Physics · Year 13

Active learning ideas

Force on Moving Charges

Active learning helps students visualize magnetic forces because the force direction and motion are hard to grasp through static diagrams alone. When students manipulate variables in simulations or build physical models, they connect the right-hand rule and force equation to real trajectories, reducing abstract confusion.

• National Curriculum Attainment Targets
• A-Level: Physics - Magnetic Fields
• A-Level: Physics - Electromagnetism

• 25–45 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Simulation Game
• 35 min · Small Groups

PhET Simulation: Particle Trajectories

Students access the PhET 'Charges and Fields' or 'Magnetic Fields' simulation. They launch electrons at varying angles into uniform B-fields, sketch predicted paths, measure radii, and adjust speeds to match observations. Groups discuss discrepancies and refine right-hand rule use.

Predict the trajectory of a charged particle entering a uniform magnetic field at different angles.

Facilitation Tip

During the PhET Particle Trajectories simulation, have students record velocity and force readouts at five points along the path to confirm constant speed despite changing direction.

What to look for

Present students with a diagram showing a proton entering a uniform magnetic field perpendicular to its velocity. Ask them to: 1. Use the right-hand rule to indicate the direction of the force. 2. State whether the particle's speed will increase, decrease, or remain constant. 3. Describe the resulting path.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 02

• Simulation Game
• 45 min · Pairs

Velocity Selector Model: Cardboard Setup

Provide bar magnets for B-field and battery-powered plates for E-field. Pairs align crossed fields, launch lightweight charged objects like pith balls, and measure undeflected speeds. They calculate v = E/B and test predictions with voltage changes.

Explain the principle behind a velocity selector in particle accelerators.

Facilitation Tip

Before the Velocity Selector Model setup, ask students to sketch their expected particle path if the electric and magnetic forces are unbalanced, then test their predictions.

What to look for

Pose the following scenario: 'Imagine you are designing a velocity selector for alpha particles (charge +2e, mass ~4 amu) moving at 10^6 m/s. If you set up a magnetic field of 0.5 T, what electric field strength and direction would you need to ensure only these particles pass through undeflected?' Facilitate a class discussion on how they would derive the answer.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 03

• Simulation Game
• 25 min · Small Groups

Right-Hand Rule Relay: Force Directions

Set up stations with velocity and B-field directions shown via arrows. Teams race to palm-thumb-finger configurations for F direction, then verify with simulation. Debrief as whole class on common errors.

Design an experiment to measure the charge-to-mass ratio of an electron using magnetic fields.

Facilitation Tip

In the Right-Hand Rule Relay, assign each group a different charge sign and have them demonstrate their palm-thumb orientation to peers for immediate feedback.

What to look for

Provide students with the formula for the radius of a circular path, r = (mv)/(qB). Ask them to explain in their own words how changing each variable (mass, velocity, charge, magnetic field strength) would affect the radius of the path for a charged particle entering a magnetic field perpendicularly.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 04

• Simulation Game
• 40 min · Pairs

e/m Ratio Design: Experiment Planning

Individuals outline a deflection experiment using a CRT tube or sim, specifying variables like B-strength and voltage. Pairs peer-review plans for controls, then simulate to compute e/m from radius data.

Predict the trajectory of a charged particle entering a uniform magnetic field at different angles.

What to look for

Present students with a diagram showing a proton entering a uniform magnetic field perpendicular to its velocity. Ask them to: 1. Use the right-hand rule to indicate the direction of the force. 2. State whether the particle's speed will increase, decrease, or remain constant. 3. Describe the resulting path.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

A few notes on teaching this unit

Start with the PhET simulation to establish the core concept visually, then move to hands-on activities to solidify understanding. Avoid teaching the right-hand rule abstractly; instead, use physical gestures and peer checks to build muscle memory. Research shows that combining kinesthetic practice with immediate feedback reduces misconceptions about force direction and charge dependence.

Students will confidently predict circular or helical paths, calculate radii using r = (mv)/(qB), and apply the right-hand rule to determine force directions. They will explain why speed remains constant during magnetic deflection and justify their reasoning with evidence from simulations or experiments.

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Watch Out for These Misconceptions

• During the PhET Particle Trajectories simulation, watch for students who assume the speed readout changes as the particle curves.

  Direct students to pause the simulation at multiple points and note that the speed value remains constant, emphasizing that the force only changes direction, not kinetic energy.

• During the Velocity Selector Model setup, watch for students who believe stationary charges experience a magnetic force.

  Have students pass a charged object through the cardboard model while it is moving and while it is stationary, observing that the path only curves when velocity is present.

• During the Right-Hand Rule Relay, watch for students who default to the left-hand rule for all charges.

  Provide a sign chart for positive and negative charges, and have students practice with both hands to reinforce the correct palm-thumb orientation based on charge sign.

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Methods used in this brief

• Simulation Game