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Re: [Phys-l] Magnetic force and work



Bob,

If I understand your comments, you are drawing attention to the "flow"
of energy from one part of the system to another. That's an interesting
way of looking at it. I vaguely remember seeing an AJP article that
might have discussed this.

Let's go back to the marble in the tube and consider the two forces
acting on the marble; namely, the normal force of the wall and the
magnetic force. It seems to me that we must consider the marble as
getting its energy from one or both of these forces.

The normal force does positive work on the marble. Is it OK to say,
therefore, that some energy "flows into the marble" via this force? The
magnetic force does zero total work. But, you can break the magnetic
force into x and y components. The x-component does negative work on
the marble. Is it OK to say that energy flows out of the marble through
this component of force? The y-component of magnetic force does
positive work on the particle. Is it OK to consider energy flowing into
the particle through this component?

If the answer is yes to the above questions, then how do we follow the
net energy flow? It seems to me that this could be done in different
ways.

One way: We could say that the energy flow into the marble by the
y-component of magnetic force is equal to the energy flow out of the
marble by the x-component of magnetic force. We then have a "loop" of
flow of energy into and out of the marble due to the magnetic force with
no net energy gained by the marble in this loop of energy flow. The
energy gained by the marble then comes from the flow of energy due to
the normal force alone.

Another way: We could consider a loop of energy flow going into the
marble via the normal force but out of the marble via the x-component of
magnetic force. This loop gives zero net energy to the marble. Then we
would say that the energy gained by the marble is the energy "flowing
in" due to the y-component of magnetic force.

Is one way preferable to the other? Or am I completely missing your
point?

Thanks,
Tom

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bob
Sciamanda
Sent: Monday, March 27, 2006 5:22 PM
To: Bob Sciamanda; Forum for Physics Educators
Subject: Re: [Phys-l] Magnetic force and work

I think I need to emphasize the important idea in this exchange withh
Tom
Snyder:

The situation in which two different agents exert equal and opposite
forces
on each other or on a particle can (but does not always) represent a
transfer of energy from one agent to the other (perhaps through the
intermediary particle).

1) If I push on a car and accelerate it, the set of equal and oppoite
forces
represents a transfer of energy from me to the car.

2) I f I push a crate against a frictional force so as to maintain its
constant velocity, the positive work which I do (against equal and
opposite
fricional forces) represents a transfer of energy from me (through the
crate
motion) to the various thermal energy modes of the crate and the floor.

3) If I (on frictionless ice skates) push off from an infinitely rigid
wall,
there is no energy transfer between the wall and me. The work energy
theorem
here guarantees that the work done by the wall's push is numerically
equal
to my increase in kinetic energy, but it says nothing to identify the
source
of my kinetic energy increase.

Bob Sciamanda