DC Circuits and Ohm’s Law Physics 132 Abstract This experiment was to prove Ohm’s laws through mathematical calculations as well as a physical experiment on light bulbs and other resistors. We used the equation for Ohm’s law, which uses voltage and current to determine resistance either through an actual resistor or a light bulb. We measured the input and output of the generator, and the difference between the two. Overall, our experiment was very successful. Questions and Answers 1. 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? a. ΔV = I • R b. V=electric potential or potential difference, measured in volts c. I=current, measured in amps, which are coulombs/second d. R=resistance, measured in ohms 2. From Part 3.1.1 (Power Supply): Produce a graph of output voltage Vout as a function of the sliding contact position x, measured from the bottom end of the slide wire resistor. Verify that the data points fall on a straight line rather than a curve ( don’t actually compute the slope). Did the current change while you varied the sliding contact positionx? ( Hint: Your current meter was switched off during this part, but this question can be answered using Ohm’s law and your graph.) a. As you can see from the trendline on this graph, the data does fall relatively linearly, with minimal discrepancy. The charge did differ from one side of the slider to the other. You can see that it rises dramatically the farther the position is from zero. 3. From part 3.1.2 ( 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. In the lab, you should also have deduced R from the color-coding on the side of the resistor itself (recall in-lab question 1). Does your value for R fall within the manufacturer's expectations? a. The closes slope would be 0.039, as shown by the trendline on the graph below. The R value of the first resistor is 36.7, and the R value for the second is 11.2.4. From part 3.1.5 (resistors in series/parallel): Using the resistor color code in your manual (part 3.2), write the value of each resistor used in this part of the experiment. From this, compute the total resistance Rtot (i) resistors in series, and (ii) resistors in parallel. You will need to use the following two equations for computing total resistance: Rseries=R1+R2 1Rparallel=1R1+1R2 a. R1 (color code) = (3*10+6)*10^0 R2 (color code) = (1*10+0)*10^0 R1 (ohmmeter) = 36.7 (+)/-5% R2 (ohmmeter) = 11.2 (+)/-10% R1 and R2 in Series R1 and R2 in Parallel Voltage V (V) = 3 Voltage V (V) 3 Current I (A) = 0.07 Current I (A) 0.38 Resistance (ohm) = 42.85 Resistance (ohm) = 7.89 Column1 r1 r2 m 3 1 n 6 0 p 0 0 tolerance 5% 10% 36 105. From part 3.1.5 (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 those calculated in the previous question. That is, compute the percent difference for each case. a. Expected R1=36.7 R2=11.2 b. Calculated R1=42.85 R2=7.89 c. Percent Error R1=16.8% R2=41.9% 6. 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. a. Carbon resistors are Ohmic, due to the linearity of the slopes of both graphs above. 7. From part 3.1.4 (Light Bulb): In reality a light bulb is not an ideal resistor. As the light bulb heats up its resistance increases. Pretend you measured the current through a light bulb at different voltages and arrived at this data: Voltage (V) 0.5 1.0 1.5 2.0 2.5 3.0 Current (mA) 13.1 18.2 21.9 25.1 28.4 31.0 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? a. The resistance of the light bulb increases as voltage and current increase.8. When a typical circuit is switched on, the net displacement of electrons along a wire is about 1mm per second. If electrons have such a low net speed, why do lights and devices appear to work instantaneously when switched on? a. Electrons move in a group, which allows them to gain more momentum to move faster. Since they are all pushing each other, the time for the light to work is nearly none. Conclusion In this experiment, we sought to test and prove Ohm’s laws using both experimental and mathematical methods. The calculated values and the actual values are not that far off from what was expected. Any discrepancy would be attributed to measuring or equipment error in this lab. Overall, our testing of Ohm’s law was successful, which was the goal of this
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