Ampere

Electric current flows through the circuit. To measure and access the performance of the circuit a unit of measurement is required. Ampere is the required unit of measurement of electric current and it is denoted by ‘A’. To honour the French scientist Andre Marie Ampere who is the father of electromagnetism, the unit of current is ampere. In the circuit when we say that the electric current is flowing then it means that electrons (a negatively charged particle) are flowing.

Ampere is the unit to measure the flow rate of electrons flowing in a circuit. If we say that one ampere of current is flowing then it represents one coulomb of electric charge carriers are moving per second from a specific point. One coulomb of electric charge is the number of electric charge carriers which equivalent to \(6.24×10^{18}\) charge carriers.

Definition of Ampere

Ampere or Amp is the unit of measurement of the electric current flowing in a circuit. One ampere of current is the movement of \(6.24×10^{18}\) charge carriers moving per second from a specific point and this quantity of charge carriers is one coulomb. In the electrical and electronic world, ampere represents constant and instantaneous current while a coulomb represents the storage of charge over some time in a device like a battery. Mathematically, the relation between ampere and coulomb is

1 A = \(1 \frac{C}{s}\)

In other terms, if current ‘I’ is flowing for ‘t’ seconds through a battery then the charge Q accumulated in the battery is

Q = I t

According to the Ohm’s law, in a circuit the voltage (V) is directly proportional the current (I) passing through it and constant of proportionality is resistance (R) which is measured in Ohm \(\Omega\). In other words, to transfer 1 A (ampere) current between two points 1V (volt) of potential energy is required to overcome 1 Ohm \(\Omega\) resistance of the circuit. Mathematically,

V = I.R

Thus, current (I) is

I =  \(\frac{V}{R}\)

As per the SI, until 2019, the standard official definition of the ampere was,

“The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to \(2×10^{-7}\) newton per meter of length.”

General Conference on Weights and Measures had conducted a meeting in November 2018 and changed the official definition of ampere. From 20 May 2019 onward, the official definition of ampere is:

“One coulomb of electric charge is the number of electric charge carriers which is equivalent to \(6.24×10^{18}\) charge carriers and when one coulomb of electric charge carriers move per second from a specific point is term as 1 ampere.”

Types of Current

Electric current is the flow of charged particles and precisely, electrons. The types of current depend on the flow of electrons. There are two types of current,

Direct Current: Direct current, abbreviated as DC, is the type of current in which electrons flow in one direction. They flow from high potential to low potential and get utilised. Almost all the electronic devices, big or small, use this current to operate. An advantage of this current is that it can be stored and can be used whenever or wherever the requirements are. These storage devices are batteries.

Alternative Current: Alternative current, abbreviated as AC, is the type of current in which electrons move back and forth several times in a second between the starting source and ending source. In other words, the electrons oscillate between the sources. This way, they change the direction several times in a second. This oscillation of electrons is measured in hertz (Hz). In India, the AC is available at 50Hz. On almost all the electrical instruments, this rating is mentioned.

Measurement of Ampere Current

The flow of current is measured in terms of ampere. To calculate the flow of current in a circuit, a device named Ammeter is used. This device is connected to the circuit in a series in which the current is to be measured. While measuring the current in the circuit, ammeter uses a wire of low resistance to avoid the voltage drop.

There are different types of ammeter currently in the use. The basis of their classification is how much current they can measure. Ammeters which can measure smaller currents are milliammeters, microammeters and picoammeters. As their name suggests they can measure milliampere, microampere and pico ampere current respectively.

There are different types of ammeters in the market. Their names are from their method of operations. Some of these are:

Moving-coil Ammeter: The moving-coil ammeter uses the magnetic deflection method in such a way that when the electric current passes through it the coil placed in the magnetic field of a permanent magnet, the coil moves and captures its movement in the form of reading. Since these types of ammeters can give an average mean of varying current through it, they are not useful for AC. These devices are useful for DC only.

Moving Magnet Ammeter: Moving-magnet ammeters operate on the same principle as of the moving coil ammeters but in this case, the coil mounts on the permanent magnet and the permanent magnet moves the needle. These ammeters are used where the measurement of large current is necessary.

Electrodynamic Ammeters: These ammeters also operate on the same principle as of the above two ammeters but instead of using permanent magnet they use electromagnet. Because of the use of electromagnets, these instruments can be used to measure both AC and DC.

Other instruments which in use to measure the flowing current are moving –iron ammeter, hot wire ammeter, digital ammeter, etc.

FAQs on Ampere

Q.1: What is current and how does it measure?

Answer: Electrical current is the flow of electrons. The current flows in the opposite direction of flow of electrons. It is measured in terms of ampere.

Q.2: Define ampere?

Answer: Ampere is the unit of measurement of electric current to honour the French physicist Andre Marie Ampere. The official definition of the current is given by SI (International System of units), which states that One coulomb of electric charge is the number of electric charge carriers which is equivalent to 6.24×10^{18}charge carriers and when one coulomb of electric charge carriers move per second from a specific point is term as 1 ampere.

Q.3: How the current is measured? Explain any two of them.

Answer: Current is the movement of electrons from one place to another. There are several instruments which are in use to measure the current. These instruments are ammeters. These instruments measure the flow of current on basis of the same principle which is the change in the magnetic field of either a permanent magnet or an electromagnet. Some of them are Moving-coil ammeter, Moving-magnet ammeter, Moving-iron ammeter, Electrodynamic ammeter, Digital ammeter, Hot-wire ammeter.

1. Moving –coil ammeter: This instrument measures the current on the magnetic deflection. In this instrument, a permanent magnet is fixed and the metal coil rounded over it moves and gives the reading. As the moving coil gives the average of the current, hence it is used to measure DC and not AC.
2. Moving-magnet ammeter: In this instrument, as the name suggests the metal coil is fixed and magnet moves to give the reading.