 |
| Magnetic Effect of Electric Current |
|
| Major progress in understanding magnetism came after the relationship between electricity and magnetism was established by Hans Christian Oersted in 1820. He found that an electric current moves a compass needle and this effect lasts as long as the current flows through the wire. It is then possible to produce magnetism without any magnetic substance at all. |
| |
| A coil of wire could produce a magnetic field exactly like the field around a permanent magnet. |
| |
|
| A magnetic field is defined as a region in which a magnetic force is present. In a magnetic field, the magnetic dipole (two equal and oppositely charged or magnetized poles separated by a distance) experiences a turning force, which tends to align it parallel to the direction of the field. The concept of a magnetic field can be understood with the help of the following activity: |
| |
 Place a piece of cardboard over a magnet |
| |
Sprinkle some iron filings onto the cardboard |
| |
 Tap the cardboard gently and draw what you see |
| |
 The iron filings show the magnetic field of the magnet |
| |
|
| The direction of the magnetic field due
to a current may be studied by drawing the magnetic lines of force. A
vertical wire AB is passed through a horizontal cardboard PQRS. Ion filings
are sprinkled on the cardboard. Current is passed through it by connecting a
battery to it. Iron filings spread evenly on the cardboard. When a compass
needle is placed on the cardboard, the direction of the needle will show the
direction of the magnetic field. The point on the cardboard where the north pole of the needle is siturated is marked. The needle is shifted a little so that its south pole takes the same position where the north pole was situated previously. The position of the north pole is marked. If the current is strong the lines will be circular. The arrows on the circular lines show the direction of the magnetic field. |
| |
 |
| |
| Magnetic Field Lines Due to Straight Wire |
| |
| If the direction of the current is reversed, the lines will still be circular, but the directions of the lines will be reversed, which can be verified using the compass needle. |
| |
|
| The direction of the magnetic field
around a current carrying conductor can be explained by a simple rule known
as Maxwell's right hand grip rule. If we hold the current carrying wire in
our right hand in such a way that the thumb is stretched along the direction
of the current, then the curled fingers give the direction of the magnetic
field produced by the current. |
| |
 |
| |
| Maxwell's Right Hand Grip Rule |
| |
|
| When a long wire is coiled in the shape of a spring so that the turns are closely spaced and insulated from each other it forms a solenoid. Generally, a wire is coiled over a non-conducting hollow cylindrical tube. An iron rod is often inserted inside the hollow tube. This rod is called the core. |
| |
 |
| |
| Magnetic Field due to a Solenoid |
| |
| The free ends of the solenoid are
connected to a battery to pass current through the solenoid. This produces a
magnetic field. The magnetic field inside the coil is almost constant in
magnitude and direction. The current carrying solenoid produces magnetic
field similar to that of a bar magnet. One end of the solenoid becomes the
north pole and the other end becomes a south pole. |
| |
| The magnitude of the field depends on the following factors. The magnetic field is directly proportional to: |
| |
 the amount of current passing through the solenoid |
| |
 the number of turns of the solenoid. It also depends on the core material. |
| |
| Since the magnetic field formed by the solenoid is temporary it is used to make electromagnets. Electromagnets
are used in electric bells, cranes, etc. |
| |
|
| The process of producing electricity by magnetic field is called electromagnetic induction. |
| |
| Electric current can also be induced
through a wire loop, by moving it near a fixed magnet. So a current is
induced either by moving a magnet near the loop or by moving the loop near a
magnet. It is the relative motion between the two which is important. It
does not matter which of the two is moved. Thus the electromagnetic
induction takes place because of the relative motion between a magnet and a
coil. The induced current exists as long as there is a relative motion
between the coil and the magnet. |
| |
| When the magnet is moved faster, then the amount of current induced is found to be higher. Normally moving the magnets in a linear fashion is difficult. Hence a different arrangement is used. |
| |
| The figure given below shows a wire loop, a section AB of which lies in a magnetic field. A galvanometer is connected to the loop. |
| |
 |
| |
| Electromagnetic Induction |
| |
| The wire is directed along south-north direction and the magnetic field is from west to east. When the loop is pulled up such that the wire AB moves upwards in the field, a current is induced in the loop as shown in the figure. The direction of the current will be from A to B, i.e., from south to north. If the loop is pushed down vertically, the direction of the current in the wire will be from B to A. |
| |
 |
| |
|
| |
| The direction of the current in a wire moving perpendicular to itself and to a magnetic field may be found by Fleming's right hand rule. If the thumb, forefinger and middle finger of the right hand are stretched in a mutually perpendicular direction, in such a way that the forefinger directs towards the magnetic field, the thumb shows the motion of the wire, then the middle finger shows the direction of the induced current. |
| |
| So the phenomenon electromagnetic induction paved us the way to generate current without the electrochemical cells. It formed the principle underlying the working of dynamos. |
| |
