Magnetic force turns motion without doing work
You will be able to: Compare parallel and perpendicular motion and explain why magnetic force does no work on a point charge.
Can a magnetic field alone speed up a particle?
A moving particle can turn without speeding up. A magnetic force is sideways relative to its instantaneous motion, so it changes direction while doing no work on the particle.
A useful starting point: Find the third perpendicular direction →
Words and symbols before equations
- Angle θ
- Angle between velocity and B, not between force and B.
- Work
- Energy transfer by a force acting along displacement.
- Kinetic energy K
- Energy of motion, K=½mv².
- Perpendicular component
- The part of velocity at right angles to B; it controls magnetic force magnitude.
What this picture assumes
Fixed positive q=2 μC, speed 5 m/s, B=0.20 T upward. Numerical angle cases limited to 0°, 90°, 180°, matching the AP Physics 2 boundary. Other angles are discussed qualitatively.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- θ=90°: |F_B|=2 μN. Magnetic force alone does no work; speed and kinetic energy stay constant.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
The general magnitude relation is |F_B|=|q|vB sinθ. AP Physics 2 quantitative force-angle calculations are limited to θ=0°, 90° and 180°; other angles are analyzed qualitatively. The controls use these three cases.
At 0° or 180°, motion is along the field and the magnetic force is zero. At 90°, the force magnitude is largest for fixed |q|, v and B. At intermediate angles, only the perpendicular motion contributes.
At each instant, F_B is perpendicular to v, so magnetic power F_B·v=0. In a magnetic-only model with no other forces, speed and kinetic energy stay constant even though velocity may change direction. An electric field, friction or another force can change speed.
A worked example, step by step
A +2 μC charge moves at 5 m/s in a 0.20 T field. Compare θ=0°, 90° and 180°.
- At 0°, v is parallel to B: F_B=0.
- At 90°, |F_B|=(2×10⁻⁶)(5)(0.20)=2 μN.
- At 180°, v is antiparallel to B: F_B=0.
- In all cases magnetic force alone does no work, so it cannot change kinetic energy.
Zero work does not mean zero force: a perpendicular force can continuously turn a velocity vector.
Can velocity change if speed is constant?
Compare with an explanation
Yes. Velocity includes direction, which can change under a perpendicular force.
Predict. Change one thing. Explain.
Choose each AP angle case. Compare force magnitude while keeping speed, charge and field fixed; explain why kinetic energy remains unchanged.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
θ=90°: |F_B|=2 μN. Magnetic force alone does no work; speed and kinetic energy stay constant.
Fixed positive q=2 μC, speed 5 m/s, B=0.20 T upward. Numerical angle cases limited to 0°, 90°, 180°, matching the AP Physics 2 boundary. Other angles are discussed qualitatively.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant field, force, flux or energy relationship to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA charged particle moves perpendicular to a uniform B with no other forces. (a) Describe force direction relative to v. (b) State magnetic work. (c) Predict speed. (d) Explain whether velocity is constant.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Force is perpendicular to v.
- 1 point: Magnetic work is zero.
- 1 point: Speed remains constant.
- 1 point: Velocity changes direction, so the velocity vector is not constant.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1When is magnetic force zero on a moving charge?
When v is parallel or antiparallel to B, or q or B is zero.
RECALL 2What does the force-angle θ compare?
Velocity with magnetic field.
RECALL 3Why can a magnetic field turn a charge without speeding it up?
The force is always perpendicular to its instantaneous velocity.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Magnetic force turns motion without doing work
- General relation: |F_B|=|q|vB sinθ.
- AP numerical angle cases here: 0°, 90°, 180°.
- Magnetic-only point-charge power F_B·v=0.
Remember: Zero work does not mean zero force: a perpendicular force can continuously turn a velocity vector.
Conditions: Fixed positive q=2 μC, speed 5 m/s, B=0.20 T upward. Numerical angle cases limited to 0°, 90°, 180°, matching the AP Physics 2 boundary. Other angles are discussed qualitatively.
Refresh Kid · AP Physics 2 Unit 4 (official Unit 12) · Objectives 12.2.B · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 12.2, objectives 12.2.B. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. Refresh Kid calls this the fourth AP Physics 2 unit; College Board numbers it Unit 12; the first unit in this course is official Unit 9. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
Want to work through this with a tutor?
Bring your question about Magnetic force turns motion without doing work. Your explanation and answers remain free to access.