In this lab, we practiced making combinational circuits using the given resistors. We also learnt how to use a potentiometer in a simple circuit. For the combination of resistors, the voltage drops across individual resistors, along with the currents were measured. The current values were compared with the current values predicted by Ohm’s law, which states that V=IR, where V is the voltage, I is the current, and R is the value of resistance.
In the first part of the lab, a potentiometer was connected in series with a 100Ω resistor and a LED, connected to a 10V source. Initially, the potentiometer was tuned to the maximum using a multimeter. Then gradually, the potentiometer was adjust (reduced value), till the LED began glowing. The corresponding value of resistance of the potentiometer was measured using the multimeter and recorded.
In the second part of the lab, the circuit as shown in Figure 1 was connected. The voltage drops and the current through each of the resistors were measured using multimeter and tabulated. We observed that the measured current values more or less coincide with the current values predicted by Ohm’s law.
Figure 1: circuit diagram
Sample Calculations – Predicted current based on Ohm’s law:
I = 1.73 V220 Ω= 7.86 mA
Percent error (%)=|Imeasured-Ipredicted|Imeasured×100
Percent error %= |7.71-7.86|7.71 ×100=1.9%
1) Knowing the potentiometer has a maximum resistance of 100k, if the resistance between terminals 1 and 2 were measured to be 25k, what would you expect the resistance to be between terminals 2 and 3?
Answer: The total resistance of the potentiometer is the sum of the resistances across terminals 1 and 2, and that across 2 and 3. Hence when the multimeter is used to measure the resistance between the terminals 2 and 3, it would measure 100 -25 = 75k.
2) Voltages across R1 and R2 in part B are seen to be equal. This can be expected since R1 and R2 are essentially in parallel, and hence will have the same voltage drops across them.
Error/Conclusion: The calculations show that the predicted current values are more or less the same as the measured current values. Further, this is also indicative of the fact that Ohm’s law holds good even in a combination circuit. From the values of voltages, it can be observed that the voltage drop across R1 and R2 are equal, as expected of parallel resistors. Similarly the currents in R1 and R2 add up to the current in R3. This is also expected since the combination of R1 and R2 is in series with R3. Finally, it is evident that the voltage drops of the resistor combination add up to the source voltage. However, there is an error of 1.9% -4.8% in the measured current values. This error can be attributed to the internal resistance offered by the multimeter, and the resistance of the connectors, which are not considered in the calculations. Further, there could be an error in the resistance value used according to color codes. These are given by the tolerance of each resistor.