INTERACTIVE EXPERIMENTS AND EXHIBITS
This is the name given to the forces exerted between conductors through which electricity flows and also between magnets and conductors. Let’s examine the objects we will use: a ring magnet, mobile wires.The poles of these magnets are on the opposite flat faces.
In this photo we see a piece of straight wire suspended over the pole of the magnet. Let’s see what happens when we electrify the wire.
In this case the force acting on the wire is perpendicular both to the wire and to the field, thus directed either to the right or left of the apparatus, as we have seen. The force with which a magnetic field acts on an electrified wire is called the “Ampere current”.
The elementary Ampere current dF that acts on a conducting element of length dl through which current I flows is given by I times the vectorial product of the conducting element for the vector of magnetic induction:
dF = I * dl x B
(The magnitudes in boldface are vectors)
The following "left-hand rule" supplies the direction of the Ampere current when current and field are perpendicular:
Here we have two small DC motors connected to each other. They were taken from two broken CD-Rom drives found in a computer.
These small electric motors, the working of which is based on the phenomena seen above, are in fact perfectly reversible.
To see what happens click on the photo
Another experiment in which we see what happens when a DC current flows through a mobile round coil placed in front of the pole of a cylindrical magnet.
It was evident that a translation had to take place. But now let’s see what happens if the coil can turn freely around a vertical axis.
The coil behaves like the magnetic needle of a compass.
For confirmation, let’s examine another experiment.
Another far more delicate experiment: in place of the magnetic needle of Arago’s disk, we put our coil through which electricity passes.
This experiment is also quite interesting.
ACTIONS BETWEEN PARALLEL CURRENTS
This is a closeup of another apparatus that shows what happens when electricity passes through two parallel conductors. The complete apparatus is shown below. The way in which the two conductors are connected appears in the photo below:
The magnetic force of attraction between the two conductors is very slight.
THE LORENTZ FORCE
Hendrik Antoon Lorentz (1853-1928), Holland, a great of Physics.
This is a cathode ray tube. A thin beam of electrons is focused on the inside of a fluorescent screen and produces the luminous green dot that can be seen slightly below the centre.
Still another experiment: the effect of a magnetic field on two parallel conductors with concordant currents flowing through them.
The Lorentz force:
This is the first Magnetron, an historical laboratory apparatus, an electronic tube that generates coherent microwaves of great power, just as it was created in 1940 by Randall and Boot in Birmingham.
In it, an axial magnetic field forces the electrons emitted by the cathode to bend into circular orbits and cede energy to the anode in the form of an intense signal at an extremely high frequency. This is the actual object that was sent in great secrecy to the United States so that it could be produced industrially as the core of RADAR, an apparatus that gave the Allies a great tactical advantage during the Second World War.
This is the form that the magnetron has now assumed for use in household microwave ovens: it usually supplies 800 W continuous at the frequency of 2.45 GHz, wavelength 12.2 cm. The magnetic field is created by the two ring magnets in the space at the sides of the finning of the anode, coaxial to the “antenna” of the exit of the radio frequency energy visible at the top. The connector at the bottom right is for powering the filament.
The Ampere current that we saw in the experiment of the conductor that moves on the magnet is originated by the Lorentz force that acts on the charges that move inside the wire.
The Corkscrew Rule: the direction is that of the advancement of a corkscrew when this rotates in the same direction as the current.