Ohm’s Law and Resistance

The relationship between an electric current and a potential difference is described by Ohm's law. The current flowing through most conductors is directly proportional to the voltage applied to them. Georg Simon Ohm, a German physicist, was the first to experimentally prove Ohm's law.

Ohm's law is one of the most fundamental and important laws of electric circuits. If all physical parameters and temperature stay constant, Ohm's law asserts that the voltage across a conductor is directly proportional to the current flowing through it.

In this article, we will discuss Ohm’s Law formula and also the equation that is used to state Ohm’s Law. We have also provided a few Ohm’s law examples to help you understand better.

Georg Ohm

Georg Simon Ohm was a German scientist and mathematician who lived from 16 March 1789 to 6 July 1854. Ohm began his investigation with the new electrochemical cell, designed by Italian scientist Alessandro Volta, as a schoolteacher. Using his own devices, Ohm discovered that the potential difference (voltage) applied across a conductor and the resulting electric current are directly related. Ohm's law is the name given to this relationship, and the ohm is the standard unit of electrical resistance.

Ohm's law was initially published in 1827 in Die galvanische Kette, mathematisch bearbeitet (The Galvanic Circuit Investigated Mathematically), in which he presented his whole theory of electricity. He asserted in this work that the electromotive force acting between the extremities of any part of a circuit is equal to the product of the current strength and the resistance of that part of the circuit.

Ohm’s Law Equation

Ohm’s law is considered to be one of the most fundamental electrical laws. It helps in the calculation of the efficiency, power, current, voltage, and resistance of an element in an electrical circuit.

The mathematical equation for Ohm’s law is given below:

Voltage (V) = Current (I) × Resistance (R)

Where, V= voltage, I= current and R= resistance

The SI unit of resistance is ohms, which is represented by the symbol Ω

A potential difference of 1V is produced when a current of 1A is passed through a conductor with a resistance of 1 Ohm. Ohm’s equation is named after the scientist Georg Ohm, who performed numerous experiments to find out the relationship between the applied voltage and the current passing through a conductor.

Ohm’s Law Circuit Diagram

Given below is a circuit diagram for demonstrating Ohm's law. A parallel connection is made between a voltmeter and a resistor. An ammeter is connected in series to measure the current in the circuit. A variable resistor is attached to the circuit. Changing the resistance changes the potential drop across the resistor, affecting the amount of current flowing through it. We can deduce from the reading that I and V vary linearly.

What is the Ohm’s Law Magic Triangle?

You can recall the different Ohm's law equations needed to solve for different variables (V, I, R) by using the Ohm's law magic triangle.

Suppose in a given question, the value of voltage is asked and the provided values are of current and resistance, then the value of voltage can be calculated by simply covering V at the top. As a result, we will be left with the values of I and R, i.e., I X R. Hence, the equation to calculate the voltage is Voltage = Current (I) X Resistance (R).

The table below shows the S.I.Unit and roles of Voltage, Current and Resistance in a circuit.

How to Calculate Electrical Power Using Ohm’s Law?

Electric power is defined as the rate at which energy is transformed from the electrical energy of moving charges to another form of energy such as mechanical energy, heat, magnetic fields, or energy stored in electric fields. A watt is a unit of power. Ohm's law can be used to compute electrical power by using the values of voltage, current, and resistance.

Ohm’s Law Formula to Find Power

The value of electrical power can be estimated by substituting the values of voltage, resistance, and current into Ohm's law formula.

• If the values for voltage and current are provided, the power can be calculated by using the formula, P = VI

• If the values for voltage and resistance are provided, the formula to calculate the power will be P = V2 ÷ R

• If both the current as well as resistance values are provided, the formula to calculate the power will be P = I² × R

The table below shows different formulas to calculate the electrical power based on the provided values:

Solved Ohm’s Law Examples

1. Determine the resistance of an electrical circuit with an 8 volt supply and a 4 mA current.

Sol: Given,

Voltage (V) = 8 V

$Current (I) = 4 ~mA = 4\times10^{-3}A$

Now, we are asked to determine the resistance of a given electric circuit. We know that, from ohm’s law resistance of any electrical circuit is given by:

$\Rightarrow R=\dfrac{V}{I}$

Where,

V- The voltage supplied to the electrical circuit

I- The total current flowing through the circuit

Substituting the given values of voltage and current in the above equation, we get:

$\Rightarrow R=\dfrac{V}{I}=\dfrac{8}{4\times 10^{-3}}=2\times10^3~\Omega=2~k\Omega$

Therefore, resistance of the given circuit is 2 KΩ.

2. A fully resistive electrical equipment is connected to an EMF source of 8.0 V. It is powered by a 2.0 A electric current. Assuming that the conducting wires have no resistance, determine how much resistance the electrical appliance provides.

Sol:

Given,

The voltage supplied to the given electrical equipment (V) = 8 V

The current supplied to the given electrical equipment (I) =2 A

Now, we are asked to determine how much resistance the electrical appliance provides. We know that, from ohm’s law, resistance of any electrical circuit is given by:

$\Rightarrow R=\dfrac{V}{I}$

Where,

V- The voltage supplied to the electrical circuit

I- The total current flowing through the circuit

Substituting the given values of voltage and current in the above equation, we get:

$\Rightarrow R=\dfrac{V}{I}=\dfrac{8}{2}=4~\Omega$

Therefore, the resistance provided by the electrical appliance is 4 Ω.

Applications of Ohm’s Law

Some of the important applications of Ohm’s law are given below:

• Ohm’s law is majorly applied to find out the resistance, current as well as voltage of an electric circuit.

• Another major application of Ohm’s law is maintaining the required voltage drop across any electronic components.

• Most of the DC ammeters as well as DC shunts uses Ohm’s law to divert the current.

• The general operation of electrical components is controlled by Ohm's law, which provides variable output voltage based on resistance.

• Ohm's law also regulates the operation of heaters, kettles, and other appliances. DC current is used in mobile phone and laptop chargers. The electrical voltage in a typical household is 120 volts.

Limitations of Ohm’s Law

Apart from being applied on a daily basis, there are also certain limitations of Ohm’s law:

• Since unilateral electrical elements like diodes and transistors only allow current to flow in one way, Ohm's law does not apply to them.

• Voltage and current will not be constant with respect to time for nonlinear electrical elements with factors like capacitance, resistance, and so on, making Ohm's law difficult to apply. Non-linear elements are ones in which the current flowing through them is not directly proportional to the applied voltage.

• It indicates that the values of resistance of non-linear elements change as the values for voltage and current change. One of the best non-linear elements is the thyristor.

Did You Know?

Conclusion

Finally, we can conclude that Ohm’s law is a relation between current flowing in a conductor and applied voltage. All material does not follow Ohm’s law because of the nature of material. We can say that Ohm’s law is not a universal law which can be applied on any material. It is a relation which is applicable to the conductors.