Charged Particle in a Magnetic Field
Classroom worksheet · Interactive simulation: https://lkforge.com/tools/physics/charged-particle/
Name: Date:
Learning objectives
- Describe the magnetic force on a moving charge.
- Relate the circular path radius to speed, mass, charge, and field.
- Predict the direction of the force.
Variables to change
- Charge sign
- Speed
- Magnetic field strength
- Entry angle
Procedure
- Send a charge into the field perpendicular to it and observe the circular path.
- Increase the speed and observe the radius.
- Increase the field strength and observe the radius.
- Flip the charge’s sign and note the direction of curving.
Observations
Record how the path radius changes with speed and field strength, and how sign affects direction.
Questions
- Write the magnetic force on a moving charge.
- Why does the charge move in a circle (v perpendicular to B)?
- Write the radius of the circular path.
- What happens to the radius if you double the field?
- What path results if the charge enters at an angle to the field?
Answer key (instructors)
- 1. F = qv×B — magnitude qvB when v is perpendicular to B.
- 2. The force is always perpendicular to the velocity, so it changes direction but not speed — centripetal motion.
- 3. r = mv/(qB).
- 4. It halves — radius is inversely proportional to B.
- 5. A helix — circular motion perpendicular to B plus constant drift along B.
A magnetic field exerts F = qv×B, always perpendicular to the velocity, so a charge moving across the field circles with radius r = mv/(qB). Faster charges circle wider, stronger fields tighten the circle, and an angled entry gives a helix.