physics Lab 2

Abstract:

In this experiment, we are going to calculate the unknown resistance of simply by using the balance wheatstone circuit. Also, our goal is to calculate the average and standard deviation of . Then we are going to compare our results for our Resistance to the results obtained in Exp. 3 (simple D.C. circuit) just to get the error or % error, since it is mentioned that using a Wheatstone bridge is more accurate. By choosing a resistance value for and galvanometer should reads zero current, so we are able to measure length “X” in centimeter since it represents and as a uniform slide wire as mentioned in the lab manual; afterwards we can calculate the value for through the following equation:

()

Introduction:

In this experiment, we are going to observe how our results/method are accurate and more precise to achieve the resistance using the Wheatstone bridge instead of Simple DC circuit. Basically, we are going to use equation no 4 in our lab manual which is: () in which we set a value for in Ω and X1 is the length of and that we are going to measure in centimeters. There will be no current going to through the galvanometer so our circuit will remain balanced (point C & D are not connected).

Procedure:

Step 1: We connected the wheatstone according to figure 1, the only difference is that and are shown connected together as a uniform slide wire where point C is in between so we can control it to move right and left to measure the length.

Step 2: We turned on our Power supply and we set our voltage to 2 Volts. For Resistor 1, we chose three different values for our standard resistor, including the measurement of length (X). We did the same procedure for Resistor 2.

Step 3: We calculated the Average and Standard Deviation for Resistor 1 and Resistor 2.

Apparatus:

DC power supply, slide wire potentiometer, galvanometer, decade resistor box and two unknown resistors mounted in a box.

Data:

The Voltage is set at 2.0 Volts, while we gave our a specific resistance between (10 & 200)Ω. Our data is going to show the three different values for our standard Resistor we used in Resistor 1 and Resistor 2.

Resistor 1:

Resistor 2:

Analysis:

()

Rx = 20 () = 16.4

Average = = 17.2

Standard Deviation = = 1.18 1

Conclusion:

The Wheatstone bridge helped us to calculate an unknown resistance Rx. We concluded that our calculations in this experiment did quite match the results from experiment 2, which is the whole purpose of this experiment. This experiment is meant to be obtain more accurate and precise values in which we introduced the galvanometer, while in DC experiment 2 we did not, also the resistor box could be the difference from experiment 2. Because there were some errors during the experiments such as measuring the resistances or changing the lengths from each trial, the final results for both resistance 1 and 2 were not accurate as expected (the uncertainties were too large).

Questions:

1, The uncertainty is the estimated reliability of a value. (an uncertain

Percentage of uncertainty = ( )* 100

= [1/(10*10) ] * 100 = 1%

a) X=10cm ; uncertainty is 1%

b) X=50cm ; uncertainty is 0.2%

c) X=95cm ; uncertainty is 0.11%

2, If you increased the Voltage from 2 Volts to 6 Volts, you would higher current flowing through the resistors Rx and Rs because Voltage and current are proportional to each other according to Ohm’s law V=I*R. Also it would affect our values for our Resistors.

3, Yes, the galvanometer sensitivity was a limiting factor, because the k-key was very important to us in order for the galvanometer to read at exactly zero current by moving the slide wire left or right in order to get a more accurate/precise measurements for the length.

4, Fluctuations in Voltage would definitely change the Resistance values for our Resistors that we recorded, because we are have a different voltage every time. So your value is going to change every time you measure specific point.

5, The contact point should be moved close to point A. First if the current is flowing from C to D then the current has to move to the left towards Rx; because it cannot move towards Rs because of the contradiction with ; it would be two current with same direction moving towards each other which does not happen. Also according to Kirchhoff, the sum of the current into the node should equal the sum of the current that goes out of the node.

6, The observation we made is that our experimental values for the Resistor 1 and Resistor 2 in this experiment is quite different than our values of R1 and R2 from previous experiment (2). Exp. 4 (wheatstone bridge) would be more accurate since we depended on a galvanometer.

Comparison between Experiment 3 and Experiment 4:

Experiment 2:

Experimental Value for R1 is: 23.70 Ω

Standard Deviation for R1 is: 0.1 Ω

Experimental Value for R2 is: 23.45 Ω

Standard Deviation for R2 is: 0.1 Ω

Experiment 4:

Experimental Value for R1 is: 17.2 Ω

Standard Deviation for R1 is: 1 Ω

Experimental Value for R2 is: 20.90 Ω

Standard Deviation for R2 is: 0.3 Ω