Lab 2 - Ohm's Law 

Comments about the lab reports ---


                        About the graphs and slope etc. --

        I.   In any graph, you should always connnect the points by either
a 
             line or a curve depending on what you expect the graph to
look
             like. 

        II.  To find the slope, always draw a line, choose two points 
             ON THAT LINE, and find the slope. Never use just two data
             points ( unless they happen to fall exactly on the line ).

        III. Always show which points you've chosen to find the slope and
show
             the calculations.

        IV.  Using the V vs. I graphs for light bulb and resistor, you had
             to show which of the two is ohmic and which is non-ohmic.

        V.   Comparison of the V vs I graphs and P vs I^2 graphs -

             Using the eqution    V = (R)(I) , we see that the slope of
             V:I graph is R. Also, Using   P = (R)(I^2), we can infer that 
             the slope of P:I^2 graph is also R. Further, for a given
resistor,
             they should look the same - because, if the resistor is
ohmic,
             both  V:I  and  P:I^2   graphs will have constant slope,
while if
             it's non-ohmic, they'll both be similar looking curves.

        XIII.  Some of you had a wrong scale on the x-axis for the P vs
I^2 
               graph. ( Well, i can't explain what's meant by a wrong
scale
               without a graph paper and as my computer familiarity is
pretty
               limited, i can't include one in this document. So see me if
you
               have further question. )


        VI.  You had to mention why you chose the points that you chose to
             to find the slope with. 
             I expected you to see that the light bulb graph is a curve
and
             it has two linear parts, one before it starts glowing and one 
             after. If you find the slope in the part after it starts
glowing,
             the value is vastly different from the value measured by the
DMM.
             So, that should give you a hint that you should find the
slope 
             in the first part of the graph and compare that value with
the DMM value.

        B2.  This was bonus for mentioning the correct reason for the
non-ohmic
             behaviour of the light-bulb. When the light bulb starts
glowing,
             it's temperature rises and change in temperature changes the
             resistance. So you'll see that the slope before it starts
glowing 
             is nearly equal to the value found from DMM when the bulb was
cold.

        
                About the calculations and verifying Kirchoff's laws --

        VII.  For both the circuits, you had to find the value of
individual
              resistances for R1, R2, R3 and compare it with the one you
              measured with DMM.

        VIII. Calculating equivalent resistance correctly -

              For circuit 2, it was just the sum of R1, R2, R3.

              For circuit 3,  1/R = 1/R2 + 1/R3   gives the equivalent
              
              resistance of R2 and R3. Then, the total equivalent
resistance
        
              is the sum of R1 and R. i.e.  Req = R + R1.

        IX.   The total resistance of the circuit as measured was

                Rt =  Vps / Ips   where Vps and Ips are voltage across and
                                        current through the power supply.
                             
        X.    You had to compare the Req (see VIII.) with the Rt(see IX.)

        XI.   Whenever you compare two values, you must mention the %
error
              ( or % discrepency or % difference or whatever else you
might
               choose to call it,) because "5.2 is close to 5.6" is not 
              very meaningful.

        XII.  You had to show that the data DIRECTLY verifies the
Kirchoff's
              laws. There were three main checks -

               1.  The sum of voltages in a loop should  add up to zero,
                   which means that
                   for circuit 2, sum of voltages across R1, R2 and R3
should
                   add up to voltage of the power supply.
                   for circuit 3, Sum of voltages across R1 and R2 should
add
                   up to voltage of the power supply.

               2.  Since R2 and R3 and in parallel in circuit 3, the
voltage
                   law implies that the voltages across R2 and R3 should
be same.

               3.  Using the current law, the current in R1 must be equal
to
                   the sum of currents through R2 and R3. ( for circuit
3).