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The screen above a 2D section through circular coil or solenoid.

The blue circles indicate the current flowing out of the plane of the screen. The red circles indicate the current flowing into the plane of the scree.

A magnetic field is a region where a magnetic force is exerted on a moving electric charge. For electric and gravitational fields, we can define field strength as the force per unit charge or force per unit mass, however in the case of magnetic fields there is no parallel definition, since there is nothing that corresponds to a unit charge or mass. Consequently magnetic fields are defined in terms of the force, F acting on a charge, Q moving at a speed v. The magnetic field strength (flux density) B is then
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Magnetic fields are vector fields. i.e. each point in the field has a magnitude and a direction. The Field lines show

  • The direction of the field - a tangent to the line. (The direction that a compass needle would point if it were placed in the field)
  • Strength of the field - the closer the lines the stronger the field.

B is measured in units called Tesla