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Ohm’s law:Definition,formula,applications & problems

what is ohm’s law?

Ohm’s law states that “The current flowing through a conductor is directly proportional to the potential difference across its ends provided the physical state such as temperature etc. of the conductor remains constant.”Ohm describes mathematically how voltage,current and resistance in a circuit are related.It is used in the equivalent form depending on which quantity you need to determine.In this section you will learn each of these forms.After completing this section,you will be able to

  • Explain Ohm’s law
  • Describe how V,I and R are related
  • Express I as a function of I and R
  • Express V as a function of I and R
  • Express R as a function of V and I

Ohm determined experimentally that if the voltage across a resistor is increased,the current through the resistor will also increase;and ,likewise if the voltage is decreased ,the current will decrease.For example ,if the voltage is doubled,the current will double.If the voltage is halved ,the current will also be halved.

Ohm’s law also states that if the voltage is kept constant,less resistance results in more current ,and,also,more resistance in less current.For example,if the resistance is halved ,the current doubles.If the resistance is doubled,the current is halved.


Symbolically ohm’s law can be written as:


It implies that


Where R, the constant of proportionality is called the resistance of conductor .The value of the resistance depends upon the nature,dimensions and the physical state of the conductor.V is applied voltage in volts and I is current in amperes.

Formula variations of ohm’s law:

The three formula variations of ohm’s law are as:

  1.           I=V/R        to find current
  2.                R=V/I        to find resistance
  3.            V=IR            to find voltage

These formulas are an important and to the understanding of circuit behaviour.behaviour. They enable us to determine the value of any of the three quantities involved if the values of the other two are known.It makes it possible to design electronic circuits and to determine the value of various components mathematically,thereby avoiding unnecessary measurements of experimentation and wastage of time. The above formulas may be memorized  by using the circle divided as in fig..

To use the circle,all you need to do is to cover the factor you want and read the remaining formula.

Calculating current using ohm law:

In this section you will learn to use Ohm’s law to determine current values when you know the values and resistance.You will also see how to use quantities expressed with metric prefixes in circuit calculations.After  completing this section,you should be able to

  • Calculate current in a circuit
  • Use Ohm’s to find current when you know voltage and resistance values
  • Use voltage and resistance values expressed with metric prefixes

In the following examples ,the formula I=V/R is used.In order to get current in amperes,you must express the value of voltage and the value of resistance in ohms.

Example: Calculate the current in following figure.current in ohm's law

solution:In the above figure R=10 KΩ and V=50 V then from ohm’s law:



                                                                                   I=50V/10 KΩ


Calculating voltage using ohm’s law:

We can find voltage by using relation of ohm’s law V=IR , if we know current and resistance of a circuit.

Example:Find voltage if current of 5mA is flowing through following circuit.finding voltage by using ohm's law

In the above figure R=10KΩ,I=5mA,by using ohm’s relation V=IR:






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