What is a magnetic field?

• A. The region around a magnet where a force acts on another magnet or on a magnetic material
• B. A region where electric charges move at high speed
• C. A field that stores electrical energy in capacitors
• D. A region that can convert mechanical energy into heat

Answer: (A)

A magnetic field is the region around a magnet where its magnetic influence can exert a force on other magnets or on magnetic materials. This field is described by the vector B and tells you both the direction and strength of the magnetic force that would act on moving charges or on magnetic materials placed in that region. Field lines illustrate this influence, typically looping from the magnet’s north pole to its south outside and closing around inside different magnetic circuits. The force on a moving charge arises from the cross product of the charge’s velocity and the magnetic field, which explains why moving charges experience sideways forces and compasses deflect in a magnetic field. The other statements refer to electric charges in motion, electric energy storage in capacitors, or conversion to heat, none of which define a magnetic field.