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Kasturi Talukdar

Updated on 17th July, 2023 , 11 min read

Ohm's Law: Statement, Formula, Equation, Triangle, Solved Example

Ohm's law Introduction

Ohm's law is a basic principle in electrical engineering which describes the relationship between current, voltage, and resistance in an electrical circuit. It states that the amount of current in a circuit is directly proportional to the voltage applied and inversely proportional to the resistance in the circuit. Georg Simon Ohm discovered Ohm's law through an experiment in 1826. He noticed that if the temperature is constant, the current flowing through a circuit is exactly proportional to the voltage and inversely proportional to the resistance. The relationship between electric current and potential difference is stated by Ohm's law. Most conductors' current is directly inversely proportional to the voltage applied to them. The German physicist Georg Simon Ohm was the first to experimentally confirm Ohm's law.

Also Read About: Gauss Law 

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What is Ohm's Law?  

Ohm's Law asserts that, under normal circumstances and with a constant temperature, the potential difference at the ends of a metallic wire is precisely proportional to the current that flows through it.

  1. The connection between the current (I) and the potential difference (V) between a conductor's terminals is given by Ohm's Law.
  2. To determine the value of resistors needed in electrical circuits, Ohm's Law is employed in all disciplines of electrical engineering.
  3. In circuits where the voltage can be easily measured across a given resistor, it may also be used to calculate the current flowing through the system.
  4. Ohm's law does not apply everywhere. Ohmic conductors are covered, while non-ohmic conductors, such as semiconductors, are not.
  5. Only when the temperature and other physical variables are constant does Ohm's law apply.
  6. The temperature can sometimes rise as the current increases.
  7. For instance, as the current is raised, the temperature of a light bulb's filament increases. Ohm's law cannot be used in this example due to the circumstances. Thus, the filament of the lamp breaks Ohm's Law.

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What is Ohm's law Explanation

Ohm's law is one of the most fundamental and significant laws governing electrical circuits.

Under the assumption that all physical parameters and temperatures remain constant, Ohm's law states that the voltage across a conductor is directly proportional to the current flowing through it.

To find the Voltage, ( V )

[ V = I x R ]      V (volts) = I (amps) x R (Ω)

To find the Current, ( I )

[ I = V ÷ R ]      I (amps) = V (volts) ÷ R (Ω)

To find the Resistance, ( R )

[ R = V ÷ I ]      R (Ω) = V (volts) ÷ I (amps)

Resistance, abbreviated R in the equation, is the proportionality constant that has ohms as its unit of measurement, with the symbol Ω. 

To students ofelectronics, Ohm's Law (E = IR) is as fundamentally important as Einstein's Relativity Equation (E = mc²) is to physicists.

E = I x R

When spelled out, it means voltage = current x resistance, or volts = amps x ohms, or V = A x Ω.

Only when the given temperature and the other physical variables remain constant does Ohm's law apply. Increasing the current causes the temperature to rise in some components. The filament of a light bulb serves as an illustration of this, where the temperature increases as the current increases. Ohm's law cannot be applied in this situation. The lightbulb filament violates Ohm's Law. 

Quantity

Ohm's Law

symbol

Unit of measure

(abbreviation)

Role in circuits

In case you're wondering:

Voltage

E

Volt (V)

Pressure that triggers electron flow

E = electromotive force (old-school term)

Current

I

Ampere, amp (A)

Rate of electron flow

I = intensity

Resistance

R

Ohm (Ω)

Flow inhibitor

Ω = Greek letter omega

Note: Resistance cannot be measured in an operating circuit, so Ohm's Law is especially useful when it needs to be calculated. A technician can calculate R using the aforementioned application of Ohm's Law without having to shut off the circuit to measure resistance.

In order to calculate the remaining two symbols (see the middle pyramid above), X out the symbol for current (I) if you already know the voltage (E) and resistance (R) and want to know the current (I).

Additionally, you can calculate voltage (E) by multiplying the pyramid's bottom halves (see the third or far-right pyramid above) if you know the values of current (I) and resistance (R).

