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Ampere-Turns: Measurement of Magnetic Force

Ampere-turns is a unit used to quantify the magnetomotive force (MMF) generated by an electric current flowing through a coil of wire, typically wound around a magnetic core. It is a fundamental concept in electromagnetism and plays a crucial role in understanding and designing electromagnets and magnetic circuits.

Key Concepts


  • Magnetomotive Force (MMF): Similar to electromotive force (EMF) in electrical circuits, MMF is the force that drives magnetic flux through a magnetic circuit.
  • Ampere-Turns (At): The product of the current (in amperes) flowing through the coil and the number of turns of wire in the coil.


The magnetomotive force (MMF) in ampere-turns is given by:
\[ \text{MMF (At)} = I \times N \]

  • ( I ) is the current in amperes (A) flowing through the coil.
  • ( N ) is the number of turns of wire in the coil.



  • Magnetic Field Strength: Ampere-turns determine the strength of the magnetic field produced by an electromagnet. Increasing either the current or the number of turns increases the MMF and thus the magnetic field strength.
    \[ \text{MMF (At)} = I \times N \]

Magnetic Circuits

  • Magnetic Flux: Ampere-turns are used to calculate and control the magnetic flux in magnetic circuits, influencing the performance and efficiency of devices such as transformers and inductors.

Transformer Design

  • Primary and Secondary Windings: In transformers, the ratio of ampere-turns in the primary winding to those in the secondary winding determines the voltage transformation ratio.
    \[ \frac{N_{\text{primary}}}{N_{\text{secondary}}} = \frac{I_{\text{secondary}}}{I_{\text{primary}}} \]

Practical Example

Consider an electromagnet with the following specifications:

  • Coil with ( N = 500 ) turns of wire
  • Current ( I = 2 ) amperes flowing through the coil

The MMF (ampere-turns) produced by the electromagnet would be:
\[ \text{MMF} = 2 \text{ A} \times 500 = 1000 \text{ At} \]

This means the electromagnet generates a magnetomotive force of 1000 ampere-turns, contributing to the magnetic field strength according to the magnetic properties of the core material and the design of the coil.


Ampere-turns \(At\) is a critical measure in electromagnetism, representing the magnetomotive force produced by an electric current flowing through a coil of wire. It determines the strength of the magnetic field in electromagnets and influences the operation of magnetic circuits in various electrical and electronic devices. By controlling ampere-turns, engineers can optimize the performance of electromagnets and magnetic devices, ensuring efficient operation and desired magnetic field characteristics.

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