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ohm law

Ohm’s Law

Ohm Law

What is ohm’s law

In the electrical field, Ohm’s law is a very important topic. This law is asked in almost all interviews and as basic fundamentals of electrical engineering for all who related to electrical engineering. This is a very important fundamental law that describes the Relation between Current, Voltage, and Resistance.

There are three main quantities present in Electricity. An electrical circuit is made by these three Quantities Current (I), Voltage(V), and Resistance(R). There is no imagine the electrical circuit without anyone thing of them. And one other essential quantity is Load. Because the current is flowing only if there is any load connected with the circuit.

Ohm’s law is given by a German mathematician George Simon Ohm, who formulated the law known after his name as Ohm’s law.

Ohm’s law definition

Ohm’ law states that if physical conditions (like- temperature, pressure, length, etc) are constant, then the voltage across the two terminals of conducting material is proportional to the current flowing through it.

Ohm’s law formula

Ohm’ law is expressed as V∝I

In the given fig when we connect a battery across the conductor then an electric field E is created and electrons move to the reverse direction of the electrical field.

ohm's law

If I consider a cross-sectional area in a conductor, Then the number of total charges passed across a cross-sectional area in one second, which is called Current in the conductor. Now if I increase the potential difference across the conductor, The electric field and magnitude are also increased and the forces of electrons are also increased and the magnitude of electrons is so increased. Now the all free electrons will move with more speed. That means the number of electrons that were crossed across the cross-sectional area will now be increased and a large number of electrons cross across the cross-sectional area of a conductor in one second. And we can say that the current is increased across that cross-sectional area.


If the potential difference increased by doubled then the current will also be doubled and the potential difference is increased 3 times then the current will also be increased by 3 times. so we can say that the Voltage (potential diff) is directly proportional to the current (I). V∝I if we replace the proportionality sign by = we need to place a constant. here R is the constant of proportionality that can be placed here. So V= I.R, R is resistance. Means R is in the circuit provided R is kept constant. Resistance is a fixed quantity of a particular conductor.

ohm law

But if i change the conductor material then the Resistance will also be changed. Also when I changed the length or thickness of the conductor then Resistance will also be changed.

V is the s.i unit of potential difference and Ω is the s.i unit of resistance. And I is the si unit of current. Ohm’s law is used to find the current, Voltage, and Resistance in a circuit.

How we use the ohm’ law

For example, in the given diagram a 9v battery is directly connected with an LED. This Yellow LED works with the 3v (only for example actual voltage for the yellow LED may different) and 20ma current, But in the diagram, the applied voltage is higher than required.  If I give more power than this the LED will burn as shown in fig. So I need to drop the voltage and current as suitable with LED.

ohm's law


LED voltage = 3v

Supply voltage = 12v

LED current = 20ma

We need to drop the voltage for LED = 12-3 = 9v, It means we need to reduce 9v from 12v battery. So Take here V= 9v,

Then V = I.R

20ma = 0.02A  (1A=1000ma)

9= 0.02 . R, and R = 450Ω, It means we need to add a minimum  of 450 Ω resistor with the LED for Proper working. 


ohm law


This Relationship between current-voltage, Resistance is not apply to all non-metallic conductors. It does not apply to non-linear devices such as Zener diodes.



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