Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Physics
Lesson Topic: explain how electric and magnetic fields can be used in velocity selection
Learning Objective/s:
  • Describe the magnetic force on a current‑carrying conductor using the vector cross‑product.
  • Explain how crossed electric and magnetic fields create a velocity selector and derive the condition v = E⁄B.
  • Apply the velocity‑selection principle to predict particle deflection and to balance forces on a conductor.
  • Analyse a typical experimental set‑up (plates, Helmholtz coils) and identify possible sources of error.
Materials Needed:
  • Projector and screen for slides
  • Whiteboard and markers
  • Printed handout with diagrams and key equations
  • Helmholtz coil set with power supply (demonstration)
  • Parallel‑plate capacitor plates with variable voltage source
  • Small electron/ion beam source or simulation software
  • Rulers/meter sticks for measuring plate separation
 Introduction:
Begin with a short video of a particle beam curving in a magnetic field and ask how the beam could be made to travel straight. Recall that students have already studied the Lorentz force and drift velocity in conductors. Today they will determine the condition where electric and magnetic forces cancel and predict the selected velocity.
Lesson Structure:
  1. Do‑now (5'): Solve a brief problem on magnetic force on a wire (quick check).
  2. Mini‑lecture (10'): Derive F = I L × B, introduce crossed‑field concept, and derive v = E⁄B (guided questions).
  3. Demonstration set‑up (10'): Show Helmholtz coils and parallel plates, explain how to vary E and B (observation).
  4. Guided activity (15'): In groups calculate the voltage needed for a given magnetic field to select a specific speed, record predictions (peer check).
  5. Simulation/real‑world test (10'): Run the beam (or simulation) and adjust V until the beam is undeflected; compare observed speed with calculation (discussion).
  6. Exit ticket (5'): Write the selector formula and name one real‑world application (collect).
Conclusion:
Summarise that a velocity selector works by balancing electric and magnetic forces, giving the simple relation v = E⁄B. For the exit ticket, students state the formula and cite an application such as a mass spectrometer. Homework: complete a worksheet calculating selector settings for different particle charges and masses.