OHM'S LAW

Ohm's Law

The relationship between Voltage (V), Current (I) and Resistance (R) was discovered by scientist Georg Ohm.
This discovery is known as Ohm's Law.

Ohm's Law states that the Current (I) flowing through a conductor is directly proportional to the Potential Difference (V) applied across it's ends. That is provided the temperature remains constant.

For current to flow, there must be a voltage.  Ohm's law shows this relationship as :-

Voltage (V)  =  Current (I)   x Resistance (R)

When we know the voltage and current, we can use this formulae to calculate the resistance. Since current is set by the values of the voltage and resistance, using Ohms' law formula, it  can be shown that :-
  • If we increase the voltage, current will increase.
  • Increase in resistance, current will reduce.
Ohm's Law Triangle



Example 1



    

Fig. 1
Refer to Fig. 1
Find V if  Current ( I )  = 0.2 A  and Resistance (R) = 30 ohms

        V =  I x R
        V =  0.2 A   x  30 Ohms
        V =  6 V

Next, if a load of 200 ohms is connected to a supply of 120V, what will be the current drawn I ?

        V  =  I x R
        I   =   V  /  R
        I   =   120 / 200
        I   =   0.6 A 

Example 2
                   


 Fig. 2
Refer to Fig. 2

Let R1 = 30 ohms, R2 = 20 ohms and V = 24 V. Find the current I.

Step 1
Total Resistance RT is determined by :-
1/RT    =    1/R1    +    1/R2
            =    1/30    +    1/20
            =    2/60  +   3/60
1/RT    =    5 / 60
Therefore  RT = 60 /5 = 12 Ohms

Step 2

Using Ohm's Law
    V      =     I    x    R
    24    =     I    x    12
    I        =    24 /    12
    I        =    2 A