Steering charges with magnets

Science · Forces & Motion · ages 17-18

Name ______________________ Date ____________

A magnetic field increases the speed of a charge moving across it.
Circle one: True False
Positive and negative charges enter the same field side by side. What happens?
- Both bend the same way
- They bend apart
- Both fly straight on
What is the force on charge Q at speed v across field B?
- B Q v, sideways
- Q times v over B, forward
- B over Q times v, backward
Two particles share a field. One is fast and heavy, the other highly charged. Which curls tighter?
- The fast heavy one
- High-charge one
- Both curl the same
A particle has m equals 2, v equals 3, B equals 1, Q equals 2 in matching units. What is its circle radius?
Answer: ______________
Why does kinetic energy stay constant in the field?
- The charge has no mass
- The field points forward
- The field does no work on the charge
Kai watches his particle bend and says the magnet speeds it up. What is his error?
- Magnets cannot touch charges
- Fields turn particles but never speed them
- Speed always doubles in circles
The same setup fires a particle with double the mass, all else equal. What is the new radius?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

False · The sideways push does no work, so speed and kinetic energy stay fixed.
They bend apart · The sign of charge steers the curve, so opposite signs bend opposite ways.
B Q v, sideways · F equals BQv, and it pushes sideways to both velocity and field.
High-charge one · Radius is momentum over charge, so big charge shrinks the circle.
3 · Radius is m v over B Q: 2 times 3 over 1 times 2 is 3.
The field does no work on the charge · A purely sideways force cannot feed energy in, so the total stays put.
Fields turn particles but never speed them · Bending proves steering, and steering by a sideways push leaves pace untouched.
6 · Radius grows with mass, so doubling mass doubles 3 to 6.