Georg Simon Ohm (physicist) 1787 – 1854 Found the relationship between the current (I) flowing through a conductor and potential difference (V) across the terminals of a conductor using the circuit diagram. The graph between V and I is always straight line.
Note that, there is some printing error or something sort of in figure 12.3 in supporting of topic 12.4 Ohm’s Law of Chapter-12 (Electricity) in class 10th NCERT Science (Physics) book. In figure 12.3, Applied Voltage (V) is the cause and current (I) is the effect. As per the International standard and Georg Simon Ohm, cause (Input) must be along with the X-axis and Effect (Output) along with Y-axis. Therefore, Potential difference (Voltage in V) must be along with X-axis and Current (A) along with Y-axis. Applied Potential difference (V) is the only Cause that produces Effect in form of Current flowing in Resister or circuit.
OHM'S LAW
Ohm’s Law states that the potential difference between two points is directly proportional to the electric current.
This means; potential difference V varies as electric current, i.e. Voltage V is proportional to the Current I.
Where R is constant for the given conductor at a given temperature and called resistance.
Resistance is the property of conductor which resists the flow of electric current through it. Component that is used to resist the flow of electric current in a circuit is called resistor. It is represented by ‘R’ and symbol is . SI Unit of resistance is ohm. Ohm is denoted by Greek letter ‘Ω’.
Fig: Resistor Symbol
1 ohm (Ω) of Resistance (R) is equal to the flow of 1 A of current through a conductor between two points having potential difference equal to 1 V.
From the expression of Ohm’s Law it is obvious that electric current through a resistor is inversely proportional to resistance. This means electric current will decrease with increase in resistance and vice versa.
In practical applications, resistors are used to increase or decrease the electric current.
Variable Resister: The component of an electric circuit which is used to regulate the current; without changing the voltage from the source; is called variable resistance.
Rheostate (Variable Resister) –This is a device which is used in a circuit to provide variable resistance.
So to increase or decrease the current accordingly in the circuit a component is used is called “Rheostat”, that regulates the current without changing potential difference. Represented by “Rh”
Its
symbol is 
or 
.
Example 1: Calculate the resistance if 5 A of electric current flows through a conductor having potential difference between two points is equal to 15 V.
Solution: Given, electric current (I) = 5 A
Potential difference (V) = 15 V
Resistance (R) =?
We know from Ohm’s Law that
Example 2: If the potential difference between the terminals of an electric motor is 220 V and an electric current of 5 A is flowing through it what will be the resistance of electric motor?
Solution: Given, electric current (I) = 5 A
Potential difference (V) = 220 V
Resistance (R) =?
We know from Ohm’s Law that
Example 3: An electric current of 15 A is flowing through an electric fan. If the potential difference between two terminals of electric fan is 240 V, what will be resistance?
Solution: Given, electric current (I) = 15 A
Potential difference (V) = 240 V
Resistance (R) =?
We know from Ohm’s Law that
Example 4: If the resistance of an electric iron is 48 Ω and an electric current of 5 A is flowing through it, what will be the potential difference between two terminals of electric iron.
Solution: Given, electric current (I) = 5 A
Resistance (R) = 48Ω
Potential difference (V) =?
We know from Ohm’s Law that
Example 5: Calculate the potential difference between two points of a terminal, if an electric current of 10 A is flowing through it having resistance of 20Ω.
Solution: Given, electric current (I) = 10 A
Resistance (R) = 20Ω
Potential difference (V) =?
We know from Ohm’s Law that
Example 6: If the resistance between two terminals of an electric heater is 15Ω and an electric current of 15 A is flowing through it then what will be the voltage of electric current?
Solution: Given, electric current (I) = 15 A
Resistance (R) = 15Ω
Voltage, i.e. Potential difference (V) =?
We know from Ohm’s Law that
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