Ohm’s Law Physics 132 ILC S110 8/10/17 Abstract This lab allowed us to experiment with circuits, resistors, and the application of Ohm’s Law. We tested how resistors differed in a parallel vs series circuit and found the values of the resistors both through our own mathematical and experimental abilities (by finding the values of voltage and currents) and the actual value using an Ohmmeter. Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points, so for our experiment to prove Ohm’s law our graph of our data of Voltage vs current should be proportionally and stably increasing in a straight line. Questions & Answers 1. Think about a string of holiday lights. When one bulb burns out, the other bulbs continue to shine. What does that mean for the nature of wiring in the string? (2 points) The nature of the wiring of the string is parallel because when one light burns out, it does not affect the other ones. This means each bulb has it’s own circuit 2. Consider two resistors of equal resistance. You can combine them in a circuit either in series or in parallel. Which combination would have the higher effective resistance: series or parallel? Which would carry more current: series or parallel? (2 points) The higher effective resistances would occur in a series circuit, a parallel circuit would carry a more effective current since it has higher power. 3. State the equation for Ohm’s law. What do the variables V, I, and R stand for, and what are the units of each? Of the units listed, which one is equivalent to coulomb/second? (1 point) The equation for Ohm’s law is R=V/I. V stands for voltage, R stands for resistance and I stands for the current that flows through the resistor. The units for V are Joules per Coulomb (J/C), the units for R is Volts/Amps, and the units for I is C/s or Amps. Of the units listed, I is equivalent to coulomb/second. 4. From part 5.1.2 ( Ohm’s law ): Produce a graph of voltage V as a function of current I. Compute the slope, which will give the resistance R ( in ohms) of the resistor. (2 points)Slope=.0954 which equal 95.4 Ohms=R 5. From part 5.1.3 (resistors in series/parallel): Use the measured voltage V and current I for the two cases: Rtot to calculate (i) resistors in series, and (ii) resistors in parallel. Compare your measured values for the series and parallel cases to the calculated resistance using formulas (2 points) (i) Using the values we obtained for the Voltage and Current we were able to find our experimental Rtot value for the series which is equivalent to V/I. Using our numbers from our data sheet this transferred to 3V/.019A=158 Ohms. Using the Ohmmeter, we were able to find the values of R1 and R2 which were both 98.9 Ohms so using the expression Rseries=R1+R2 (Rseries=98.9+98.9) We were able to find the actual value of the resistor to be 197.8 this was different from the measured value which could be accounted for by simple measuring human error. (ii) Using the values we obtained for the Voltage and current we were able to find our experimental Rtot value for the parallel which is equivalent to V/I. Using our numbers from our data sheet we obtained for V and I, our equation ended up being 3V/.065A=46.1 ohm. Using the Ohmmeter and the values for R1 and R2 we used last time (98.9 and 98.9) we were able to use the R parallel formula to find the actual value of the resistor: 1/Rparrellel=1/98.9+1/98.9= 49.5 ohms which was very close to our experimental value but again was slightly off probably due toa human error in measurement or rounding. 6. Which configuration (parallel or series) consumes more power (P = VI)? Explain why. (2 points) Power is determined by the I or resistor multiplied by the number of volts, in the case of our experiment volts were the same in both cases of 3 V, so we can determine it based on which has the larger resistant. In the case of parallel I= .065 Amps while in the series, I=.019. With that in mind, parallel has more power. 3V X .065A = 195 ohm. 7. An Ohmic device is one that obeys Ohm’s Law. According to your data, are carbon resistors Ohmic devices? Support your answer with references to your graphs and/or equations. (1 point) Yes because our resistors created a straight line with no curves or deviations which means it captures the Ohm’s accurate and was inline with the theory of Ohm’s Law and what we predicted the graph should look like if Ohm’s Law was maintained in this experiment (a linear graph.) 8. From part 5.1.4 (Light Bulb): In reality a light bulb is not an ideal resistor. As the light bulb heats up its resistance changes. Pretend you measured the current through a light bulb at different voltages and arrived at this data: Plot a voltage as a function of current for this data. Please connect the dots instead of adding a best fit line. What can you say about the resistance of the light bulb? (1 point) The resistance of the lightbulb is consistent with Ohm's law as the line of best fit is for the most part straight and increasing as currents and voltage increase. 9. When a typical circuit is switched on, the net displacement of electrons along a wire is about1mm per second. If electrons have such a low net speed, why do lights and devices appear to work instantaneously when switched on? (1 point) Although electrons have a low net speed, lights and devices appear to work instantaneously when switched on because the electrons move as a wave pushing each other further on. 10. Calculate the total power (P = VI) for two light bulbs in series and in parallel. Which uses more power? Explain why. (2 points) In the case of the parallel set up, the power would be equivalent to 3 V*.183 A=549 ohm. Comparing this to the series set up which would equal to 3V*.066 A=190 ohms, it is clear the the parallel set up requires more power. This is mostly accounted for by the resistance always being greater in parallel set ups as opposed to series ones. Conclusion Our experimental results and the actual results differing in this experiment could have been caused by a number of factors including mathematically or rounding errors or errors with recording the measurements of the currents both human and from the software we were using to record it. However, we were still able to prove Ohm’s law as applying to our experiment as seen in the data we recorded and the straight and increasing line of the
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