From this experiment we can conclude that the potential
difference between the ends of the iron spoke (conductor) is
directly proportional to the current passing through it (assuming
the temperature of the iron spoke is constant during the flow
of current through it).
Draw a graph between V and I taking the current (I) along
y-axis and potential difference (V) along x-axis with appropriate
scale. You will get a straight line graph passing through the
origin as shown in figure 11.
Repeat the process by using a LED instead of iron spoke.
The long terminal of the LED is connected to the positive
terminal of the battery and short terminal of the LED is
connected to negative terminal of the battery. Note the values
of current ‘I’ and potential difference ‘V’ in each case and record
the values in table 1 (draw this table in your notebook). Find
V/I for each set of values I and V. You will notice that the ratio
V/I is not a constant. Draw a graph between V and I for LED.
You will get a curved graph as shown in figure 12.
From the above lab activity we can conclude that the ratio between V
and I is constant for some materials at constant temperature. This fact was
established by German Physicist, George Simon Ohm and it is popularly
known as Ohm’s law.
We can define Ohm’s law as follow
The potential difference between the ends of a conductor is directly
proportional to the electric current passing through it at constant
temperature.
Let V be the potential difference between the ends of the conductor
and I be the current passing through it.
V ∝ I (temperature is constant)
This constant is called resistance of the conductor. It is denoted by
‘R’. Then we get V/I = R.
V = IR
The SI unit of resistance is ohm. The symbol of ohm is Ω.
