Pushing wires with magnetism

Science · Forces & Motion · ages 16-18

Name ______________________ Date ____________

Conventional current flows due north in a wire. The field points straight down. The left hand rule gives a force due west.
Circle one: True False
In F equals BIL sin theta, when is F largest?
- Wire at 90 degrees to the field
- Wire lying along the field lines
- Wire at 30 degrees to the field
What is one tesla?
- One joule per coulomb
- One newton per amp metre
- One volt per metre
What force does an 8 cm wire carrying 15 A feel in a 0.5 T field at right angles?
- 0.6 N
- 0.06 N
- 60 N
A 0.5 m wire carries 3 A at right angles in a 2 T field. F equals BIL. Give F in newtons.
Answer: ______________
A pupil uses electron flow in the left hand rule. What happens?
- The size of the force doubles
- The size and direction are unchanged
- The predicted direction flips
A pupil says a tesla is one newton per coulomb. What is wrong?
- Per coulomb is fine because current is charge
- A tesla is a unit of charge, not of field
- It confuses charge with current and omits L
A 2 m wire carries 4 A at 30 degrees to a 3 T field. Sin 30 is 0.5. Give F in newtons.
Answer: ______________

Answer key

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

True · First finger down along the field, second finger north along the current, and the thumb points due west.
Wire at 90 degrees to the field · Sin 90 is one, the biggest value it can take.
One newton per amp metre · The tesla is read straight from F equals BIL.
0.6 N · 0.5 times 15 times 0.08 gives 0.6 N.
3 · Multiply 2 by 3 by 0.5 to get 3 N.
The predicted direction flips · Electron flow runs in the opposite direction, so the thumb predicts the opposite force direction.
It confuses charge with current and omits L · F = B I L gives B = F/(I L). The pupil replaces current I with charge and leaves out wire length L. These are two separate errors.
12 · Multiply 3 by 2 by 4 by 0.5 to get 12 N.