Try out a few simple series circuit calculations using just one voltage source (the battery) and one resistance source (the light). In each example, two values are known. The third can be calculated using Ohm's Law. 

Also read about- Difference between Interference and Diffraction

Similar Read: Brewster Law

What is Ohm's Law Applications

Ohm's law is primarily utilized in:

  1. the measurement of an electric circuit's voltage, resistance, or current.
  2. The desired voltage drop across the electronic components is maintained by Ohm's law.
  3. In DC ammeters and other DC shunts, the current is also redirected using Ohm's law.

Also Read About- Moseley Law & SI Unit of Resistance

Limitations of Ohm's Law

The limitations of Ohm's law are as follows:

  1. Electrical components that only allow one direction of current flow, such as diodes and transistors, are not subject to Ohm's law.
  2. The ratio of voltage to current won't be constant with respect to time for non-linear electrical elements with parameters like capacitance, resistance, etc., making it challenging to apply Ohm's law.

Read More about- Limitations of Ohm's Law, Drift Velocity Formula and Electrical Insulator.

Water Pipe Analogy for Ohm's Law

Ohm's Law describes the current flow through a resistance when various electric potentials (voltages) are applied to either end of the resistance. We cannot see electrons, so the water-pipe analogy helps us understand how electrical circuits work. Water flowing through pipes is a good mechanical system that can be compared to an electrical circuit.

The force acting on the pipe is represented by the voltage, the amount of water flowing through it is represented by the current, and the pipe's dimensions are represented by the resistance. Greater water flow through the pipe (current) will result from higher pressure (voltage) and a larger pipe (less resistance).

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Also read about- Law of Variable Proportion

What is the voltage in the circuit?

E = I x R = (5A)(8Ω) = 40 V

Ohm's main discovery when he published his formula in 1827 was that the voltage applied to a conductor directly relates to the amount of electric current flowing through it. In other words, it takes one volt of pressure to push through a resistance of one ohm.

Experimental Verification of Ohm's Law

The following experiment makes it simple to confirm Ohm's Law:

Apparatus Required:

  1. Resistor
  2. Ammeter
  3. Voltmeter
  4. Battery
  5. Plug Key
  6. Rheostat

Procedure:

  1. The rheostat is initially set to get the lowest reading in the ammeter A and voltmeter V while the key K is closed.
  2. By adjusting the rheostat's sliding terminal, you can gradually increase the current flowing through the circuit. The process involves recording the current flowing through the circuit and the corresponding potential difference across the resistance wire R.
  3. Different sets of voltage and current values are obtained in this manner.
  4. The ratio of V/I is computed for each set of values for V and I.
  5. V/I = R, which is a constant, because you will notice that it is almost the same when you calculate the ratio V/I for each case.
  6. A straight line will appear on a graph of the current against the potential difference. This demonstrates how the current and potential difference are related.

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Also read about- SI Unit of Electric Flux

What to validate using Ohm's Law

The static values of circuit components, current rates, voltage sources, and voltage drops can all be verified using Ohm's Law. A test instrument may indicate that resistance has decreased or voltage has increased, resulting in a high-voltage situation, if it detects, for instance, a higher than usual current measurement. This could indicate a supply or circuit issue.

A lower than usual current reading in a direct current (dc) circuit could indicate that the voltage has dropped or the circuit resistance has risen. Poor or loose connections, corrosion, and/or damaged components are potential causes of increased resistance.

Electrical current is drawn by the loads in a circuit. Any type of component, including small electrical devices, computers, home appliances, or a sizable motor, can be a load. The majority of these parts (loads) are marked with nameplates or informational stickers. These nameplates offer multiple references numbers and safety certification.

When learning about standard voltage and current values, technicians consult the nameplates on components. Technicians can use Ohm's Law to identify the component of a circuit that is failing and from there determine the potential location of a problem if they discover during testing that standard values do not register on their digital multimeters or clamp meters.

Also read about- Difference between Real Image and Virtual Image

Ohm's Law Magic Triangle

You can use the Ohm's law magic triangle to help you remember the various equations for solving for the different variables (V, I, and R).

