High School Physics

Magnetic Moment – definition, formula, questions class 12

In this post, we will cover the Magnetic Moment, with its definition, formula, concepts, and questions.

Magnetic Moment – definition, formula, and Concepts class 12

The quantity of a magnetic moment is important in the study of magnets and magnetic dipoles because it helps us to describe the magnetic strength of magnets, not only simple bar magnets and magnetized materials but also magnetic dipoles generated from current loops or spinning electrons. 

Magnetic Moment Definition

The magnetic moment is a measurement of the strength of the magnetic field produced by a magnet. It defines the torque experienced by the magnet in an externally applied magnetic field.

Magnetic moment SI unit

 The magnetic moment is usually measured using the units Ampere-square meters [Am2]

Magnetic Moment Formula (in terms of torque and magnetic field)

A magnet’s magnetic moment can be defined by considering placing the magnet in an externally applied magnetic field. When placed in an external magnetic field, magnets experience a turning force or torque due to the fact that magnets are always dipoles with opposite polarity at either end.

The Torque formula contains magnetic moment m in it in the following way

Magnetic Moment Formula from the torque formula

Hence the formula of Magnetic Moment m can be written in terms of the torque and the magnetic field.

where τ is the torque experienced by the magnet measured in units of Nm, m is the magnetic moment measured in units of Am2, and B is the magnetic field vector measured in units of teslas T. This formula shows us that the magnetic moment is a vector quantity, this vector is taken to point from the south to the north pole of the magnet.

Magnetic moment formula for a current-carrying loop

For a current carring loop, the magnetic moment formula can be expressed as m = NIA
Here, N = number of loops, I = current flowing through the loop, and A = area of the loop

See this post on Torque on the current carrying loop in a magnetic field

Practice Problem [question] on Magnetic moment

A bar magnet is placed flat on a flat table, with an applied magnetic field moving upwards through the table, meaning the field is perpendicular to the bar magnet. The magnetic field strength is B = 1 T, if this causes the magnet to experience a torque of τ=4×10−3Nm what is the magnet’s magnetic moment?


See also  Force on a current carrying conductor in a magnetic field
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