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# UNIT OF VOLTAGE

## VOLTAGE

Though there are numerous ways in which voltage can be defined, the most popular description is that voltage is the total work required to move a unit of charge between two points in a static electric field. It can also be defined as the electric potential difference between two points in the same electric field. Voltage is otherwise also known as electric potential difference, electric pressure, electromotive force (emf) or electric tension. The voltage present between two points in the same electric field can be created physically by means of a buildup of electrical charges or imbalances using static capacitors or electronic capacitors, by electric current using a magnetic field, and by time-varying magnetic fields using a dynamo or a generator.

Macroscopically, the voltage can be created using electrochemical processes as seen in batteries, pressure-induced piezoelectric effect and heat-induced emf across metal-metal junctions. However, the working principles of all these methods are essentially the same. A voltage can refer to either the energy source (emf) or the loss in energy (potential drop). Voltage is denoted using the symbol V, and the unit of voltage is volt. The device used to measure the voltage values are called voltmeters.

## Ohm’s Law

Ohm’s law was named after Georg Ohm, a German physicist. Ohm’s law states that the current passing between two points on a conductor is directly proportional to the voltage between those two points. The constant of proportionality that equates voltage with current is the resistivity, which, as the name suggests, is the resistance offered to current flow by the conductor. The mathematical equation of Ohm’s law is

I=V/R

where
- I is the current flow (SI unit - amperes A)
-V is the voltage between the two points (SI unit – volt V)
- R is the resistance of the conductor (SI unit – ohm Ω)

Ohm’s law also states that the resistance offered by the conductor is independent of the current that flows through it. The vector form of the law, which is also used in physics, is

J=σE

where

• J is the current density measured at a point on the conducting material
• E is the electric field at that location
• σ is the conductivity of the conductor. Conductivity measures how well a material conducts electricity, and so is the opposite of resistivity.

## Kirchhoff’s Circuit Laws

Introduced in 1845 by German physicist Gustav Kirchhoff, Kirchhoff’s circuit laws gave rise to two principles, one each for current and voltage in an electrical circuit. To form the basis of network analysis, these laws can also be applied in the time and frequency domains. Kirchhoff’s laws, or Kirchhoff’s rules as they are also known as hold true for all DC circuits.

Kirchhoff’s current law or KCL, states that in an electric circuit, the sum of all the incoming currents at a node is equal in magnitude to the sum of all the currents that flow out of that node, or in other words, the net resultant of all the currents meeting at a node is zero. KCL is also called Kirchhoff's first law/ point rule/ junction rule/ nodal rule.

Kirchhoff’s voltage law or KVL states that the net resultant of all the voltages (with the direction of flow of the voltages taken into consideration) in the loop of a closed circuit is equal to zero. KVL is also called Kirchhoff’s second law/ loop rule/ mesh rule/ voltage rule.

## Unit of Voltage

The standard unit of voltage is volt (V), named after the Italian physicist Alessandro Volta, who is credited with the invention of the first battery. Volt is a derived SI unit. One volt is defined as the voltage taken to move power of one watt.

V = A. Ω

= W/A

= J/C

= eV/e

It can also be described in terms of the potential difference that exists between two points when the energy of one joule (J) per coulomb (C) of charge flows through the circuit.

V = J/V

= kg.m2/A. s3

That is, 1 V = 1 kg m2 s-3 A-1 (one-kilogram meter squared per second cubed per ampere).

## Water Flow Analogy

Let us better understand the concept of voltage with the help of an analogy. Consider a closed system of pipes, where water is pumped through them using an external water pump. The pressure differences between two points in the pipe is similar to the potential difference found in an electrical circuit. Because of this pressure difference, the water moves from the point of high pressure to the point of low pressure, as is the case with an electric current.

The topic of Voltage is widely discussed in NCERT Class 10 Science Chapter 12, which one of the most crucial chapters for Class 10 students,       Talk to our expert
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