Magnetic Fields & Forces on Conductors

[Jimmy87]

Homework Statement

Multiple Choice Question

Relevant Equations

F = BIL sin (θ)

Hi,

Here is a multiple choice question I am stuck with and would appreciate some guidance:

The mark scheme for this paper says it is option A - 0N. I didn't get that at all. If a current carrying wire is in a magnetic field with some component perpendicular to the field then how can it be zero?! I got option B by doing

F = BIL sin (θ) where θ is the angle between B and I.
F = (0.08T x 0.5A x 0.01m) sin (30) = 2.0 x 10^-4 N (to 2 sig figs)

Have I missed something?

Thanks.

 

[TSny]

The wording of the problem is not particularly clear. It asks for the force on a section of the loop, not the force on a section of the wire. A section of the loop might be interpreted as containing two sections of the wire.

 

[Cutter Ketch]

In my opinion this is almost deceptively tricky. However note that they said “1cm section of the loop”, not “1cm section of the wire”. I believe they mean you to understand that a “section of the loop” includes both wires. While there is indeed a force on the outgoing and returning wires, the forces on each are in opposite directions. The net force is zero. There would be a torque on the loop, but the wires are close together making the lever arm small, so the torque is also small.

 

[Jimmy87]

In my opinion this is almost deceptively tricky. However note that they said “1cm section of the loop”, not “1cm section of the wire”. I believe they mean you to understand that a “section of the loop” includes both wires. While there is indeed a force on the outgoing and returning wires, the forces on each are in opposite directions. The net force is zero. There would be a torque on the loop, but the wires are close together making the lever arm small, so the torque is also small.

Thanks. Wow that’s crazy! I see what you mean now. The resultant force on a section of the LOOP is zero. That’s mean . They should have at least but loop in bold font.

Thanks guys.