Magnetic Force

the magnetic force on a charge q, moving with velocity v:

\[\vec F=q\vec v \times \vec B\]

• There is no magnetic force on a charge at rest
• There is no magnetic force on a charge moving parallel to the magnetic field lines.
• The force is maximum when the velocity is perpendicular to the magnetic field.
• The force is perpendicular to both the direction of motion and to the magnetic field, and its direction is given by the so-called “right hand rule”.

This means that the component of motion parallel to the field lines is not altered, but the component perpendicular to the field lines is. So, charged particles will move so that they spiral around magnetic field lines.

Through the forces that it exerts on moving charges, a magnetic field will exert a torque on a current loop.

The current loop has a magnetic dipole moment, and the torque will cause the dipole to line up with the field lines. In this way, the dipole moment of a compass needle lines up with the Earth’s magnetic field.

Measure the Sun’s magnetic field?

For certain atoms placed in a magnetic field, energy levels will shift. This is known as the Zeeman effect. Different energy levels shift by different amounts. Some transitions that normally appear as one line split into a group of lines. The amount of splitting is proportional to the strength of the magnetic field.

by checking the amount of splitting in different atoms in various magnetic fields in the laboratory.

Sunspots exhibit stronger magnetic fields, and their occurrence is in pairs, with one corresponding to the north pole and the other to the south pole of a magnet. The leading spot in a pair is determined by the Sun's rotation and exhibits consistent polarity within a solar hemisphere, with a reversal in polarity between hemispheres every 11 years. This suggests a 22-year cycle in the Sun's magnetic field. Unlike Earth's stable magnetic field, the Sun's magnetic field arises below the surface, and the Sun does not rotate as a rigid body, with equatorial material having a faster rotation period than material at higher A latitudes.

As the Sun rotates differentially, the magnetic field lines become distorted. This is because the charged particles in the matter cannot move across field lines, so the field lines are carried along with the material. The magnetic field is frozen into the material. As the field lines wind up, the field becomes very strong in places. Kinks in the field lines break through the surface.