al phy mc d

(1)

the key point is that v is constant
why??
i)the magnetic force acting on it is perpendicular to its velocity
ii)the magnetic force cannot change the speed of the charge
iii)the work done by the magnetic force is used to change the velocity(direction)
that is changing the direction of motion

w is changing as r changes
(v is constant)

(2)

(A)
obvious...

(B)
NO
B is positive, negative charge is induced in the inner side
so field lines as pointing radially outwards

(C)
NO
the effect of the positive charge inside is shielded


(3)

A)NO
copper has definite m.p. ->don't melt it
glass has no definite m.p. ->melt it
the above two statements are not cause-reslut

B)
No...
rubber has a smaller Young modulus but we just pull it by hands

C)
yes...
because if you hammer a piece of glass, it breaks

D)
No...
If galss could have plastic deformation, we can make glass pates by exerting LARGER force/pressure... we have machines now, why not??


2010-02-20 23:19:43 補充：
2C correction
same potential
for a conductor, the potential everywhere is the same
and the field inside it is zero, by gauss's law

Just supplement some information to the answers of the questions:

1. Basically, both equations are the same. But you should be aware that the angular speed w depends on the radius r, which is also changing with changing velocity v. Thus the other equation is more suitably used with v and r separated.

2. The tension depends on the weight of the bob and its position.

3. Copper (in fact, most metals) are ductile and mellable. Glass is brittle. As such, copper shows a plastic deformation region, whereas glass doesn't.