I am so glad I found the link to this test, so I can reference questions with figures.(adsbygoogle = window.adsbygoogle || []).push({});

let me write the problem first, and provide the link at the end, and in between explain where I was stuck.

87) two small pith balls, each carrying a charge q, are attached to the ends of a light rod of length d, which is suspended from the ceiling by a thin torsion-free fiber, as shown in the figure above ( in the link). There is a uniform magnetic field B, pointing straight down, in the cylindrical region of radius R around the fiber. The system is initially at rest. If the magnetic field is turned off, which of the following describes what happens to the system?

I found the emf induced around the loop of Radius R, and I integrated from 0 to R to find the torque on the bar, and I multiplied my answer by 2, because there are two fields on the bar of opposite direction ( a loop), which will add to the torque, so I got that the bar will rotate with angular momentum = qBR^2/2,

but the correct answer is twice as much.

here is it with the figure:

http://phys.columbia.edu/~hbar/Physics-GRE.pdf [Broken]

thankyou so much!

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Gre Problem #87

**Physics Forums | Science Articles, Homework Help, Discussion**