Week 3

Blog sheet week 3

1.     Compare the calculated and measured equivalent resistance values between the nodes A and B for three circuit configurations given below.

figure 1. three combinations of resistors

The resistors’ values: R1=1.491kΩ, R2=5.57kΩ, R3=0.988kΩ, R4=1.208kΩ.

Group	Calculated value (Ω)	Measured value (Ω)
a	536.95	537.8
b	2330.15	2328.6
c	3066.03	3064.7

Table 1. Calculated and measured value for resistor combinations

The measured values are all close to the calculated value.

2.     Apply 5V on a 100 Ω resistor. Measure the current by putting the multimeter in series and parallel. Why are they different?

      With the DMM in series we measured 39.491 mA, we expected to see 41.7 mA. With the DMM in parallel we measure the current at out of limits. We got out of limits when the DMM was in parallel because when you are measuring current with the DMM the internal resistance of the DMM is zero. With zero resistance in the DMM this shorts out the circuit.

3.     Apply 5 V to two resistors (47 Ω and 100 Ω) that are in series. Compare the measured and calculated values of voltage and current values on each resistor.
     

Table 2. Calculated and measured values for 47Ohm and 120Ohm Resistors

      The measured current and voltage was very close to the calculated values. The error could have come from the resistors not being completely accurate. The current for both resistors are the same since they are in series.

4.     Apply 5 V to two resistors (47 Ω and 100 Ω) that are in parallel. Compare the measured and calculated values of voltage and current values on each resistor.

Table 3. Calculated and measured values for 47Ohm and 120Ohm Resistors

      The measured current and voltage was very close to the calculated values. The error could have come from the resistors not being completely accurate. The voltage across both resistors is the same when they are measured in parallel, but the current is different.

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

a.     Current on 2 kΩ resistor, 
Calculated current = 1.962mA, Measured current = 1.9993mA.

                   b. Voltage across both 1.2 kΩ resistors
                   As shown in the figure 2:
                   For R1=1.2kΩ, Calculated voltage = 0.84V, Measured voltage = 0.859V.
                  For R2=1.2kΩ, Calculated voltage = 0.689V, Measured voltage = 0.705V.
                   

Figure 2. Resistors combination.

Figure 3. Showing measuring the current for the 2 kΩ resistor

Figure 3 we have the DMM in series with the 2 kΩ

6.     What would be the equivalent resistance value of the circuit above?

The total measured resistance for the circuit is 2524.5 Ω, the calculated total resistance is 2548.1Ω.

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

      We measured the equivalent resistance as 2524.5 Ω without the 5 V applied, with the 5 V applied we measured OL. They are different. They are different because when you measure the resistance With the DMM, the DMM generates a small amount of voltage and measures the current to calculate the resistance of the resistor. If there is a voltage already applied to the circuit the DMM will be attempting to calculate the resistance with and unknown voltage.  

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 1. The operation of a potentiometer

A potentiometer is a 3 terminal resistor that is controlled by a rotating knob. As you turn the knob either right or left you adjust the resistance for all 3 terminals. This are commonly used to control speed of motors.

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.

The maximum voltage would be 5 V since that is was is being supplied. If we reduce the resistance all the way down to 0 Ω the voltage would be 0 V. By reducing the resistance down to zero, we are basically shorting the circuit, causing no potential difference. If the resistance is not zero, or the circuit is not shorted the voltage will always be 5 V.   

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

Video 2. The relationship between voltage and different position of the knob of the pot

V1=5V, and V2 will increase if we increase the resistor of potentiometer, V2 will decrease if we decrease the resistor of potentiometer.

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

You should not decrease the potentiometer all the way down to zero because you will be shorting out the circuit. When you short out the circuit you are generating an infinite amount of current. This will damage the power supply and also the potentiometer.

12.     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? (photo)

Video 3. The relationship between current and different position of the knob of the pot

We took a video instead of photo since video can show the changes more directly.

Current for 1k resistor = 5mA, and current for potentiometer will decrease if we increase the resistor of potentiometer; current for potentiometer will increase if we decrease the resistor of potentiometer. The current for the 1K resistor will not be affected by the potentiometer until the resistance of the potentiometer is zero. Once the potentiometer is zero ohms the current of the 1k resistor will be zero amps.

13.     Explain what a voltage divider is and how it works based on your experiments.
      When two resistors are in series they divided the total voltage of that circuit, the amount of voltage that each resistors uses is dependent on that individual resistance in relation to the total resistance. By adjusting the resistance of the potentiomer you can adjust the voltage drop across each resistor.

14.     Explain what a current divider is and how it works based on your experiments.
      Current divider is similar to voltage divider, but now the resistors are in parallel. The voltage for each resistor is the same, but each one has a different current going through it. The total current of the circuit is divided between all the loads in the circuit. How much current each load gets depends on the amount of resistance that each load had in relation to the total resistance and total current. If the potentiometer is turned down to zero ohms you can eliminate part of the circuit.