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Dry cells are made of to parts where one part gives the coulombs an ideal amount of energy while the other part takes energy from the coulombs as they move through the cell. This is called Internal Resistance. It acts much like an invisible load inside the cell. This is because all material used in putting up dry cells have some resistivity which is main factor behind internal resistance. According to Fitzpatrick (2007, p.1), a pure voltage supply is generally known as an electromotive force.
In a complete circuit where a battery is connected to a resistor, Ohm’s law will be respected in respect to “voltage drop across the loadwhich implies that the drop in voltage across a resistor , carrying a current , is in the direction in which the current flows”( Fitzpatrick 2007, p. 2). Therefore, the voltage of the dry cell is connected to its internal resistanceand electromotive force by the following expression (Nilsson 2010, p.4):
The following apparatus was used in the experiment; Power pack-standard laboratory 12v, Leads-copper insulated wires, ammeter-0-10A, voltmeter-0-10V, and rheostat-5 amps, 20 Ohms maximum.
Experimental apparatus were set u as shown in the diagram below with the ammeter in series with the variable resistor while voltmeter in parallel with variable resistor. The variable resistor was adjusted until the ammeter showed 0.25A. At this point, the reading on the voltmer and ammeter were taken and recorded in the table. Resistance of rheostat was varied, six values and the corresponding current and voltage recorded in the table. For more accurate results, the experiment can be repeated using different voltage on the power pack. From the results, a graph of V against I was plotted and gradient of the graph is calculated. By extrapolating, the V-crossing was determined (Colombia Physics 2008, p.4-5).
From the graph, the slope of the graph is given by:
Slope= (3.8-3.73)/0.25-0.5= -2.8 Ohms
Percentage error is given by:
If the resistance of rheostat is set at maximum, 20 Ohms
Total resistance is 20+2.8=22.80Ohms and actual current=22.80/5=4.56A
While calculated current=20/5=4A and percentage error is given by=4.56-4/4.56=0.12%
A plot of current against voltage gave a straight line where the gradient of the straight line representing the internal resistance of the dry cell. If we apply the equation for a straight line is Y = MX + C, which can be applied to the EMF formula V = E-Ir, given that the voltage is on the Y-axis, the intercept (c) equals the electromotive force and the gradient (m) is equal to the internal resistance r (Randall 2004, p. 54) .
The internal resistance of dry cell is 2.8 Ohms. The value of internal resistance is negative because it is an opposing force i.e. EMF is the reverse of potential difference (Randall 2004, p. 55).
Since the percentage error was not big and fell within acceptable range, the experiment was successful with a few setbacks. These include unsteady ammeter and voltmeter, which lead to reading error.
From the experimental data, it is clear that current decreased with increase in voltage. Since the graph is a dropping one, it is clear indication of decline in resistance. This agreed well with Ohms law, as the decline in the resistance is comparative to the other computed answers. The internal resistance of the dry cell pack was found to be 2.8Ohms and the percentage error was 0.12%.