Monday, January 23, 2017

Daniel Bruce and Moaad Alzahrani

Week 3



      1.     Compare the calculated and measured equivalent resistance values between the nodes A and B for three circuit configurations given below. Choose your own resistors. (Table)


Part A R1 R2 R3 R4 REQ Calculated REQ Measured
121 ohm 48 ohm 100 ohm 25.6 ohm 27.1 ohm
Part B R1 R2 R3 R4 REQ Calculated REQ Measured
121 ohm 48 ohm 100 ohm 152.2 ohm 153.4 ohm
Part C R1 R2 R3 R4 REQ Calculated REQ Measured
121 ohm 48 ohm 100 ohm 2K ohm 167.9 ohm 142 ohm

Table 1.1 Shows the caculated and measured resistance values of our circuits.

Most of our calculated readings were very close to the measured readings, except for part C. I assume we may have had some wires touching unknowingly decreasing our resistance values.

      2.     Apply 5V on a 120 Ω resistor. Measure the current by putting the multimeter in series and parallel. Why are they different?

      When measuring the current in series we are measuring the current flowing through the resistor.   When measuring the current in parallel we are shorting the circuit causing almost no current to flow through the resistor, and almost all of the available current to flow through the meter resulting in a wrong measurement.

3    3.     Apply 5 V to two resistors (47 Ω and 120 Ω) that are in series. Compare the measured and calculated values of voltage and current values on each resistor.
   
      Table 3.1 Below is a table of our measurements of each resistor's current values and voltage values.
    
120 ohm  48 ohm
 Calculated voltage 3.57 V 1.42 V
 Measured voltage 3.56 V 1.39 V
 Calculated current 29 mA 28 mA
 Measured current 28 mA 28 mA

      4.     Apply 5 V to two resistors (47 Ω and 120 Ω) that are in parallel. Compare the measured and calculated values of voltage and current values on each resistor.


120 ohm 48 ohm
 Calculated voltage 5V 5V
 Measured voltage 5V 5V
 Calculated current 41 mA 88 mA
 Measured current 37 mA 85 mA

      5.     Compare the calculated and measured values of the following current and voltage for the circuit below: (breadboard photo)



Figure 5.1 Shows the connections on our breadboard for measuring current and voltage

a.     Current on 2 kΩ resistor, 

Calculated
I=V/R ,  5.0V/2.5K=2.0mA
      Measured
I=V/R , 5.3V/2.5K=2.5mA
b.     Voltage across both 1.2 kΩ resistors. 

Calculated

1.2K Resistor A ;V=I*R , .15mA*1.2K ohm= .18V
1.2K Resistor B ;V=I*R , .88mA*1.2K ohm= 1.056V
      Measured
1.2K Resistor A ;V=I*R , .148mA*1.2K ohm=.1776V
1.2K Resistor B ;V=I*R , .885mA*1.2K ohm=.1.062V

      6.     What would be the equivalent resistance value of the circuit above (between the power supply nodes)?
        
       We measured 2.06K ohms of the circuit between the power supply nodes, this is the equivalent resistance value between the power supply nodes.

      7.     Measure the equivalent resistance with and without the 5 V power supply. Are they different? Why?

       Yes, because the power introduced voltage to the circuit. The meter is supposed to produce a small current and read the voltage; when the power supply added voltage to that reading the meter is assuming the resistance is very high in the circuit; this gave us a reading of "over limit" on the meter.

      8.     Explain the operation of a potentiometer by measuring the resistance values between the terminals (there are 3 terminals, so there would be 3 combinations). (video)

       
Video 8.1 Showing the operation of the potentiometer

   The video above shows the variable resistance between the middle terminal and any one of the other terminals. Also we wanted to point out that maximum resistance is always obtained by connecting the two outside terminals regardless of the position of the knob on the potentiometer.

      9.     What would be the minimum and maximum voltage that can be obtained at V1 by changing the knob position of the 5 KΩ pot? Explain.

We used a 10K ohm potentiometer and the minimum and maximum voltage that can be obtained at V1 is the same at any position of the 10 K ohm potentiometer. Changing the resistance value does not change the voltage drop across the potentiometer. The full 5V will always be dropped in this circuit because it is the only resistor in the circuit.

     10.  How are V1 and V2 (voltages are defined with respect to ground) related and how do they change with the position of the knob of the pot? (video)



Video 10.1 Shows the voltage drop vs resistance relationship of a series circuit using a potentiometer and resistor

     11.  For the circuit below, YOU SHOULD NOT turn down the potentiometer all the way down to reach 0 Ω. Why?

We should not turn down the potentiometer all the way because the resistance of the potentiometer will be almost zero. This is a parallel circuit, and if the potentiometer has too low of resistance, this will in effect become a short to ground; causing maximum current to flow through it and may burn up the potentiometer.

