Physics HL · Chapter 19: Motion in Electric and Magnetic Fields
How to Read This Motion in Fields Chapter
Set up a trajectory-first workflow for switching between force equations, component kinematics, and energy checks without losing direction sense.
Estimated time: 14 minutes
Why Chapter 19 Is a Mechanics-Electromagnetism Bridge
This chapter is where the mechanics methods from early chapters and the force laws from Chapter 18 become one unified toolset. In a uniform electric field, charged particles behave like projectiles under constant acceleration. In a uniform magnetic field, they curve because force stays perpendicular to velocity. In crossed fields, those two effects compete and can be tuned to cancel. The same particle can therefore move in a straight line, parabola, circle, or helix depending only on field geometry and initial velocity orientation.
Students often lose marks here not because they forget formulas, but because they skip model selection. Before substituting numbers, decide which velocity component changes, which stays constant, and whether field work is possible. That sequence prevents almost every sign and direction error in this chapter.
Learning Targets
By the End of Chapter 19 You Should Be Able To
- Classify electric-field trajectories as straight-line acceleration or parabolic deflection depending on entry velocity orientation.
- Use component equations in uniform electric fields exactly as in projectile motion, with qE/m replacing g.
- Derive and apply magnetic circular-motion relations for radius, period, and frequency.
- Explain why magnetic forces do zero work while still changing trajectory shape.
- Predict and calculate helical motion when velocity has both parallel and perpendicular components to B.
- Solve crossed-field selector conditions and connect them to beam instrumentation and q/m measurement logic.
Recommended Problem-Solving Workflow
Step 1: draw v, E, and B before writing any equations. Step 2: resolve velocity into components parallel and perpendicular to the relevant field. Step 3: write force directions from geometry first, magnitudes second. Step 4: choose whether you need kinematics, circular-motion, or energy methods. Step 5: run a final physics check: does your answer predict the expected bend direction and a plausible scale?
No simulation is embedded in this orientation section because this stage is method setup. Interactivity begins in Section 19.1 once trajectory equations are established and we can test predictions against dynamic path plots.