Lorentz Magnetic Force and Direction
For a point charge q moving with velocity v in magnetic field B, the magnetic part of the Lorentz force is q(v cross B). Its magnitude is |q|vB sin theta and it is perpendicular to both v and B.
Why this shows up in the exam
Finding deflection directions in beam diagrams · Calculating instantaneous force vectors · Comparing positive and negative particle tracks
Learn the idea
A moving charge feels a sideways magnetic force set by the cross product of velocity and field. Imagine velocity and magnetic field as two arrows. Only the part of motion across the field is turned; motion along the field is untouched. The right-hand rule gives the force for a positive charge, and a negative charge reverses it.
🧠 Memory hook: Right hand for positive charge; reverse it for negative charge.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- F_B = q(v cross B) — vector force; reverse the right-hand-rule direction when q is negative
- |F_B| = |q| v B sin(theta) — magnitude when theta is the angle between v and B
How to approach it
- 1Write v and B as vectors
- 2Compute v cross B before multiplying by q
- 3Reverse the direction if the charge is negative
Common slip-ups that cost marks
- •Applying the right-hand rule directly to an electron
- •Using cos theta instead of sin theta
- •Ignoring vector signs in component-form data
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A charge of 2 microC moves perpendicular to a 3 T magnetic field at 4 x 10^5 m/s. Find the magnetic force.
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