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Jai Ganesh

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Registered: 2005-06-28

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Voltage

Voltage

Gist

Voltage is the pressure from an electrical circuit's power source that pushes charged electrons (current) through a conducting loop, enabling them to do work such as illuminating a light. In brief, voltage = pressure, and it is measured in volts (V).

Summary

Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to move a test charge between the two points. In the International System of Units (SI), the derived unit for voltage is volt (V).

The voltage between points can be caused by the build-up of electric charge (e.g., a capacitor), and from an electromotive force (e.g., electromagnetic induction in generator, inductors, and transformers). On a macroscopic scale, a potential difference can be caused by electrochemical processes (e.g., cells and batteries), the pressure-induced piezoelectric effect, and the thermoelectric effect. Since it is the difference in electric potential, it is a physical scalar quantity.

A voltmeter can be used to measure the voltage between two points in a system. Often a common reference potential such as the ground of the system is used as one of the points. A voltage can represent either a source of energy or the loss, dissipation, or storage of energy.

Definition

The SI unit of work per unit charge is the joule per coulomb, where 1 volt = 1 joule (of work) per 1 coulomb (of charge). The old SI definition for volt used power and current; starting in 1990, the quantum Hall and Josephson effect were used, and in 2019 physical constants were given defined values for the definition of all SI units.

Voltage difference is denoted symbolically by

, simplified V, especially in English-speaking countries. Internationally, the symbol U is standardized. It is used, for instance, in the context of Ohm's or Kirchhoff's circuit laws.

The electrochemical potential is the voltage that can be directly measured with a voltmeter. The Galvani potential that exists in structures with junctions of dissimilar materials is also work per charge but cannot be measured with a voltmeter in the external circuit.

Voltage is defined so that negatively charged objects are pulled towards higher voltages, while positively charged objects are pulled towards lower voltages. Therefore, the conventional current in a wire or resistor always flows from higher voltage to lower voltage.

Historically, voltage has been referred to using terms like "tension" and "pressure". Even today, the term "tension" is still used, for example within the phrase "high tension" (HT) which is commonly used in thermionic valve (vacuum tube) based electronics.

Details

Voltage, also called electromotive force, is a quantitative expression of the potential difference in charge between two points in an electrical field.

The greater the voltage, the greater the flow of electrical current (that is, the quantity of charge carriers that pass a fixed point per unit of time) through a conducting or semiconducting medium for a given resistance to the flow. Voltage is symbolized by an uppercase italic letter V or E. The standard unit is the volt, symbolized by a non-italic uppercase letter V. One volt will drive one coulomb

charge carriers, such as electrons, through a resistance of one ohm in one second.

Voltage can be direct or alternating. A direct voltage maintains the same polarity at all times. In an alternating voltage, the polarity reverses direction periodically. The number of complete cycles per second is the frequency, which is measured in hertz (one cycle per second), kilohertz, megahertz, gigahertz, or terahertz. An example of direct voltage is the potential difference between the terminals of an electrochemical cell. Alternating voltage exists between the terminals of a common utility outlet.

A voltage produces an electrostatic field, even if no charge carriers move (that is, no current flows). As the voltage increases between two points separated by a specific distance, the electrostatic field becomes more intense. As the separation increases between two points having a given voltage with respect to each other, the electrostatic flux density diminishes in the region between them.

Voltage describes the “pressure” that pushes electricity. The amount of voltage is indicated by a unit known as the volt (V), and higher voltages cause more electricity to flow to an electronic device. However, electronic devices are designed to operate at specific voltages; excessive voltage can damage their circuitry.

By contrast, too low a voltage can cause issues, too, by preventing circuits from operating and making the devices built around them useless. An understanding of voltage and of how to rectify associated issues is necessary in order to handle electronic devices appropriately and identify the underlying issues when problems occur.

The difference between voltage and current

As introduced above, a simple description of voltage would be “the ability to cause electricity to flow.” If you’re like most people, you have trouble envisioning what voltage is since you can’t view it directly with your eyes. To understand voltage, you must first understand electricity.

Electricity flows as a current. You can imagine it as a flow of water, like in a river. The water in rivers flows from mountains upstream to the ocean downstream. In other words, water flows from places with a high water height to places with a low water height. Electricity acts similarly: the concept of water height is analogous to electric potential, and electricity flows from places with high electric potential to places with low electric potential.

Electricity resembles the flow of water
.
The potential difference between two places can be expressed as a voltage. Voltage is the “pressure,” as it were, that makes electricity flow. In physics, voltage can be calculated using Ohm’s Law, which tells us that voltage equals resistance times current.

Resistance indicates the difficulty with which electricity flows. Imagine a water main. As the pipe grows smaller, resistance increases, and it becomes more difficult for the water to flow; at the same time, the strength of the flow increases. By contrast, as the pipe grows larger, water flows more readily, but the strength of the flow decreases. A similar situation applies to current. Resistance and current are proportional to voltage, meaning that as either increases, so too will voltage.

Method for measuring voltage

Multimeters (multi-testers) are used to measure voltage. In addition to voltage, multimeters can perform continuity checks and measure parameters such as current, resistance, temperature, and capacitance. Multimeters come in both analog and digital variants, but digital models are the easiest to use without mistakenly reading values since they display values directly.

To measure voltage with a multimeter, you connect positive and negative test leads and select a voltage measurement range. You then place the leads in contact with both ends of the circuit you wish to measure. When using an analog tester, you start with the largest voltage measurement range.

If the instrument does not respond, you then try progressively lower measurement ranges until you reach a range that can measure the circuit’s voltage. When using a digital tester, many models simplify the measurement process by adjusting the measurement range automatically.

Additional Information

Volt is the unit of electrical potential, potential difference and electromotive force in the metre–kilogram–second system (SI); it is equal to the difference in potential between two points in a conductor carrying one ampere current when the power dissipated between the points is one watt. An equivalent is the potential difference across a resistance of one ohm when one ampere is flowing through it. The volt is named in honour of the 18th–19th-century Italian physicist Alessandro Volta. These units are defined in accordance with Ohm’s law, that resistance equals the ratio of potential to current, and the respective units of ohm, volt, and ampere are used universally for expressing electrical quantities.

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