(20 14? As shown in the figure, there is a uniform magnetic field B bounded vertically downwards, and a square wireframe abcd with a single turn length of L and a mass of M..

(1)ab side is equivalent to power supply. According to the cutting formula, there are:

E=BLv

According to the ohm's law of closed circuit, there are:

I=ER=BLvR？

According to ohm's law, there are:

Uab=I×34R=3BLv4

(2) According to the equilibrium conditions, there are:

F=FA+μmg=B2L2vR+μmg

After the tension is removed, the wireframe decelerates at a uniform speed with an acceleration of-μ g. According to the velocity-displacement relationship, there are:

x 1=v22μg

So: d=L+v22μg

(3) Joule heat generated when entering the magnetic field:

Q 1=I2Rt 1=B2L3vR

Heat generated by friction:

Q2 =μmg(L+v 22μg)=μmgL+ 12mv 2

When the wireframe rotates around the ab axis, Em=BL2ω, valid values:

E=BL2ω2

t= 14T=π2ω

Joule heat generation:

Q3 = I2Rt = E2Rt =πb2l 4ω4R；

So the heat generated by the whole process is