D.3 · IB Physics
Velocity selector
Crossed electric and magnetic fields push a charge in opposite directions. Only one speed balances the two forces exactly; every other speed curves away and misses the exit slit. Try different particles to see that the selected speed does not depend on charge or mass.
Predict first
Start from the defaults (carbon ion, v = 300 km s⁻¹, E = 60 kV m⁻¹, B = 0.20 T), where the ion goes straight through. You raise v to 400 km s⁻¹. Which way does it bend?
The electric force qE is unchanged, but the magnetic force qvB grows with v. At 400 km s⁻¹ qvB > qE, so the ion bends in the direction of the magnetic force, towards the + plate.
Check it in the model: Move the Particle speed, v slider from 300 to 400 km s⁻¹ and watch the path after the ion is fired.
electric force qE magnetic force qvB path of the particle. Magnetic field points into the page (×).
net force: F = q(E − vB) along the field
e = 1.60 × 10⁻¹⁹ C, u = 1.66 × 10⁻²⁷ kg
Try this
- Choose 'Balanced (v = E/B)', then switch between the four ions. Does the particle still go straight through, and why does the selected speed not contain q or m?
- Keep E = 60 kV m⁻¹ and B = 0.20 T. Set v = 250, then 350 km s⁻¹ with the carbon ion. Which way does it bend each time, and which force is larger?
- Double B from 0.10 T to 0.20 T with E = 60 kV m⁻¹. What speed must the particle have for the path to be straight now?
Common error: saying that heavier or more highly charged particles are selected differently; the condition qE = qvB cancels q, and m never appears, so only the speed matters.