Tom and Josh:
1) Tom's statement: " The energy gained by the marble then comes from the
flow of energy due to the normal force alone." didn't parse well for me
because it seems to imply that the x directed normal force can DIRECTLY
affect the marble's y velocity component. It cannot; the details of the
indirect mechanism must be stated: It accomplishes energy transfer to the
marble's y velocity component by using the magnetic force as a "go between".
Josh puts it this way: "B transfers velocity from the x- dim. to the y-dim.,
and the normal force "replenishes" that x velocity."
By pushing in the x direction to maintain the charged marble's x velocity
component, the tube provokes the qVxB force into existence to push the
marble in the y direction.
2) From the standpoint of energy flow:
The tube's normal force pushes the marble against the opposing x component
of the B force. This represents a transfer of energy from the pushing agent
to the tube, to the marble (x motion) and to the B field. The y component
of the B force simultaneosly transfers this energy to the marble's y motion.
All of this goes on continuosly and simultaneously, so that the B field
neither gains nor loses energy, the agent pushing the tube in the x
direction loses energy, and the marble gains this energy in its increasing y
velocity.
3) A loosley related instance of the operation of an energy transferring
agent:
Suppose I lift a bowling ball at a constant vertical velocity against the
equal and opposite weight force of the ball. This represents an energy
transfer from me to the ball, but that is not the whole story. The force
between the ball and the earth simultaneously and continuosly transfers this
energy to the gravitational potential energy of the ball-earth system.
There is no change of direction here, but it does illustrate how the bowling
ball's interactions with both me and the earth, can act as an energy
transfer agent. Energy flows from me to the ball to the gravitational
earth-ball system. I'm sure there are better examples - that's what comes
to mind now.