If the voltage value is requested and the current and resistance values are provided, then the voltage can be calculated by simply covering V at the top. Thus, all that is left is the I and R or I  R. So, the equation for voltage is current multiplied by resistance.

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What is a power triangle?

When we know the values of the other two parameters, we can use the power triangle to calculate the values of electric power, voltage, and current. Power (P) is at the top of the power triangle, while current (I) and voltage (V) are at the bottom.

The formula for calculating power when current and voltage values are given is,

P = V x I.

The formula for calculating current when power and voltage values are given is,

I = P/V.

The formula for calculating voltage when power and current values are given is,

V = P/I.

Also read about- Father of Physics

 Ohm's Law Matrix Table

The Ohm's law matrix table is a compact representation of the relationship between current (I), voltage (V), and resistance (R) in an electrical circuit. The table provides a quick reference for determining any one of the three variables if the other two are known.

The table below is a detailed explanation of each row and column:

Known ValuesResistance (R)Current (I)Voltage (V)Power (P)
Current & Resistance--R = I X RP = I2 X R
Voltage & CurrentR = V/I--P = V X I
Power & CurrentR = P/I2-V = P/I-
Voltage & Resistance-I = V/R-P = V2/R
Power & Resistance-I = √P/RV = √Z X R-
Voltage & PowerR = V2/PI = P/V--

What is Ohm's Law Pie Chart

The Ohm's Law Pie Chart or Ohm's Law Wheel, which looks like the following, can be used to understand the relationship between the several parameters of Ohm's Law.

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Calculating Electrical Power Using Ohm's Law

Electric power is the rate of energy conversion from the electrical energy of moving charges to other forms of energy such as mechanical energy, heat energy, energy stored in magnetic fields, or energy stored in electric fields. The watt is the measurement of power. Using Ohm's law and the values of voltage, current, and resistance as substitutes, the electrical power can be calculated.

Formula to find power

When the values for voltage and current are given,

P =  V x I

When the values for voltage and resistance are given,

P =  V² ➗R

When the values for current and resistance are given,

P =  I² R

The basic science of circuits

Atoms make up all matter, including circuits. Atoms consist of subatomic particles:

  1. Protons (which have an electrical charge of positive)
  2. electron-free neutrons
  3. (Negatively charged) electrons

The forces of attraction between an atom's nucleus and the electrons in its outer shell keep atoms joined together. Atoms in a circuit start to reform when voltage is applied, and the components of the circuit exert an attraction force known as a potential difference. Loose electrons that are attracted to one another move toward protons to form an electron current. Any material in the circuit that restricts this flow is considered resistance.

Also read about- Surface Tension Formula

Uses of Ohm's Law

Ohm's law has a wide range of applications in electrical and electronics engineering. Some of the main uses of Ohm's law are:

  1. Circuit analysis: Ohm's law is used to calculate the current, voltage, and resistance in an electrical circuit, which allows engineers to analyze the performance of the circuit and make necessary modifications to optimize it.
  2. Design of electrical systems: Ohm's law can be used to design electrical systems by determining the required resistance, voltage, and current to achieve a desired power output.
  3. Power calculation: Ohm's law can be used to calculate the power (P) in an electrical circuit, which is given by the equation P = IV. This allows engineers to determine the power requirements of a circuit and design it accordingly.
  4. Electrical safety: Ohm's law can be used to calculate the maximum safe current that can be passed through a conductor, which is important for ensuring electrical safety in circuits.
  5. Fault analysis: Ohm's law can be used to diagnose faults in electrical circuits by measuring the voltage, current, and resistance in the circuit and comparing the results to the expected values.

Sample Questions

Ohm's Law Sample Ques 1: What is the current if Volt = 6 V and Resistance is 2 Ω?

Ans: According to Ohm's law, 

V = R*I

So, 6 = 2(I)

or I = 6/2 = 3 A

Ohm's Law Sample Ques 2: Is Ohm's law universally applicable?

Ans: Ohm's law does not apply to non-ohmic conductors, only to ohmic conductors like iron and copper, hence it cannot be said to be universally applicable.

Ohm's Law Sample Ques 3: What is the Resistance if Current = 8 A and Voltage is 10 V?

