Ohm’s Law and Resistance

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What is Ohm's Law?

According to Ohm’s law, the electricity that passes within a conductor has direct proportionality with respect to the potential difference through which it flows.

Moreover, since the electric resistance of a conductor is constant. The mathematical equation of Ohm's law is: 

R = V/I

Where,

I = current in amperes.

V = voltage in volts.

R = resistance in ohms.

To illustrate: When a current of 1A is passed through a conductor having a resistance of 1 Ohm, it produces a potential difference of 1V. This equation is named after the scientist Georg Ohm. He published his findings in 1827.

He performed a number of experiments to determine the relationship between the applied voltage and the current passing through a conductor.


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The law is empirical. Ohm's law is one of the fundamental laws of electrical engineering. 

SI unit of electrical resistance = Ohm.

Kirchhoff's circuit law (given by Gustav Kirchhoff) made a generalization of Ohm's law and is widely used.

\[\sigma = \frac{J}{E}\]

Where,

σ = Conductivity of the material (material specific),

J = Current density in a particular location of that material, and

E = Electric field in that location


Ohm’s Law Formula

Known Values

Resistance (R)

Current (I)

Voltage (V)

Power (P)

Current & resistance

---

---

V = I × R

P = I² × R

Voltage & current

R = V ÷ I

---

---

P = V × I

Power & current

R = P ÷ I²

---

V = P ÷ I

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Voltage & resistance

---

I = V ÷ R

---

P = V² ÷ R

Power & resistance

---

I = (P ÷ R)½

V = (P × R)½

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Voltage & power

R = (V²) ÷ P

I = P ÷ V

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Quantity

Ohm's Law Symbol

Unit of Measure (Abbreviation)

Role in Circuits

In Case You're Wandering

Voltage

V

Volt V

Pressure that triggers electron flow

E = electromotive force

Current

I

Ampere, amp A

Rate of electron flow

I = intensity

Resistance

R

Ohm

Flow inhibitor

Ω = Greek letter omega


Ohm’s Law Equation

The formula of Ohm's law can be used when any two out of three variables are known. There are different relations of resistance, current, and voltage. Ohm’s law calculator can be used to remember these relations. 

Ohm’s Law Triangle

If the values of current and resistance are known and it is required to calculate the value of voltage, then simply cover the V at the top to calculate the voltage.

Now, we are left with I and R which should be multiplied to get the value of voltage.

An example to use the magic triangle to determine voltage is mentioned below.

\[R = \frac{V}{I}\]


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or

V = IR

or

\[I = \frac{V}{R}\]

These were the Ohm’s law equations.


Ohm’s Law Chart

For a better understanding of the relationship between various parameters, we can simply summarize the equations used to calculate voltage, current, resistance, and power into an Ohm's Law pie chart as mentioned below.


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Ohm’s Law Graph

Ohm’s law depicts that at a constant temperature, there is a direct proportionality within current and potential difference across its ends.


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Based on the statement, the voltage should determine current. If we plot voltage in the x-axis and current in the Y-axis of a graph, we will get a straight line that represents resistance.


Application of Ohm's Law

Some of the major applications of Ohm's law are:

  1. To determine the voltage, current, and resistance of an electric circuit.

  2. Ohm's law is used to maintain the voltage drop across the components of electronics.

  3. Ohm's law is used to divert current in dc ammeter and dc shunts.

Some other applications of Ohm’s law are:

  1. Ohm's law controls the overall working of the electrical components by providing variable output voltage based on the resistance.

  2. The working principle of heaters, kettles, and other equipment are also based on Ohm's law. Mobile phone and laptop chargers use DC current.

  3. For normal household purposes, the electrical voltage is 120 volt.

Limitations of Ohm's Law

  1. Ohm's law doesn't apply for unilateral networks. In unilateral networks, the current flows only in one direction. The unilateral networks consist of elements like diodes and transistors.

  2. Ohm's law is not applicable to non-linear electric elements. The non-linear elements are those elements in which the current flowing through it is not exactly proportional to the applied voltage.

  3. It means that the resistance values of the non-linear elements change with the change in the value of voltage and current. A thyristor is one of the best non-linear elements.

Calculating Electrical Power

The rate at which electrical energy is converted into some other form of energy like mechanical energy, energy stored in electric and magnetic fields, heat energy is known as electric power.

The unit of power is the watt.

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

How to Find Power?

  1. In case the current and voltage values are given, the value of power is calculated by the formula

P = V * I

  1. In case the resistance and voltage values are given, power is calculated by the formula

P = V² ÷ R

  1. When the values for voltage and current are given, power is calculated by the formula

P = I² × R


Do You Know?

Resistance cannot be measured in a circuit under function. That's why Ohm's law is useful when resistance needs to be calculated.

Rather than shutting off the circuit, Ohm's law can be calculated by using the above formulas.

FAQ (Frequently Asked Questions)

Q1. How do we Establish a Current-Voltage Relationship?

Ans: To establish the relationship between current and voltage, the ratio of V/I should remain constant for a given resistance. 

The graph for the current, and potential difference are obtained, which is a straight line.

Q2.  The given parameters are the resistance of an electric iron as 40 Ω and 4.2 A. Current flows through the conductor. Find the voltage between two points.

Ans: To calculate voltage for a given value of current and resistance, we have to use the formula V = I × R

Given, Resistance (R) = 50 Ω

Current (I) = 3.2 A

Therefore,

Voltage (V) = I X R = 4.2 A x 40 Ω =168 V

Q3. Let's take a 10 Ω resistor in a circuit, subject to a current of 2 Amperes and a voltage of 120V. What is the voltage drop across the resistor?

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Ans: Using the Ohm's triangle shows us that:

V = IR = 2 * 10 = 20

The voltage at the end terminal is calculated by the difference, i.e. 120 - 20 = 100 V.

Q4. How to Evaluate Different Parameters Using Ohm’s Law?

Ans: The formula of Ohm's law is used to calculate the current, resistance, and potential difference. If any two parameters are known, the third one can be calculated by the following equations.

To find Voltage (V),

V = IR

To find Current (I),

I = V/R

To find Resistance (R),

R = V/I

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