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Electric Current: Introduction to Ohms law

Grade 9
Aug 22, 2022

Key Concepts

  • Ohm’s law
  • Resistance
  • Voltage


By shifting the regulator to and fro, we can regulate the speed of the fans in our house. By controlling the resistance via the regulator, the current flowing through the fan is managed. In this section we are going to learn about the relation between resistance, voltage and current. 


Ohm’s law: 

Ohm’s law states that in an electrical circuit, the current passing through most materials is directly proportional to the potential difference applied across them. 

V/I graph


I ∝VI ∝V

IR = V 


Where R is the constant of proportionality known as resistance.

ohms law

There are three forms of Ohm’s Law:  

 I = V/R   

V = IR   

R = V/I   



I = Current  

V = Voltage  

R = Resistance 

In practical units, this law can be written as 

Voltage, V = Current (A) × Resistance (Ω) 

Other form of Ohm’s law which can be used to define the relation between Voltage, Current and Resistance are: 

  • 1 ampere = 1 volt / 1 ohm  
  • 1 ohm = 1 volt / 1 ampere 

In the electric circuit resistance is represented as


Resistance which can be varied has the symbol

varied resistance

One Ohm: 

The SI unit of resistance, Ohm (Ω), is named after the German physicist Georg Simon Ohm.  

One ohm is equal to the resistance of a conductor through which a current of one ampere flows when a potential difference of one volt is applied to it. 


A few prefixes of Ohm’s are used as multiples and submultiples of SI unit such as: 

1 microohm = 1 μΩ =

1 × 10−61 × 10−6


1 milliohm = 1 mΩ =

1 × 10−3


1 ohm = 1 Ω  

1 kiloohm = 1kΩ =

1 × 103


Questions and answers 

Question 1: Find the missing value.

Fig 1


Fig 2


  1. Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

a. I ∝ V

b. IR=V

  1. The SI unit of resistance is “ohm” written as Ω.
  2. In the electric circuit resistance is represented as
Fig 3
  1. Resistance which can be varied has the symbol as
Fig 4


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