Through Faraday's law of electromagnetic induction? e =△φ△t = B△S△t = bl×l△t？ ①

And I=q△t is defined by current? ②

I=ER comes from ohm's law of closed circuit? ③

L=qRBL is obtained from ① ② ③.

(2) According to the functional principle, -Wf+(-Q)=0- 12mv20.

And wf = μ GL = μ g? qRBL

∴Q= 12mv20？ μmgqRBL

So the answer is: qRBL 12mv20? μmgqRBL。