# Is this solution in Conquering the Physics GRE incorrect? [closed]

In Conquering the Physics GRE 2nd edition, Exam 3 problem 20, the statement is as follows: A metal bar is pulled at constant velocity $$v\mathbf{\hat{x}}$$ along two metal rails a distance $$d$$ apart connected by a resistor or resistance R, as shown in the diagram. There is a magnetic field, pointing into the page, of magnitude $$B = Cx$$, where $$x=0$$ is the initial position of the bar. At time $$T$$, how much energy has been dissipated in the resistor thus far, as a function of $$T$$?

The solution is to first find the flux as a function of time, then the EMF, then the power dissipated by the resistor, then the energy. My method is to compute the flux as:

$$\Phi = \iint \mathbf{B}\cdot d\mathbf{S} = \iint (Cx\mathbf{\hat{z}})\cdot(dx\,dy\,\mathbf{\hat{z}}) = \int_0^d dy \int_0^{vt} dx\,Cx = \frac{1}{2}Cd(vt)^2 = \frac{1}{2}Cv^2t^2d$$

However, the solution says "Since the magnetic field is perpendicular to the loop, the flux through the loop is $$\Phi = BA = C(vt)(vtd) = C v^2t^2 d$$." The errata states that one of the answers was misprinted but that the solution is correct. Am I crazy or is the solution wrong? $$\Phi = BA$$ should only be true if $$B = const.$$ correct?

• On the very last page of the linked errata, their correction is precisely what you say here. – J. Murray Dec 22 '18 at 22:28
• I didn’t read your question, but I found an error in one of their solutions and just sent them an email. They’re very quick to respond – InertialObserver Dec 22 '18 at 22:43
• Oh I did not realize that the errata was separated into versions, I only saw the version 2 correction. Thanks for pointing that out @J.Murray, glad I'm not crazy – Kai Dec 22 '18 at 22:52
• It seems to depend on what is meant. Perhaps they mean there is a time changing (but uniform) B field? Because the way you have interpreted it B is not divergence-less (and the problem is also sensitive to the location of the origin, which is not well-specified). – Green Apples Dec 22 '18 at 22:58