Find the third perpendicular direction
You will be able to: Use the right-hand rule for magnetic force and reverse it for negative charge.
How does charge sign change magnetic force?
A positive charge moves right through an upward magnetic field. Its magnetic force points out of the page. This is a three-direction relationship: force is perpendicular to both velocity and field.
A useful starting point: A moving charge produces a magnetic field →
Words and symbols before equations
- Velocity v
- Direction and speed of the charge’s motion, in m/s.
- Magnetic force F_B
- Force on a moving charge due to the external magnetic field, in N.
- Right-hand rule
- Point fingers along v, curl toward B; thumb gives force for positive charge.
- Reference axes
- +x right, +y up, +z out of the flat page. Rotating the 3D view does not change these physical labels.
What this picture assumes
v=3 m/s right, |q|=2 μC, |B|=0.50 T vertical. Force=3 μN perpendicular to the flat page. 3D vectors are normalized direction arrows in an orthographic projection, not equal physical magnitudes. View rotation does not alter the physics.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- F_z=3 μN: out of the fixed reference page. Rotating the 3D view to 30° changes only the projection.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Start with the positive-charge result F_B=q(v×B). For v right and B up, v×B is out of the page. A negative charge reverses that direction. Reversing both v and B leaves their cross-product direction unchanged.
The 3D view rotates the camera around three labeled arrows. It shows their spatial relationship; it does not change the field or velocity when you rotate the view. The fixed flat diagram and direction sentence remain the primary reference.
For perpendicular motion the magnitude is |q|vB. A stationary charge or a charge moving parallel or antiparallel to B feels no magnetic force from that field. The force direction is not generally the same as B.
A worked example, step by step
A −2 μC charge moves right at 3 m/s in a 0.50 T upward field. Find magnetic force.
- Velocity is perpendicular to B, so magnitude is |q|vB.
- F=(2×10⁻⁶)(3)(0.50)=3×10⁻⁶ N=3 μN.
- For positive charge, right crossed with up gives out of page.
- The negative charge reverses the force into the page.
Use the right hand for the positive-charge result, then reverse for negative charge. Do not reverse twice.
If both q and B reverse while v stays fixed, what happens to F?
Compare with an explanation
It stays the same: the two sign reversals cancel.
Predict. Change one thing. Explain.
Reverse the charge sign, then the field direction. Rotate the 3D view to see the perpendicular arrows; verify that camera rotation never changes the stated physical force.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
F_z=3 μN: out of the fixed reference page. Rotating the 3D view to 30° changes only the projection.
v=3 m/s right, |q|=2 μC, |B|=0.50 T vertical. Force=3 μN perpendicular to the flat page. 3D vectors are normalized direction arrows in an orthographic projection, not equal physical magnitudes. View rotation does not alter the physics.
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 +1 μC charge moves at 4 m/s to the right in a 2 T upward field. (a) Find magnitude. (b) Find direction. (c) Reverse q and state the new direction. (d) State whether rotating the viewing camera changes the physics.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: F=8 μN.
- 1 point: Out of the page.
- 1 point: Into the page.
- 1 point: No; camera rotation only changes the drawing projection.
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 1Which direction is magnetic force relative to v?
Perpendicular whenever the magnetic force is nonzero.
RECALL 2What reverses for negative q?
The positive-charge force direction.
RECALL 3Does a dot or cross encode force magnitude?
No; it encodes an out-of-page or into-page direction.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Find the third perpendicular direction
- F_B=q(v×B), with direction found by the right-hand rule.
- Perpendicular: |F_B|=|q|vB.
- Negative charge reverses the positive-charge force.
Remember: Use the right hand for the positive-charge result, then reverse for negative charge. Do not reverse twice.
Conditions: v=3 m/s right, |q|=2 μC, |B|=0.50 T vertical. Force=3 μN perpendicular to the flat page. 3D vectors are normalized direction arrows in an orthographic projection, not equal physical magnitudes. View rotation does not alter the physics.
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.
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