Chapter 21: Magnetism

Magnetic Force

The force experienced by a moving charged particle in a magnetic field.

Formula for Magnetic Force

$F=\left|q\right|vBsin\theta$ Where:

• $q$: charge of the particle
• $v$: velocity of the particle
• $B$: magnetic flux density
• $\theta$: angle between velocity and magnetic field

Right-Hand Rule

A method to determine the direction of magnetic force: Point fingers in the direction of velocity ($v$), curl towards magnetic field ($B$), and thumb points in the direction of force ($F$).

Key Concepts

• A charged particle at rest experiences zero magnetic force.
• A charged particle moving parallel to magnetic field lines also experiences zero magnetic force.
• Magnetic force acts perpendicular to both the velocity vector and magnetic field vector.
• Magnetic force does no work since it does not change the particle's kinetic energy.

Applications of Magnetic Force

• Velocity Selector: Uses perpendicular electric and magnetic fields to select particles of specific velocity.
• Mass Spectrometer: Measures masses of particles based on the radius of their path in a magnetic field.
• DC Motors: Magnetic force interacts with current in loops to create rotation.

Questions for Students

1. What is the formula for magnetic force? Define each term.
2. Explain the right-hand rule for magnetic force direction.
3. Describe how a velocity selector works.
4. Why does a particle moving parallel to magnetic field lines experience zero force?
5. Explain the role of magnetic force in the operation of a mass spectrometer.