When carrying out my experiment it will be set up as below
However because the power supply has an internal resistance the diagram could be drawn like this instead
What is displayed in the red circle is actual one single thing: the power source, or in my case the battery. However it can be drawn as two separate items for illustration purposes. It represents a ‘perfect cell’ of e.m.f. (E) with a separate resistor, but in actual fact it is all one item.
R and r are in series with each other, they therefore have a combined resistance of R + r, therefore we can write the equation
E = I(R + r) or E = IR + Ir
However we are unable to measure only the e.m.f. because I am only able to attach the voltmeter across the whole cell so the internal resistance is included in that. The voltmeter will then measure the terminal potential difference. This can be calculated by the equation below.
V = IR
However we know that the terminal potential difference will be less than the e.m.f. as voltage is lost, the amount lost is known as the lost volts. The currents multiplied by the internal resistance of the cell can calculate the lost volts.
When combining these equations we can create the equation below
V = E – Ir
(Terminal p.d. = e.m.f. – lost volts)
My experiment will be set up as below
I will alter the variable resistor, so the current in the circuit changes. I will record V and I so that I can make a graph of Voltage against Current, for each cell.
The graph will have an equation of y =mx +c and my equation is V= E – Ir. If I then plot V on the y-axis and I on the x-axis, the gradient will be –r and the y intercept will be E.
I will have top be very careful when carrying out my experiment because if the temperature increases the internal resistance will decrease, as there is more electron mobility, and so therefore the results I produce we be biased. Therefore I will make sure that when carrying out my experiment I do it very quickly so that none of the equipment heats up and a wait a while for things to cool down before taking some more results.
Preliminary experiment
I did I preliminary experiment to decide how man readings I would need to take for each battery, which batteries to use and what equipment to use.
The equipment I used was
- Ammeter
- Voltmeter
- Wires, some with crocodile clips, some with plugs
- Variable Resistor (rheostat)
- Different types of cells
I set up the experiment as below. Then I put different cell in and changed the resistance on the rheostat and recorded the results.
My results were as follows
When doing my preliminary experiment I discovered that it would be better to use digital voltmeters and ammeters rather than one I used as it would be more accurate to use a digital ammeter and voltmeter. This ensures there will be no errors when reading the values. In my real experiment will also take repeats and also I will use more cells.
The Real Experiment Method
I will set up the experiment as below. I will implement different cells and change the resistance to get at least 8 readings for each cell. I will wait for the equipment to cool down between each reading. I will also use the same equipment throughout to attempt to stop any anomalies occurring.
Safety
The experiment is very safe and so all I need to do is ensure the batteries do not get hot and that they do not leak as the contain very harmful and corrosive chemicals.
