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Ohm - Unit of Electrical Resistance

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Ohm is the unit of electrical resistance in the SI system. It was named in the honor of the famous German physicist Georg Simon Ohm. It is mathematically equal to the resistance of a circuit in which a potential difference of one volt can produce a current of one ampere or, the resistance in which one watt of power is dissipated when one ampere of current starts flowing through it. 

Ohm’s law provides a direct relationship between electric current and potential difference. The current which is flowing through any conductors is directly proportional to the voltage applied to it. In this topic we have discussed what is Ohm, let’s understand Ohm definition and laws and some numerical examples.


Ohm Definition

Ohm’s law states that the voltage across any conductor is directly proportional to the current flowing through it. Assuming all the physical conditions and temperature remain constant.

Ohm’s law is valid only if the temperature provided and other physical factors remain constant. Ohm’s SI unit is rho (Ω), In certain components, the current raises the temperature. Eg: The filament of a light bulb where the temperature rises as the currents are increased. In this case, Ohm’s law will fail. The lightbulb filament is violating Ohm’s Law.


Calculating Electrical Power Using Ohm’s Law

The rate at which one form of energy is converted from the electrical energy of the moving charges to some other form of energy Eg: mechanical energy, heat, magnetic fields, or energy which is stored in electric fields, is known as electric power. The electrical power can be calculated by using Ohm’s law and by substituting the values of voltage, current, and resistance.

When the values of current and voltage are given, the formula for finding power will be: P = V I

When the values of power and voltage are given, the formula for finding current will be: I = P / V

When the values of power and current are given, the formula for finding voltage will be: V = P / I


Experimental Verification of Ohm’s Law

Aim: To verify the Ohm's law.

Apparatus Required: 

  • Resistor

  • Ammeter

  • Voltmeter

  • Battery

  • Plug Key

  • Rheostat

Theory: Ohm’s law states that the voltage across any conductor is directly proportional to the current flowing through it, Assuming all the physical conditions and temperature remain constant.

Procedure:

1. The key K is closed in the first step and the rheostat Rh is adjusted to get the minimum reading in the ammeter A and the voltmeter V.

2. The sliding terminal of the rheostat is then moved slowly to increase the current gradually and each time the value of current I flowing in the circuit and the value of potential difference V across the resistance wire is recorded. So, different sets of values of V and I are recorded.

Then for each set of values of Voltage and Current, the ratio V / I is calculated.

The ratio of V / I gives a constant value called R which is called the resistance of the conductor.

Plot a graph between current and the potential difference, it will be a straight line. This brings us to the conclusion that the current is proportional to the potential difference.


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The Main Applications of Ohm’s Law are:

The main applications of Ohm's law are:

  • It is used to determine the voltage, resistance, and current of an electric circuit.

  • Ohm’s law is majorly used in order to maintain the desired voltage drop across any electronic components.

  • Ohm’s law is to find its application in dc ammeter and other dc shunts to divert the current.

Following are the Limitations of Ohm’s Law:

Ohms law has a certain limitation:

  • Ohm’s law is not applicable for some electrical elements like diodes and transistors as they allow the current to flow through them in a single direction only.

  • For those electrical elements which don’t follow linear relation with parameters eg: capacitance, resistance, etc the voltage and current won’t be constant with respect to time making it difficult to use Ohm’s law.


Solved Example

Example 1: If the resistance of an electric iron is 60 Ω and a current of 3.2 A flows through the resistance. Find the voltage between two points.

Answer: It is asked to calculate the value of voltage provided current and resistance have given to us: T, we use the following formula to calculate the value of V:

V = I × R

Substituting the values in the equation, we get

V = 3.2 A × 60 ÷ = 192 V

V= 192 V


Example 2: Calculate the resistance of an electrical circuit having a voltage supply of 100 Volts and a current of 10 mA.

Answer: V = 100 V, I = 10 mA = 0.010 A

R = V / I

= 100 V / 0.010 A

= 10000 Ω = 10 kΩ

FAQ (Frequently Asked Questions)

1. Define 1 Ohm.

Answer: 1 ohm is defined as the resistance of a conductor such that when a potential difference of 1 volt is applied to its ends, a current of 1 ampere starts to flow through it.

2. What is Resistance?

Answer: Whenever electric current is flowing through a bulb or any conductor, the conductor offers some obstruction to the current and this obstruction is known as electrical resistance and is denoted by R.

3. On What Factor Does the Resistance of a Wire Depend?

Answer: The resistance of any wire is dependent on the following factors:

  • The cross-sectional area of the conductor

  • Length of the conductor

  • The material of the conductor

  • The temperature of the conducting material