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Magnetic Fields

Magnetic Fields

A moving charge or a current creates a magnetic field in the surrounding space, in addition to its electric field. The magnetic field exerts a force on any other moving charge or current that is present in the field. Like an electric field, the magnetic field is also a vector field. At any position, the direction of the magnetic field is defined as the direction in which the north pole of a compass needle points.
A magnetic field is defined by the force that a charged particle experiences...

Magnetic Field of a Solenoid

Magnetic Field of a Solenoid

A solenoid is a conducting wire coated with an insulating material, wound tightly in the form of a helical coil. The magnetic field due to a solenoid is the vector sum of the magnetic fields due to its individual turns. Therefore, for an ideal solenoid, the magnetic field within the solenoid is directly proportional to the number of turns per unit length and the current. Conversely, the magnetic field outside the solenoid is zero.
Consider a solenoid with 100 turns wrapped around a cylinder of...

Magnetic Field Lines

Magnetic Field Lines

The representation of magnetic fields by magnetic field lines is very useful in visualizing the strength and direction of the magnetic field. Each of the magnetic field lines forms a closed loop. The field lines emerge from the north pole (N), loop around to the south pole (S), and continue through the bar magnet back to the north pole.
Magnetic field lines follow several hard-and-fast rules:

Energy In A Magnetic Field

Energy In A Magnetic Field

If a magnetic field is sustained, there must be a current in a closed circuit or loop, implying some energy has been spent in creating the field. If this energy is not dissipated via the circuit's resistance, it is stored in the field.
Take an ideal inductor with zero resistance. Although it's practically impossible, assume that the coil's resistance is so small that it is practically negligible. The loss of the field's energy to dissipate thermal energy (or heat) is thus...

Magnetic Field Of A Current Loop

Magnetic Field Of A Current Loop

Consider a circular loop with a radius a, that carries a current I. The magnetic field due to the current at an arbitrary point P along the axis of the loop can be calculated using the Biot-Savart law.

Magnetic Field due to Moving Charges

Magnetic Field due to Moving Charges

A stationary charge creates and interacts with the electric field, while a moving charge creates a magnetic field.
Consider a point charge moving with a constant velocity. Like the electric field, the magnetic field at any point is directly proportional to the magnitude of the charge and inversely proportional to the square of the distance between the source point and the field point. However, unlike the electric field, the magnetic field is always perpendicular to the plane containing the line...

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Related Experiment Video

Updated: Feb 15, 2026

Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

Published on: March 30, 2017

Control of Ultracold Photodissociation with Magnetic Fields.

M McDonald¹, I Majewska², C-H Lee¹

¹Department of Physics, Columbia University, 538 West 120th Street, New York, New York 10027-5255, USA.

Physical Review Letters

|February 6, 2018

Summary

Ultracold molecule photodissociation in weak magnetic fields dramatically alters photofragment angular distributions. This quantum control demonstrates precise manipulation of molecular reactions at extremely low temperatures.

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Last Updated: Feb 15, 2026

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Area of Science:

Quantum Chemistry
Molecular Physics
Ultracold Atoms and Molecules

Background:

Molecular photodissociation produces photofragments with spatial distributions dependent on molecular structure and light characteristics.
Quantum mechanical effects become dominant in photodissociation reactions performed at ultracold temperatures.
Weak external fields can control photodissociation reactions in the ultracold regime.

Purpose of the Study:

To investigate the control of molecular photodissociation reactions using weak magnetic fields at ultracold temperatures.
To observe and analyze changes in photofragment angular distributions under controlled conditions.
To validate theoretical models describing ultracold photodissociation dynamics.

Main Methods:

Photodissociation of ultracold diatomic strontium molecules.
Application of weak magnetic fields (below 10 Gauss).
Precise quantum-state control of molecules.
Comparison with a multichannel quantum chemistry model incorporating nonadiabatic effects.

Main Results:

Striking changes in photofragment angular distributions were observed.
Experimental results showed excellent agreement with theoretical predictions.
The study confirmed strong mixing of partial waves in the photofragment energy continuum.

Conclusions:

Weak magnetic fields can effectively control the photodissociation of ultracold molecules.
The observed phenomena are well-described by quantum chemistry models including nonadiabatic effects.
Precise quantum-state control is crucial for manipulating ultracold molecular reactions.