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

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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.
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Reading #3:

Voltage (v) (± 0.01V)

Current (A) (± 0.01A)

Resistance (Ω)

(Provided on resistor)



















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:

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Improvement 3:

To improve on graph plotting skills, we could use a computer to help us. Many computer software’s nowadays can help us plot the results and calculate the gradients directly. After the experimental results are obtained, simply copy them into the program and plot graphs of V/I. These graphs would be much more accurate then hand plotted ones, and internal resistance values would be calculated to the highest degree of accuracy.

Unfortunately, allowing the computer to do the job for you does not show any skills in data processing. Thus another improvement can be done by using calculus. Calculus is a good tool in mathematics to calculate the gradient of a known equation. The final results maybe even more accurate then the computer values. However, this would be inappropriate in this experiment, since knowing the equation would already give as the internal resistance and EMF values. This improvement maybe effective on other investigations.

By Clement Ng 12.6

Thursday, July 4 2002 05:12AM


...read more.

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