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Investigating the Emf and the internal resistance of a dry cell.

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Wendy Croft 12NB

Investigating the Emf and the internal

resistance of a dry cell

        The objective of this coursework is to find the Emf of the circuit and the internal resistance of a cell. I am going to use this by measuring the resistance (ohms), current (Amps) and voltage (volts) of a circuit.

        The below diagram shows how I am going to set up my experiment:

Apparatus:   Wires


Dry cell


Variable Resistor


By using the variable resistor above, I will measure eleven different readings on an Ammeter and a Voltmeter. I am able to do this as the variable resister measures resistance from 0 to 10 ohms. I will then reverse this procedure by taking the readings from the Voltmeter and the Ammeter when the resistance goes from 10 to 0 ohms. I will do this because by doing this I can view whether or not the original results were reliable. I predict that the results, both ways, will be very close to being the same.

        To make it a fair test, I will make sure that the cell is not left running.

...read more.



From these results in the above table I am going to produce a

y = mx + c graph. From the graph I will calculate the internal resistance and the Emf of the dry cell in my circuit.

On the graph I have plotted the above results, but due to the sensitivity of the meters used there may have been some inaccuracy in the data. Consequently I have drawn error boxes around the results. The size of these is 0.1v big.

By using the error boxes I have been able to also draw on to the graph three lines of best fit: a maximum line, a minimum line and a normal line of best fit. These will enable to do my calculations later on.

As you can see on the graph, there are four anomalous results, these have been highlighted in the results table. These anomalous may have been down to a number of problems: human error when reading the meters, the cell running down and the sensitivity of the meters.

Analysis of results and graph

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Hence, the maximum line of best fit has the bigger difference.

   Emf = 1.75 – 1.68

                        = 0.07

Therefore the Emf of the dry cell = 1.68    0.07


        Even though not all my results on my graph are in a straight line, like I had previously predicted, you can expect this due to inaccuracies and human errors when reading the meters. Although, by using a line of best fit I could still calculate the Emf and the internal resistance of a dry cell. If you do not take into account the anomalous results then the results are in a straight line.

By using a maximum and minimum line of best fit as well as normal line of best fit I was able to also calculate the error of the results. This made my final calculations even more accurate than if I had just calculated the Emf and the internal resistance by using the y-intercept and the gradient of the normal line of best fit.

As you can see from the graph the voltage and current are inversely proportional to each other. You can tell this due to the negative slope of the straight line.

...read more.

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