     12.  For the circuit above, how are current values of 1 kΩ resistor and 5 KΩ pot related and how do they change with the position of the knob of the pot? (video).


      
Video 12.1 Shows the current flow vs resistance of a parallel circuit using a potentiometer and resistor



     13.  Explain what a voltage divider is and how it works based on your experiments.
   
      A voltage divider effectively turns a higher voltage into one that is smaller by placing a resistor is series with the target device or component, causing the output voltage after the resistor to be lower to the desired level for the component. When we placed a potentiometer in front of a resistor we were able to effectively change the voltage on the resistor by changing the position of the knob on the potentiometer. When the resistance was increased on the potentiometer the voltage on the resistor was lowered.

     14.  Explain what a current divider is and how it works based on your experiments.

      A current divider allows current be adjusted to a desired level for a component by placing a resistor in parallel to the component. We used a potentiometer placed parallel of a resistor. As we increased the resistance of the potentiometer the current flow through the resistor was increased. When the resistance of the potentiometer was decreased this would direct less current flow through the resistor and more through the potentiometer.













14 comments:

  1. For sections in which there were discrepancies between measured and calculated values, I would like to know your take on why this is. This is particularly the case for question 1. Why were the calculated equivalent resistances so different in circuits A and C? Additionally, it might be useful to provide a calculated equivalent resistance for question 6; this could expand on the discussion of theory versus practice.

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    1. We did not get very different numbers between the measured and calculated except for C that is why we did not talk to much about it.

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  2. For Question 7, the wording to the response to the question is difficult to follow particularity the passage "we the power supply added voltage to that reading the meter is assuming the resistance is very high in the circuit". Another suggestion is to add the given diagrams for the questions that had them to help illustrate what you are trying to show, for example what part A is showing in problem 1. Really liked your attention to detail when explaining Question 11.

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    1. You are right! there were some spelling mistake that might case the confusion for you. Thanks for noting that.

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    2. I just fixed it.
      Thanks again.

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  3. For #3 we got the same values for the measured and calculated for both voltages and current i think you need to put more information why did you get these values and why the current doesn't change for both resistor while the voltage is different.
    Also, for #4 I think you need to write why the voltage is the same while the current is different. Other than that i think you made a good job, and I enjoyed reading your blog.

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    1. We thought it will be clear enough like this, but you might be right. We will take note. thanks.

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  4. In #5 I liked that you guys took the time out and typed up the equations and then gave the answers to them instead of just giving the answers like everybody else. I also thought your answer for #'s 13 and 14 were really descriptive and accurate.

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    1. We always try to take the time and the write the equations down to show how we did get there. I hope we were accurate in our numbers we didd our best. Thank you.

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  5. Our measured values versus our recorded values were much closer than yours in general for number 1. I believe something must have gone wrong in Part C. I believe that there is internal resistance in the DMM that allows only part of the current to flow through it when placed in parallel. We got a smaller reading than the full current because I believe the current was divided between the resistor and the DMM. Wow, your measurements and calculations for number 3 were very close. We did not read the directions correctly on that part and didn’t calculate. You may want to enlarge your photos or take closer pictures so that it is easier to see when viewing the blog. Why did you not measure the current directly in number 5? Instead you are still calculating because you do not know the exact resistance of the resistor. You should break the circuit and measure the current that way and compare that to mesh analysis calculations. Our equivalent resistance was a little different for number 6. You should include calculations so any errors can be spotted. For number 7, the voltage being introduced will not change the resistance in the circuit. We have photo evidence of this in out blog if you would like to check it out. Your video for explaining the potentiometer contained a very good explanation, I thought. Overall, I liked the blog. Just including some calculations may be helpful for those reviewing your blog in the future.

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    1. You're right about question number 1. We believe there was something wrong to for C. That is why we wrote under the table that "Most of our calculated readings were very close to the measured readings, except for part C. I assume we may have had some wires touching unknowingly decreasing our resistance values". We will check your blog for the other question.
      Thank you.

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  6. Its difficult for me to read the values for your tables, try screen-shotting a table made in excel, it will make your blog look exquisite. Make sure to add the pictures of the circuits from the question. Content wise your values seemed to follow along the same lines as ours, with the exception that some of our videos were taped outside of class. Good quality videos, and explanations, feels like I actually understand the conceptual answers for 8 - 12 now.

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    1. We were trying to learn to screen-shot tables, but it didn't work with the us. Finally we know how to do it now. If you look at the new blog now the tables will look much betters.
      Thank you.

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  7. Good responses to comments. Good blog.

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