Ohm's law

I (A)                                                        it can be seen that the slope = I/V,

                                                        which is equal to 1/R. The resistance can then

                                                        be found by inversing the slope.

                        V (V)

                Figure 1.1

Thus, a greater resistance would cause the slope in the graph I vs. V to be more gentle compared to a smaller resistance as the slope is equal to 1/R. (eg-in the cases when R= 3Ω, the slope of the graph would be 1/3 and when R= 5Ω, the slope would be 1/5. 3<5, while 1/3>1/5, which demonstrates that a greater resistance(5Ω) would have a more gentle slope graph of I vs. V when compared with a smaller resistance (3Ω).

It then follows that resistors connected in parallel (the total resistance = 1/R1 + 1/R2) would have a steeper IV graph slope compared to the same resistors connected in series (the total resistance = R1 + R2).

However, not all objects obey Ohm’s Law. Those which do are called Ohmic Conductors, whereas those that disobey Ohm’s Law are called Non-ohmic Conductors.

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Circuit Diagram:

                                          390Ω           820Ω

Aim:

To examine the relationship between the potential drop across each of the three resistors connected in series and the current through them.

Materials:

Variable DC Power Supply

Micro Lab Electronic Kit

Multimeter

Procedure:

1. The circuit was connected as show above.
2. The longer leg of the light emitting diode must be connected to the positive side of the board
3. With the DC supply set to 2V, record the potential rise applied to the circuit across the three resistors and the current through the circuit.
4. Repeat the procedure for DC supply setting to 12V in increments of 2V.
5. On one set of axes, graph V against I for each of the three resistors and calculate the slope of any straight lines obtained.

Results:

Results of Experiment

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Analysis:

For the graph of I vs. V,

Slope = m

   m = I/V =1/R

For the 390Ω resistor,

   m = (y2 –y1)/(x2-x1)

     = (1.25 * 10-3)/(0.48)

     = 1/R

Hence R = 480/1.25

       = 384Ω

Theoretical:

    390 +/- 5%

    390 +/-19.5

    370.5 → 409.5

   Since R found in this experiment is 384Ω, therefore it is within the tolerance range.

For the 820Ω resistor,

    m = (y2-y1)/((x2-x1)

      = (2.06-0.810) * 10-3/(1.68-0.620)

      = 1.25/1010

    R = 1010/ 1.25

      = 808Ω

Theoretical

     820+/- 5%

     820+/-41

     779 → 861

Since R found in this experiment is 808Ω, it is within the tolerance range.

For LED, the gradient is not constant because its IV graph is not a straight line. This means that the current flowing through the LED is not directly proportional to the voltage across it.  

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Discussion:

From the graphs of I vs V plotted for the two resistors and the LED, it follows that the two resistors are ohmic because their IV graphs formed a straight line approximately, whereas the LED is non-ohmic as its IV graphs was non-linear showing that it disobeyed Ohm’s Law.

In general, this experiment was very successful because the results obtained were quite accurate; the resistances found for the two resistors were within the tolerance range of 5%. One reason for this accuracy is due to the fact that the equipment used is digital Micro labs equipment- which can provide very accurate readings

Conclusion:

In this experiment, the potential drop across each of the two resistors is found to be directly proportional to the current through them because their I vs. V graphs are straight lines passing through the origin. This shows that they are ohmic conductors. For the LED, the potential drop across it is not directly proportional to the current through it because its I vs. V graph is not a linear one, thus it is non-ohmic.