B5 practical: EMF and internal resistance simulation
EMF and internal resistance
The ideal circuit
Using the Phet DC circuit simulation it is possible to construct the same circuit. Because this way works so well it helps understand the theory so is maybe worth doing instead of the real version.
Here the series resistors represent the resistance wire, each resistor having a resistance of 1 Ω. The EMF of the battery is 1.5 V and the internal resistance 0.5 Ω.
This leads to the following table of results.
Plotting the same graphs that were plotted in the real experiment we get
The equation for this line is R = ε/I - r so the gradient = ε = 1.5 V
The intercept is -r = -0.5 Ω
This line has the equation V = -Ir + ε so gradient = -r = -0.5 Ω
The intercept = ε = 1.5 V
So with perfect equipment the method works well.
Wires with resistance
One of the factors that might have affected the results is the resistance of the wires. In the Phet simulation it is possible to add resistance to the wires by increasing their resistivity. This makes quite a difference to the graphs.
Notice how the both methods give a significantly larger value for r, the second being the biggest of the two. This corresponds with the real situation quite nicely. The slight curvature in both graphs is not however seen in the real version.