Il Museo di Fisica di Sardegna

	Interactive experiments and exhibits News Events F G C
	Introduction Arago's disk Oersted's experiment Electromagnetic induction The Foucault current Electrodynamic actions Coils, solenoids and other things Measurement of a magnetic field Magnetic properties of materials Levitation experiment And if there's no variation in flow? The rotating magnetic field High frequency fields Photos	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. Click on the photo to see the experiment 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. To see what happens, click on the photo. 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. To see what happens click on the photo The coil behaves like the magnetic needle of a compass. It can be demonstrated (Ampère did it in 1820, immediately after learning of Oersted’s discovery) that the coil is equivalent to a magnetic dipole which has a moment of module m = I.S, direction perpendicular to the plane of the spiral and face given by the "corkscrew rule", since I is the intensity of the current and S is the surface of the coil. Thus if the coil can rotate it will orient itself in the field placing its normal in the direction of the field. For confirmation, let’s examine another experiment. In which the fact that the coil is equivalent to a dipole having a certain moment is quite evident. The current reaches the coil by means of two plaits of very thin, extremely pliable copper wires. Click on the photo to see the 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. Click on the photo to see the experiment 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: Click on the photo to see the experiment The magnetic force of attraction between the two conductors is very slight. It is given by the expression that appears in the figure and is valid for both direct and alternating current. Since µ₀ = 4π * 10^-7 H/m we can see that with a current of 10 A in two parallel conductors 1 cm apart, for each length of 10 cm the force between them is 2 * 10^-5 N amounting to about 2 mg. THE LORENTZ FORCE Hendrik Antoon Lorentz (1853-1928), Holland, a great of Physics. The deflection of cathode rays in traditional television sets and monitors (not those with liquid crystals or plasma), in oscilloscopes or of other charged particles in accelerators is an effect formalized by Lorentz, like the Hall effect in solid conductors that bears this name and many other phenomena. 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. The deviation of the electrons is caused by the Lorentz force. FTo see what happens when a magnet is brought close to it, click on the photo Still another experiment: the effect of a magnetic field on two parallel conductors with concordant currents flowing through them. Why is it that on placing a magnet so that the two opposite poles symmetrically face the two conductors the conductors deflect in opposite directions even though the currents are concordant? To see experiment click on the photo The Lorentz force: the effect of a magnetic field on a charge moving at velocity v: A force, known as the “Lorentz force”, given by the expression in the figure, acts on an electrical charge moving in a magnetic field perpendicular to the plane of velocity and the field. In the figure we can see the direction of the force for a positive charge; for a negative charge the direction is the opposite one. 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. The glass tube is evidently for emptying the apparatus; we can also see the tubes that form the cooling sleeve. The two square plates were inserted between the polar expansions of a large electromagnet. 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.