Dipole in a Uniform Magnetic Field – Class 12 | Chapter – 5 | Physics Short Notes Series PDF for NEET & JEE

Dipole in a Uniform Magnetic Field: When a magnetic dipole is placed in a uniform magnetic field, it experiences a torque that tends to align it with the direction of the magnetic field. This is because the magnetic field exerts a force on the poles of the dipole, causing it to rotate until it is aligned with the field.

Derivation of Dipole in a Uniform Magnetic Field

The magnitude of the torque τ on a magnetic dipole of magnetic moment μ placed in a uniform magnetic field B is given by the equation:

τ = μ × B

where × denotes the vector cross product.

The direction of the torque is perpendicular to both the magnetic moment and the magnetic field, and is given by the right-hand rule. If the fingers of the right hand are curled in the direction of the magnetic moment μ, and then the hand is rotated towards the direction of the magnetic field B, the thumb points in the direction of the torque τ.

The magnetic dipole experiences a net force F as well, which is proportional to the gradient of the magnetic field B. The force F is given by the equation:

F = (∇ (μ · B)),

where · denotes the dot product, and ∇ is the gradient operator.

If the magnetic field is non-uniform, the dipole will also experience a translational motion in addition to the rotational motion. In this case, the magnetic field will exert a net force on the dipole, causing it to move along the direction of the field gradient.

The behavior of a dipole in a uniform magnetic field is important in a variety of applications, such as in magnetic resonance imaging (MRI) and magnetic storage devices.

Importance of Dipole in a Uniform Magnetic Field

The behavior of a magnetic dipole in a uniform magnetic field is important in a variety of applications, including:

  • Magnetic resonance imaging (MRI): In an MRI machine, a strong uniform magnetic field is used to align the magnetic moments of the protons in the body. By applying a radiofrequency pulse and then measuring the response of the protons as they return to their original alignment, detailed images of the body can be produced.
  • Electric motors and generators: In a motor or generator, a magnetic dipole is used to convert electrical energy into mechanical energy or vice versa. The dipole experiences a torque when it interacts with the magnetic field, causing it to rotate and produce electrical or mechanical work.
  • Magnetic storage devices: In hard disk drives and magnetic tape, information is stored as tiny magnetic domains that represent 1’s and 0’s. A magnetic read/write head uses a magnetic dipole to write and read the magnetic information.
  • Magnetic compasses: A magnetic compass works by using a magnetic dipole to align itself with the Earth’s magnetic field. This allows the user to determine the direction of the Earth’s magnetic north pole.


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By Team Learning Mantras