I don't understand the mechanism by which "flux tubes" create heat, and are pinned by impurities in a type II superconductor. When Bc1<B<Bc2 I understand magnetic flux will penetrate the material in the form of flux tubes. Then the current in the superconductor exerts a force on these tubes driving them through the material and generating heat. My first question is how does this movement create heat. It's not apparent to me why this is the case. I also understand these tubes are "pinned" by impurities in the material (stopping their movement). I don't understand how these impurities keep the flux from moving. I know in the case of NbTi the Ti precipitates are responsible for blocking the flux movement, but I'm not sure how they do it. I'm having a hard time finding resources to answer these questions, so suggestions on papers/books would be great if this post is too much to answer.