Induced Currents
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-395
2025-12-13
12
Induced Currents.(a) When the Conductor is Moved.-Demonstration. -Makea coil of insulated copper wire No. 30, that will slide easily over a long bar magnet, and couple it to a sensitive galvanometer. Place the coil around the magnet at the middle and when the needle of the galvanometer is quiet, suddenly slip the coil off the + end of the magnet. The galvanometer will give a sudden throw, and then gradually come to rest at zero. Place the coil again at the middle of the magnet, and slip it off over the - end. Again there will be a throw of the needle, but in the opposite direction.
FIG.1
Taking the magnet in one hand and the coil in the other, slip the coil on the magnet from the - end. Note the direction of the deflection. Again, slip the coil on the magnet from the + end, and notice the direction of the deflection.
The currents produced in this demonstration are induced currents, and they are produced by the cutting of magnetic lines of force by an electrical conductor. The demonstration shows that the currents produced depend for their direction upon the direction in which the conductor cuts the lines of force. By varying the speed of slipping off the coil, we shall find that the amount of current depends upon the number of lines cut in a given time. The relation between the direction of motion of the conductor, the direction of the lines of force, and the direction of the induced current, is shown in Fig. 394. If a conductor AB is held horizontally and allowed to fall in a magnetic field, cutting the lines of force as shown, then there will be set up at the ends A and Ba difference of potential which will tend to send the current from A to B. This important law may be stated as follows :
If a person, holding a conductor horizontally, stands at a + pole looking in the direction of the lines of force, and lets the conductor fall, the induced current will flow toward the right hand.
Various other rules have been devised to express the law. One of these is as follows: Hold the thumb and the first and second fingers of the right hand in such a way that each shall be perpendicular to the direction of the other two; turn the hand so that the thumb shall point in the direction of the motion, and Motion the first finger in the direction of the lines of force; then the second finger will point in the direction in which the induced current flows.
The rate at which the lines of force are cut determines the induced E. M. F., 1 volt being induced in a conductor when it cuts 100,000,000 lines of force (maxwells) per second.
Since lines of force are closed circuits, it is sometimes convenient to consider a coil of wire and these lines of force as being linked with each. Other. In this case the induced E. M. F. is directly proportional to the rate of change in the number of linkages.
(b) When the Magnet is Moved. - Demonstration. - Couple a coil of small insulated wire, called a secondary coil, to a sensitive galvanometer. Thrust the + end of a long bar magnet into the coil as shown in Fig. 396, and observe the throw of the needle. Pull the + end out suddenly and compare the throw with that obtained at first. Repeat both experiments with the - end of the magnet.
This demonstration, compared with the preceding, shows that it makes no difference whether the coil or the magnet is moved. A current is induced whenever magnetic lines of force are cut by a conductor.
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