load is varied, so that you obtain a set of voltage and current readings.
  • Calculate averages and plot a graph of current against voltage. Use the graph to figure out the internal resistance and the EMF of the power supply.


Skill 3 Data Collection

Reading #1:

Reading #2:

Reading #3:

Observations: As different external loads, or resistors were plugged into the circuit, several voltage and current readings were recorded. It was also noticed that the readings on both meters kept on jumping, therefore uncertainties of ± 0.01 was deduced for both the current and voltage values.

Skill 4 Data Processing

Arranged in increasing voltage and resistance values:

Join now!

Please note that all data processing graphs are presented at the last page of this report.

Skill 5 Conclusions and Evaluations


Several things could be found out from the data processing graph. Using the equation E = V + Ir, (where E is the EMF (V), V is the useful volts across the load (V), I is the current (A) and r is the internal resistance (Ω)) we could find out what the gradient of the V/I graph plotted would represent.

Since V here is plotted on the y-axis, and I is plotted on the ...

This is a preview of the whole essay