Voltage: Ohm's Law and Kirchhoff's Rules ABSTRACT Ohm's Law and Kirchhoff's rules is fundamental for the understanding of dc circuit. This experiment proves and show how these rules can be applied to so simple dc circuits. INTRODUCTION In the theory of Ohm's Law, voltage is simply proportional to current as illustrated in the proportionality, V = RI. As shown in this relation, V represent voltage which is the potential difference across the two ends of a electrical conductor and between which an electric current, I, will flow.
The constant, R, is called the conductor's resistance. Thus by the Ohm's Law, one can determine the resistance R in a DC circuit without measuring it directly provided that the remaining variable V and I is known. A resistor is a piece of electric conductor which obeys Ohm's Law and has been designed to have a specific value for its resistance. As an extension of the Ohm's Law, two more relationship can be drawn for electric circuits containing resistors connected in series or / and parallel. For resistors connected in series, the sum of their resistance is, RTOTAL = R 1+R 2+... +Rn.
And for resistors connected in parallel, 1/RTOTAL = = 1/R 1+1/R 2+... +1/Rn. Complex dc circuit involving a combination of parallel and series resistors can be analyzed to find the current and voltage at each point of the circuit using 2 basic rules formulated by Kirchhoff. 1) The algebraic sum of current at any branch point in a circuit is zero.
2) The algebraic sum of potential difference, V, around any closed loop in a circuit is zero. These rules and equations provided by the Ohm's law and the Kirchhoff rule can be experimentally tested with the apparatus available in the lab EXPERIMENTAL METHOD The apparatus used in the experiment includes a Voltmeter, an Ammeter, some connecting wires and a series of resistors and light bulb with varies resistance. This experiment could be divided into 5 sections which value of voltage and current measured is noted in all sections for further calculation. In the first section, in order to evaluate the reliability of Ohm's law, a dc circuit was constructed as FIG 2 (on p.
4) using a resistor with an expected resistance at 2400 W 120 W. In the second section, we were instructed to determine the internal resistance of the voltmeter. Two dc circuit we reconstructed as FIG 1. and FIG 2. using a resistor with an expected resistance at 820000 W 41000 W. In the third section, we were asked to judge if the filament of a light bulb obey Ohm's law, this was done by constructing a dc circuit as FIG 1.
with a light bulb instead of a resistor. Where in the forth section of the experiment, we explored the ability of multimeter to measure resistance directly and observed the difference in total resistance when two resistor at 270 W 14 W and 690 W 35 W were connected parallel or series together. And finally, in the last section of this experiment, we were instructed to construct a circuit like the one shown in FIG 3. and test the Kirchhoff's rules where R 1, R 2, R 3 are 270 W 14 W, 690 W 35 W and 2400 W 120 W respectively. The voltage and current across and through each resistor was measured. RESULTS AND DISCUSSION Results from section 1 as we saw on Graph 1, the calculated resistance was constant at 2448 W 147 W and this was within the experimental error of the actual resistance of the resistor and so proved the accuracy of Ohm's law.
Graph 2 and 3 summarized the differences in total resistance led to the finding of the voltmeter's internal resistance in section 2. Since the calculated total resistance, R 1 total, from circuit constructed as FIG 1. was, Resistor, the resistance of the resistor alone, on the other hand, the calculated total resistance, R 2 total, from circuit constructed as FIG 2. was, 1/Resistor+1/Internal resistance, a combination of resistance of resistor and internal resistance of the voltmeter. Though a series of mathematical calculation, Internal resistance can be solved.
Our calculated Internal resistance is 18. 21 MW 0. 02 MW which was much greater than the expected value of 10 MW. This error is most likely due to 1) the inaccurate value of given internal resistance since it's unlikely that all voltmeter have the same internal resistance. 2) Un stability of power supply causes reading error. Graph 4 shown that growing light bulb did not obey Ohm's law.
Its resistance increased as it became brighter. The fact that resistance of a metal increases with temperature is largely due to the heat, or kinetic vibration built up in metal interferes with flow of electrons. In the fourth section of the experiment, the resistance measured in parallel and series is 191 W 1 W and 950 W 5 W, very similar to the calculated resistance which is 194 W 13 W and 960 W 37 W respectively. And in our last section, to verify Kirchhoff's rules, I 2+I 3 = 3. 70 mA 0. 04 mA is approximately equal to I 1 which is 3.
79 mA 0. 03 mA. Also, V battery+V 1+V 2 = Battery +V 1+V 3 where both are equal to 0 V. CONCLUSION This experiment show that most dc circuit problems can be solve by Ohm " slaw and Kirchhoff's rules which interested in voltage current and resistance.
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1991. Canada. p. 434-435 F. Hands.
First Year Physics Laboratory Manual 1995-1996 University of Toronto Bookstores. 1995. Toronto, Canada. p. 74-76.