Internal Resistance of a Battery - Lab Report Example

Internal Resistance of a Battery Abstract This experiment was carried out to look into two issues about an ordinary house battery—its internal resistance and the nature of its electrical power transfer. The hypothesis for this experiment is that the equation P=IV hold true for an ordinary house battery. Theoretical Model Like any other battery, an ordinary house battery is a possible source of voltage and has an inbuilt form of resistance known as internal resistance. In order to determine its internal resistance accurately this battery is connected in a series circuit with a variable resistor (rheostat) as shown in figure 1 below =battery =switch = rheostat Figure 1 Source: Internal Resistance of a Battery, p 2 The rheostat’s resistance is then varied a number of times, taking the current and voltage readings for each turn. The equation V = ε - Ir, is then used to determine the internal resistance of the battery, where V is the recorded total voltage drop across the battery, ε, the electromotive force or the total voltage put out by the battery, I the current flowing through the circuit and r the required internal resistance of the household battery. ε, V, and I, are measured and then the equation V = ε – Ir used to determine r which is then compared to accepted values. A possible limitation to this experiment that may lead to inaccurate results is the resistance of the connecting wires that is never taken to account. Experiment Procedure The following list of equipments were used in the experiment −New 9-volt battery −89-ohm rheostat −1-ohm, 10-watt resistors −Switch −Multipliers −Wires and crocodile clips A new 9-volt battery was obtained and the voltage it could put out measured and recorded. Next, the slide on the rheostat was put to the extreme right—its maximum level--and its resistance measured and recorded. The circuit was then connected as shown in figure two below, keeping the switch open and taping its base to the table for stability (Internal Resistance of a Battery, p 2). Figure 2: the set-up of the circuit The high current (20A) input of the ammeter was used, setting it up to the appropriate scale. Everything was now set for measurement taking. In quick moves, the switch was closed, both the voltage and current readings recorded and then the switch opened immediately. A few seconds were then to pass before the next measurements were taken (Internal Resistance of a Battery, p 2). The rheostat was the disconnected and its resistance decreased by sliding it a few centimetres to the left. This resistance was then measured and recorded. The circuit was then reconnected and the above procedure of taking measurements repeated. Next, the rheostat was disconnected and moved a few more centimetres to the left. As in the previous steps, the resistance was measured and recorded, the circuit reconnected and the current and voltage measurements repeated as before. The process of decreasing resistance by moving the slide to the left each time and subsequent taking of measurements continued until eight data points were obtained (Internal Resistance of a Battery, p 2). The rheostat was then removed from the set-up and some white 1-ohm and 0.5-ohm resistors obtained. Since these were the sated values, their actual values were measured as well. These resistors were arranged in a configuration that represented 3.0 ohms and the above stated measurements and connections repeated. After that, the resistors were rearranged such that the overall resistance decreased by 0.5 ohms and the same procedure of taking measurements repeated. This process of decreasing resistance by about half an ohm continued unitll the last data point was taken at around 0.5 ohms. All along, the resistance of each configuration was measured. Finally, with the switch left open, the resistors were removed from the circuit so that the battery was left connected only to the voltmeter and ammeter. The circuit was then reconnected without the resistors, p 3. Working with extremely high speed, the switch was closed, the current and voltage noted and then the switch was opened. This last step had the intention of measuring the ‘short-circuit current that the battery could deliver. The battery was then disconnected from everything else and once aging, its voltage was measured with a multiplier and the readings recorded (Internal Resistance of a Battery p 3). One of the possible sources of errors in this experiment could have originated in delays during the opening and closing the circuit since in between, one was expected to have taken the measurements. Two, due to an urge to complete the experiment owing to its numerous and lengthy procedures, it was possible for one to lack the patience of waiting for a few seconds before taking the next measurement thus negatively influencing the chances of obtaining accurate data. A defective nature of the battery during manufacturer that could result to say, lower voltage, could also lead one to obtaining erroneous results (Internal Resistance of a Battery, p 4). Data Analysis From the obtained data, a graph of voltage (y-axis) was plotted against current as shown in figure 3 below. Figure 3 Source: Electronics Point From the equation V = E - Ir, is is apparent that gradient of the graph will yield the internal resistance of the household battery. It is important to note that in this case, the gradient would be obtained as negative because the graph slopes down. Calculations (I) Unloaded voltage = 9 V Resistance load = 100 Ohms Voltage reading = 8.55 V The internal resistance is given will be (9V - 8.85V)*(100 Ohms)/9V = 0.0005 Ohms (ii) The accepted value is 0.00035 (Electronics Point 2006) % error = (0.0005 – 0.00035)* 100 0.00035 = 4.2% The maximum current that can be supplied is equal to 20A Summary of results The internal resistance of the battery was obtained as 0.0005 Ohms. This value however, had a percentage error of 4.2% compared to the accepted value. The maximum current that the circuit could supply was found to be 20A. Discussion of Results and Conclusion The objectives of the experiment were achieved. This is in view of the fact that the internal resistance of the battery was obtained as 0.0005 Ohms. The above stated error on the internal resistance could be attributed to potential experimental errors that are pointed out elsewhere in this lab report. From the formula, P = IV it is clear that the nature of electrical power transfer is such that current flowing through the circuit is high when the voltage is low and vice versa. From the obtained value of the internal resistance of the battery, the accuracy of the experiment was relatively high. Nevertheless, this accuracy could be enhanced by exercising more keenness when caring out the experiment couple with speed ans patience where either is necessary. Works Cited ElectronicsPoint. "Electronics point: discussion and advice on all electronics." 2006. 6 February 2012 . Internal Resistance of a Battery. [Class Handout] PHYS 1112L/2212L, 2011. Read More