Magnetic field and magnetic force

Equipment

• Model of a charge/wire in a magnetic field
• Sliders for magnetic field B and angle/speed
• Quantitative plot of F versus B and θ

Procedure

1. Vary the magnetic field

   Drag the "Field / level" slider in figure 1 and watch the particle's trajectory: larger B gives a tighter radius via $r = mv/(qB)$. Read the value displays, record two (B, r) pairs. Use each figure's "Pause" for precise reading.

2. Check the angular dependence

   Use the "Angle / speed" slider to change the angle between $\vec v$ (or current I) and $\vec B$. Check $F = BIl\sin\theta$ in figure 2: F peaks at $\theta = 90°$ and vanishes when parallel. Vary the speed to see $F \propto v$ for a single charge.

3. Hand rules and prediction

   Before moving each slider, predict the force direction with the left-hand rule ($\vec F = q\vec v \times \vec B$), then check against the simulation. Reverse the charge sign (reverse v or B) to see the force flip. Conclusion: the magnetic force is always perpendicular to velocity, so it does no work — it only bends the path.

Simulation

Experiment history

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