Ans: According to Ohm's law, 

V = R*I

So, 10 = R x 8

or, R = 10/8

R = 1.25 Ω

Ohm's Law Sample Ques 4:  What will the resistance be to limit the current from a 12 V battery to 0.04 A? 

Ans: Given a 12 V battery, the task is to find the resistance such that the current will be limited to 0.04 A.

  • Voltage V=12 V
  • Current I=0.04 A

From Ohm's law,

V=IR

We solve for the resistance and substitute the given values,

R=V/Im

= 12 V/0.04 A

R = 300 Ω

Ohm's Law Sample Ques 5: What is the unit for reciprocal of resistivity of a material?

Ans: Conductivity = 1/ resistivity

SI unit of conductivity = 1/ohmmeter

= ohm-1 m-1

Ohm's Law Sample Ques 6: Describe a few applications of Ohm's Law.

Ans: Ohm's Law has several applications, including the following:

  1. The values of current, voltage, or resistance may be calculated using the formula based on Ohm's law.
  2. The Ohm's law formula may also be used to determine power usage.
  3. Heaters, irons, and ovens are just a few electrical appliances that employ the Ohm's Law formula to determine voltage, electric current, and resistance.
  4. Circuit components, current densities, voltage sources, and voltage dips can all be statically approved.

Ohm's law Sample Ques 7: When does the Ohm's Law fail?

Ans: Ohm's law does not account for the behaviour of unilateral devices like diodes and semiconductors. If the physical conditions, such as pressure and temperature, are not maintained constant, Ohm's law may also fail.

Ohm's law Sample Ques 8: When and Who discovered Ohm's Law?

Ans:  Ohm's Law was discovered by Georg Simon Ohm in 1826.

Ohm's law Sample Ques 9: Write the value of 1 Ohm?

Ans: 1 Ohm is equal to one kilogramme metre squared per second cubed per ampere squared when converted to base units.

Ohm's law Sample Ques 10: What are Ohm's Law's 3 Forms?

Ans: Ohm's law formula can be used in 3 forms – 

  • ΔV = IR
  • R = V/I
  • I = V/R

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Frequently Asked Questions

What does Ohm’s law state?

According to Ohm’s law, the voltage across two points is directly proportional to the current flowing through a conductor between them.

What can Ohm’s law be used for?

The static values of circuit components, such as current densities, voltage sources, and voltage drops, are verified using Ohm’s law.

What is Ohm’s law as a formula?

The formula for Ohm’s law is V=IR.

What is the unit of ohms?

The SI unit of electric resistance is the ohm (Ω).

What does ohm’s stand for?

The electrical resistance unit known by the abbreviation "ohm" is named after German physicist Georg Simon Ohm, who lived in the 19th century. It is measured in meters per kilogram per second.

What is the practical application of Ohm’s Law?

Ohm’s Law is used in a wide range of electrical applications, from designing electronic circuits to calculating power consumption in household appliances. It is an essential tool for engineers and technicians working with electrical systems.

What is the practical application of Ohm’s Law?

Ohm’s Law is used in a wide range of electrical applications, from designing electronic circuits to calculating power consumption in household appliances. It is an essential tool for engineers and technicians working with electrical systems.

How does Ohm’s Law relate to circuit analysis?

Ohm’s Law is a fundamental tool in circuit analysis, allowing engineers to determine the current, voltage, or resistance of a circuit component by measuring the other two variables. It is often used to determine the power dissipated by a resistor or to calculate the voltage drop across a particular component.

What is the difference between series and parallel circuits in Ohm’s Law?

In a series circuit, the current passing through each component is the same, and the voltage across each component is proportional to its resistance. In a parallel circuit, the voltage across each component is the same, and the current passing through each component is proportional to its conductance.

Can Ohm’s Law be applied to AC circuits?

Ohm’s Law can be applied to AC circuits, but the analysis becomes more complicated due to the phase relationship between voltage and current. In AC circuits, the impedance, rather than the resistance, is used to describe the relationship between voltage and current. The impedance is a complex quantity that includes both resistance and reactance.

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