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What is Ampere (A)? Unit of Electrical Current & Calculation

Amps “A”: Definition, Formula, Measurement, Conversion and Calculation

What is Ampere?

An Ampere is the unit of electric current. It is named after the French physicist André-Marie Ampère (who is considered the father of electromagnetism) and used in physics and electrical and electronics engineering as a base unit in SI (International System) to measure the electric current.

The unit of Ampere (short form as Amp) denoted by the symbol of “A” is used to measure the flow of electrons between two points triggered by the electric pressure known as voltage. An ampere is defined as:

In a conductor, If one Coulomb of charge (C) is flowing through a point in one second (s), the amount of flowing current past that point is one Ampere (A).

In short words,

One ampere is one coulomb per second.

Amp = Coulomb ÷ Second

mathematically, it forms the equation as follow:

I = Q ÷ t


  • I = Current in Amperes
  • Q = Charge in Coulombs
  • t = Time in seconds

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What is Ampere (A)? Unit in Electrical Engineering & Physics

An ampere is also expressed as flow of electron charge (6.25 x 1018 electrons) by one volt of potential difference between two points which allows it to dissipate one Watt of power during the process between these two points.

I = P ÷ V

A = W ÷ V

Amp = Watt ÷ Volt


Good to know: electron charge “e” = 1.60217662 × 10-19 coulombs.

According to Ohm’s law, if the value of potential difference (i.e. voltage in volts) across a one ohm (Ω) resistor is one volt, the value of flowing current in amperes (A) due to the voltage in that resistor will be one Ampere:

I = V ÷ R

Amps = Volt ÷ Ohms


  • I = Current in Amperes
  • V = Voltage or P.D in Volts
  • R = Resistance in Ohms (Ω)

Ampere Equations used for Conversions to the Related Quantities

Amps from Watts and Volts

  • A = W ÷ V
  • Ampere = Watts ÷ Volts  …  (I = P ÷ V)

Amps from Coulombs and time

  • I = Q ÷ t
  • Amps = Coulombs ÷ time in seconds

Amps from Volts and Resistance

  • I = V ÷ R  …  (Ohm’s law)
  • Amp = Volts ÷ Ohms (Ω)

How to Measure Ampere?

The tool which is used to measure current in amperes is known as amperemeter or simply ammeter. In both analog and digital multimeters, there is an amps (A) mode for measuring AC and DC currents in amperes.

To measure the flowing electric current in an electrical element (such as resistor, capacitor, inductor, diode etc.), simply put the two leads of the multimeter (in series) and the display will show the exact value of current in amperes. You may follow the step by step guide posted in the previous article as “How to Measure Current using Digital and Analog Multimeter?”.

How to Calculate Amps?

Based on the above given formula and equations for current in amps (for different scenarios), we may calculate the value of electric current in amperes as follows.

Example 1:

If the applied voltage across a 4Ω resistor is 12V, find the value of current in Amps flowing in the resistor.


According to Ohm’s law:

I = V ÷ R

Putting the values

I = 12V ÷ 4Ω

I = 3 Amps

Example 2:

If the value of supply voltage across a 30W led bulb is 12V, Calculate the current in Amps in the light bulb.


We know that

I = P ÷ V

Putting the values:

I = 30W ÷ 12V

I = 2.5 Amps

Example 3:

Determine the value of required current in amps to glow a light bulb having 500W and 5Ω resistance .


The derived equation from P = V x I where putting V = I x R which becomes P = I2 R.

I = √(P ÷ R)

Putting the values:

I = √(500W ÷ 5Ω)

I = √(100)

I = 10 amps.

Example 4:

What is the value of electric current in amps if 10 coulombs of charge is flowing through a point in a conducting material in 5 seconds.


We know that I = Q ÷ t. Now put the values.

I = 10C ÷ 5 seconds

I = 2 Amps

Amp to kVA & kVA to Amp Conversion

You may use the online tools for Ampere to kVA calculator and kVA to Amperes calculator.

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