Crossed Fields: the Velocity Selector

Switch on both fields and find the speed that goes straight through the slit. Does it depend on the charge or the mass?

Speed that passes (E/B)—

Electric force (qE)—

Magnetic force (qvB)—

What happens—

Swipe the picture sideways to see all of it.

Positive chargeNegative chargeForceVelocityElectric field linesMagnetic field (× into, • out of the page)

Your shots

No shots yet. Choose the settings, make a prediction, and press Fire.

Charge (q) +1 qSpeed (v) 2 vMass (m) 2 m

Electric field onElectric field strength (E) 2 E

Direction of E

Magnetic field onMagnetic field strength (B) 2 B

Direction of B

My prediction Show the velocity and force arrows

Show the physics

The electric force on the particle is qE, and the magnetic force is qvB. With the fields set so that the two forces point opposite ways (E down with B into the page, or E up with B out of the page), they cancel when:

qE = qvB, so v = E/B

The charge q and the mass m cancel, so any particle with this speed goes straight through the slit, whatever its charge or mass. Faster particles are pushed one way by the magnetic force, and slower ones the other way, and they hit the wall. Try it with different charges and masses.

If the two fields point the wrong way, the forces add up instead of cancelling, and no speed goes straight through.

The numbers are in relative units (1 q, 2 v, 3 m and so on), so you can compare shots without needing real values.

Learn more in the Charge in magnetic fields lesson.