PHY050 · Magnetism · Class 12 + College
Lorentz force
Objective: To see how a moving charged particle (a proton) moves in a magnetic field.
🧰 Apparatus
- (Simulation) proton beam
- Uniform magnetic field region
- Detector screen
- (Real lab: cyclotron or e/m tube with Helmholtz coils)
📐 Formulas and SI units
F = qvB sin θ (N)
r = mv sin θ / (qB) (m)
T = 2πm / (qB)
💡 Theory
A charge moving in a magnetic field feels the Lorentz force F = qvB sin θ, at right angles to its velocity. It cannot change the speed, only the direction, so the particle moves in a circle of radius r = mv/(qB), or a helix if it also moves along the field.
🪜 Procedure
- Choose the particle speed and field strength.
- Watch the path: a circle at 90°, a helix at smaller angles.
- Double B and see the radius halve.
- Double v and see the radius double.
- Compare with r = mv/qB.
📋 Observations Simulated values
✅ Result and conclusion
A proton at 10⁶ m/s in 0.1 T feels 1.6 × 10⁻¹⁴ N and circles with r = 10.4 cm.
🔬 In a real lab
School labs use electron beams in a fine-beam tube; the beam glows faintly and its radius is read from a scale inside the bulb.
⚠️ Safety
Fine-beam tubes use high voltage: teacher operation only, with the tube in its stand.
❓ Quick check
1. The magnetic force on a moving charge changes its…
2. Doubling B makes the radius…
3. A charge moving parallel to B feels…
🏋️ Practice
EasyIs the speed changed by the magnetic force?
Answer: No
MediumElectron vs proton at the same speed: which has the smaller circle?
Answer: Electron (smaller mass)
ChallengeName a machine that uses this.
Answer: Cyclotron, mass spectrometer, TV tube