Beat your professor twice
kuan Peng
In my 1rst "Beat your professor", I have proposed the new problem <<The
expression of an induced electric field>> in
http://yourcenar.chez.tiscali.fr/. Many people have objected this problem. I
have answered them with success. But nobody has given the right solution.
Only one person has understood the deep sense of this problem: Mr. Kuan-Ting
Lin.
To the students who have dared to show this problem to their professor, I
say: "Congratulation! You win!"
Now, after debate, the second problem is ready: <<What force makes the
current flow?>> in the page http://yourcenar.chez.tiscali.fr/. I invite
students to debate among you and to ask your professor for solution.
If one can critic the first problem to be complicated, the second is very
straightforward: "Does the ring experience a torque or not?" If your
professors fail again to give a valuable answer, you have the right to
wonder if they were good in electromagnetism they teach.
The debate is in the news group sci.physics.electromag
--
Kuan PENG
http://yourcenar.chez.tiscali.fr/
Outline of the Contain
<<What force makes the current flow?>>
Force on a charged ring
1)A primary coil carries an AC current. A secondary coil gets an induced
current.
2)The induced current is due to the induced electric field within the
secondary coil.
3)Replace the secondary coil by a charge dielectric ring. The charges that
are not free to move transmit the electric force to the dielectric matter
resulting a torque on the ring.
4)The primary coil is neutral. It does not receive Colombian force. The ring
is fixed. It does not generate magnetic field. So, the primary coil does not
receive Lorentz' force. In consequence, no force is applied on the primary
coil.
5)The ring receive a torque, the primary coil does not receive a torque.
Newton's 3rd law is violated.
Energy on the ring
1)The ring turns at constant speed. The torque does a work
2)The constant rotation of the ring acts like a constant current. d(B of the
ring)/dt=0. So, the ring does not induce U in the primary coil where no
energy is consumed.
3)The ring receives a work, the coil does not provide energy. The energy
conservation is violated.
<<The expression of an induced electric field>>
On the surface of an emitting wire
1)A wire carries an AC current. The magnetic field is sinusoidal. The
electric field is in phase with the magnetic field as the fields of a plane
wave are in phase. So, anywhere B is variable, E is variable, including the
surface of the wire.
2)Within the conductor, the electric field is U/length. The self induced
voltage is counter balanced by the source voltage. So, the remaining voltage
is RI=0 if R=0.
3)Across the surface, the electric field is variable on the free space
side, but 0 on the conductor side. It cannot be continuous. Discontinuity
lead to infinite curl of E, i.e. infinite dB/dt.
On the surface of a pick up coil
1)Plane wave comes from the left. When B increases, a current occurs in the
coil. The electric field in the conductor is E= RI/length. In the left part
of the coil, the current goes down, in the right, it goes up. So, E within
the coil is negative in the left part and positive in the right.
2)The wavelength is very long. Suppose E is negative in free space over the
entire coil. Within the coil, E is positive in the right part. E is
discontinuous by crossing the surface of the right part from conductor into
free space. If E is positive in free space, E is negative in the left part.
E is discontinuous by crossing the surface of the left part.
3)If there is a charge gradient within the coil in order to make E positive
in both left and right part, there would be extra Q in lower part where the
current is horizontal. As E=RI, there would be horizontal force Q*E that is
not counter balanced by external or internal force.