Magnetic fields and electromagnetism

In a nutshell

Magnetic fields are fields that surround magnets and electric currents which exert a force on magnetic objects. 

Magnetic fields

A magnetic field is a field that surrounds permanent magnets and electric currents and exerts a force on magnetic objects. They are normally represented by magnetic field lines which map magnetic field patterns and are helpful to interpret the strength and direction of the force.

Magnetic field lines have a few rules:

• The arrows on a magnetic field line always points from North to South. 
  
• If the lines are parallel and equally spaced then the field is uniform.
• The denser the magnetic field lines the stronger the field.
• Like poles repel and unlike poles attract.

Below is an image showing the field lines around a bar magnet: 

Electromagnetism

Anytime a charged particle moves, it produces a magnetic field around it. In permanent magnets, the field is produced by electrons moving around the nuclei, while all the atoms are lined up.

Right-hand grip rule

A current carrying wire produces a field with circular shapes around it. The direction of the magnetic field and current can be found with the right-hand grip rule: 

• The thumb points in the direction of the current. 
• The curl of the fingers in the direction of the field.
  

The graphic below demonstrates the right-hand grip rule. 

Magnetic field lines for a flat coil 

A flat coil of current carrying wire gives a field with the North and South poles above and below the coil:

1. Magnetic field lines 2. Flat coil 3. Current going in 4. Current going out

Magnetic field lines for a solenoid  

The field around a long solenoid is similar to that of a bar magnet but with a uniform field inside it.