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Unit of Current

Unit of Current

Ampere is the basic unit of current. It is the standard unit (SI unit) used across the globe. It is named after the physicist Andre Marie Ampere, who is also called the father of modern electromagnetism. One Ampere is defined as the current produced when one Coulomb of charge passes through a conductor in one second.

The current flowing through a conductor can be measured using Ohm’s law. Ohm’s law states that the value of current passing through a conductor is equal to the ratio of the voltage produced across the conductor and the resistance of the conductor.

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Ohm’s Law

Mathematically, Ohm’s law can be written as:


Here, I is the current flowing through the conductor V is the voltage induced across the conductor R is the resistance of the conductor

As discussed above, one Ampere is then defined as the current produced when a charge of one Coulomb passes through the conductor in one second.


Commonly Used Terms Related to Current

  • Voltage: Voltage is defined as the difference in electric potential between two different points in space. This means that if some difference in potential exists between points A and B, then a charge moving from point A to B will do some work. In simple words, energy is required to move the charge from A to B. This potential difference is responsible for the flow of all the electricity we see in nature.
  • Resistance: Resistance is defined as the opposition to the flow of current that a conductor provides. Materials with high resistance provide more opposition to the flow of current. This means that to make a small amount of current pass through such objects, a high amount of energy must be expended. Similarly, if a conductor has low resistance, then the current flows easily through it, and the amount of energy needed is low.
  • Charge: Charge is defined as the property of a substance that causes it to experience force acting on it when placed in an electric field. Charge is one of the basic quantities used in physics, and it is used to measure many things like current, capacitance, voltage, etc.
  • Power: Power is defined as the amount of energy an electrical circuit produces in a certain amount of time. The SI unit of power is defined as one joule of energy produced in a second. The concept of electrical power is very important in engineering, as it is used directly in making electrical machines like AC generators and electric motors etc. The formula for calculating power can be written as: 
    Here, P is power generated by the circuit I is the current flowing through the circuit R is the resistance of the circuit
  • Capacitance: Capacitance is defined as the ratio between the amount of charge stored on a conductor to the difference in electric potential. The formula for capacitance can be written as:
     Here, C is capacitance q is the charge stored on the conductor V is the voltage difference Capacitance is further defined in two ways: mutual capacitance and self-capacitance. Mutual capacitance is calculated between two different conductors. Self-capacitance is calculated on a single conductor.

Recent Developments

The value of the elementary charge e (the charge on an electron) is fixed to be 1.602 × 10-19. This caused the value of Coulomb to become fixed. Now, as the Coulomb is defined in terms of Ampere, the new definition of Ampere is formalized in terms of these standards.

The Ammeter

Hans Christian Orsted, around the 1820s, invented the earliest device that could be called an ammeter. The ammeter is a device used to measure the current flowing through a circuit. An ammeter is usually connected in series to the circuit. Ammeters are required to be of low resistance to prevent the ammeter from causing a significant drop in voltage across the circuit. When they were first invented, the ammeters used the earth’s magnetic field to operate. The accuracy of measurement with such devices was low, and they were rejected in favor of high accuracy devices invented in the late nineteenth century.

Ammeters are of several different types. A moving coil ammeter uses a magnet that shows deflection when the current is passed. The measure of deflection is used to determine the value of the current flowing through the circuit. It is the most common type of ammeter used.

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