University Physics II · Magnetic Fields and Magnetic Forces · 8.02
Magnetic force on a moving charge
The magnetic Lorentz force, cross-product direction, speed and angle dependence, zero-work property, and sign of charge.
Build the model
Connect the measurement to the mechanism.
The magnetic Lorentz force, cross-product direction, speed and angle dependence, zero-work property, and sign of charge. Treat this course-map statement as a claim to test rather than an invitation to import a familiar equation. In Magnetic Fields and Magnetic Forces, begin from particle-orbit modelling, then state the system, observable, assumptions, and evidence before calculating.
- Simple definition
- The magnetic Lorentz force, cross-product direction, speed and angle dependence, zero-work property, and sign of charge.
- Example
- A strong response uses particle trajectories and states where the model stops being reliable.
The force is perpendicular to both v and B.
Reverse the direction for a negative charge.
The force vanishes for motion along the field.
θ is the angle between v and B.
The subsection's claim
The magnetic Lorentz force, cross-product direction, speed and angle dependence, zero-work property, and sign of charge.
How to work with it
Start from particle-orbit modelling. Then using cross products and free-body reasoning before substituting magnitudes into magnetic-force equations. Select an equation only after its variables and assumptions match the stated system.
What evidence would decide
Which electric and magnetic field settings transmit only a chosen particle velocity? Useful evidence includes vector equations, trajectory simulations, parameter sweeps, transmission windows, and analytic comparison.
Keep the boundary visible
This GioPhysics course map is an adaptable learning sequence, not academic credit, accreditation, or a universal university syllabus. Departments may redistribute weeks, laboratory hours, optics, or the modern-physics survey to match local requirements. This GioPhysics course map is an adaptable learning sequence, not academic credit, accreditation, or a universal university syllabus. Departments may redistribute weeks, laboratory hours, optics, or the modern-physics survey to match local requirements. Thermal physics appears as an unnumbered institutional extension: some universities assess it within Physics II, while others teach it in a separate course, so include the thermal extensions only where the local syllabus requires them. A result should be checked against units, signs, limiting cases, and the conditions under which its model was derived.
Change one variable at a time
Make the relationship visible.
Rotate velocity from parallel to perpendicular. Charge sign changes direction, not magnitude.
FORCE3.00 ×10⁻² N
ANGLE FACTOR1.00
Live interpretationFORCE: 3.00 ×10⁻² N. ANGLE FACTOR: 1.00
Catch the common trap
Explain before calculating.
What work does a purely magnetic force do on one charge?
Choose an answer to test the model.
Practice & worked examples
Reason from the model, then test the result.
Worked calculationA +3.0 µC charge moves at 2.0×10⁴ m s⁻¹ perpendicular to a 0.50 T field. Find the force magnitude.
- Use |F|=|q|vB sinθ with θ=90°.
- Convert q=3.0×10⁻⁶ C.
- |F|=(3.0×10⁻⁶)(2.0×10⁴)(0.50)=3.0×10⁻² N.
AnswerThe magnetic-force magnitude is 0.030 N; qv×B determines its direction.
TransferDesign one observation that separates Magnetic force on a moving charge from Charged-particle motion in uniform fields.
- Name the observable central to Magnetic force on a moving charge.
- Name the contrasting observable or condition in Charged-particle motion in uniform fields.
- Choose a graph feature, sign, scale, or limiting case that would distinguish them.
AnswerThe comparison is useful only if the proposed observation could rule out at least one of the two accounts.