AcademyMagnetic Fields and Forces
Academy
Magnetism Basics
Level 1 - Physics topic page in Magnetic Fields and Forces.
Principle
Magnetic effects come from moving charge and act on moving charge.
Notation
\(\vec B\)
magnetic field
\(\mathrm{T}\)
\(q\)
particle charge
\(\mathrm{C}\)
\(\vec v\)
particle velocity
\(\mathrm{m\,s^{-1}}\)
\(I\)
conventional current
\(\mathrm{A}\)
\(\vec F_B\)
magnetic force
\(\mathrm{N}\)
Method
Magnetism is not a separate kind of charge. It is the interaction pattern associated with moving electric charge and magnetic dipoles.
Charge source idea
\[\text{moving charge}\rightarrow\vec B\]
Force target idea
\[\vec B+\text{moving charge}\rightarrow\vec F_B\]
No magnetic monopole
\[\text{north and south poles occur as pairs}\]
Bar magnets align with an external magnetic field. A compass needle is a small magnetic dipole: its north-seeking end points along the local field direction.
Rules
Field unit
\[1\,\mathrm{T}=1\,\mathrm{N\,A^{-1}\,m^{-1}}\]
Moving charge condition
\[F_B=0\quad\text{if }v=0\text{ or }\vec v\parallel\vec B\]
Dipole direction
\[\vec\mu\text{ tends to align with }\vec B\]
Examples
Question
A stationary charge sits in a steady magnetic field. What magnetic force acts on it?
Answer
None. Magnetic force on a particle requires velocity:
\[F_B=|q|vB\sin\theta\]
Checks
- Magnetic fields are vector fields.
- A magnetic field can change direction of motion without doing work on a point charge.
- Isolated north or south magnetic poles are not observed in ordinary electromagnetism.