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A) MAGNETIC EFFECT OF ELECTRIC CURRENT: OERSTED'S EXPERIMENT ON THE MAGNETIC EFFECT OF ELECTRIC CURRENT Hans Oersted, in 1820, in his experiments observed that when an electric current is passed through a conducting wire, a magnetic field is produced around it. The presence of magnetic field at any point around the current carrying wire can be detected with the help of a compass needle. Explanation: The deflection of magnetic needle on passing current in the wire, clearly indicates the creation of magnetic field around the wire. Thus, on passing current in the wire, a magnetic field is produced around it and the magnetic needle of compass experiences a torque in this magnetic field, so it deflects to align itself in the direction of magnetic field at that point. On increasing current in the wire, the deflection of the compass needle increases which implies that the strength of magnetic field around the wire increases. On reversing the direction of current in the wire, the direction of deflection of the magnetic needle of compass reverses because the direction of magnetic field reverses. Inference: A current (or moving charge) produces a magnetic field around it. This is called the magnetic effect of current. The strength of magnetic field depends on the magnitude of current and its direction depends on the direction of current. MAGNETIC FIELD AND FIELD LINES DUE TO CURRENT IN A STRAIGHT WIRE From the magnetic field lines pattern, we note that (1) The magnetic field lines form the concentric circles around the wire, with their plane perpendicular to the straight wire and with their centres lying on the wire. (2) When the direction of current in the wire is reversed, the pattern of iron filings does not change, but the direction of deflection of the compass needle gets reversed. The north pole of the compass needle now points in a direction opposite to the previous direction showing that the direction of magnetic field has reversed. (3) On increasing current in the wire, the magnetic field lines become denser and the iron filings get arranged in circles up to a larger distance from the wire. showing that the magnetic field strength has increased and it is effective up to a larger distance. RULE TO FIND THE DIRECTION OF MAGNETIC FIELD Experimentally the direction of magnetic field at a point is determined with the help of compass needle . But theoretically the direction of magnetic field (or magnetic field lines) produced due to flow of current in a conductor can be determined by various rules. One such rule is the right-hand thumb rule. Right hand thumb rule If we hold the current carrying conductor in our right hand such that the thumb points in the direction of flow of current, then the fingers encircle the wire in the direction of the magnetic field lines. MAGNETIC FIELD DUE TO CURRENT IN A LOOP (OR CIRCULAR COIL) From the pattern of magnetic field lines, it is noted that: 1) In the vicinity of wire at P and Q, the magnetic field lines are nearly circular. 2) Within the space enclosed by the wire (i.e., between P and Q), the magnetic field lines are in the same direction. 3) Near the centre of loop, the magnetic field lines are nearly parallel to each other, so the magnetic field may be assumed to be nearly uniform in a small space near the centre. 4) At the centre, the magnetic field lines are along the axis of loop and normal to its plane. 5) The magnetic field lines become denser (i.e., the magnetic field strength is increased) if (i) the strength of current in loop is increased, and (ii) the number of turns in the loop is increased. Clock rule (clockwise current-south pole and anticlockwise current-north pole) MAGNETIC FIELD DUE TO A CURRENT CARRYING CYLINDRICAL COIL (OR SOLENOID) If a conducting wire is wound in form of a cylindrical coil whose diameter is less in comparison to its length, the coil is called a SOLENOID . It looks like a helical spring. To obtain the magnetic field lines due to a current carrying solenoid, the following experiment is performed. From the pattern of magnetic field lines, it is found that: (1) The magnetic field lines inside the solenoid are nearly straight and parallel to the axis of solenoid i.e., the magnetic field is uniform inside the solenoid. (2) The magnetic field lines become denser (i.e., a strong magnetic field is obtained) on increasing current in the solenoid. (3) The magnetic field is increased, if the number of turns in the solenoid of given length is increased. (4) The magnetic field is also increased, if a soft iron rod (core) is placed along the axis of Di solenoid. The soft iron increases the strength of magnetic field of the solenoid as soft iron has a high magnetic permeability. (5) In Fig., the end P at which the direction of current is anticlockwise behaves as a north pole (N), while the end Q at which the direction of current is clockwise behaves as a south pole (S). On reversing the direction of current in the solenoid, the polarities at the ends of solenoid are reversed because the direction of magnetic field has reversed. Similarities between a current carrying solenoid and a bar magnet : (1) The magnetic field lines of a current carrying solenoid are similar to the magnetic field lines of a bar magnet. Thus, a current carrying solenoid behaves just like a bar magnet. (2) A current carrying solenoid when suspended freely sets itself in the north-south direction exactly in the same manner as a bar magnet does. (3) A current carrying solenoid also acquires the attractive property of a magnet. If iron filings are brought near the current carrying solenoid, it attracts them. Dissimilarities between a current carrying solenoid and a bar magnet: (1) The strength of magnetic field due to a solenoid can be changed by changing the current in it, while the strength of magnetic field due to a bar magnet cannot be changed. (2) The direction of magnetic field due to a solenoid can be reversed by reversing the direction of current in it, but the direction of magnetic field due to a bar magnet cannot be reversed. Electromagnet: An electromagnet is a temporary strong magnet made by passing current in a coil wound around a piece of soft iron. It is an artificial magnet. An electromagnet can be made in any shape, but usually the following two shapes of electromagnet are in use: (1) I-shape (or bar) magnet, and (2) U-shape (or horse-shoe) magnet. Construction of I Shaped (or bar) electromagnet: Construction of U Shaped (or horse shoe) electromagnet: Ways of increasing the magnetic field of an electromagnet The magnetic field of an electromagnet (I or Ushaped) can be increased by the following two ways: (1) by increasing the number of turns of winding in the solenoid, and (2) by increasing the current through the solenoid. Permanent Magnet : A permanent magnet is a naturally occurring magnet. Since it is not strong enough and also not of the required shape for many purposes, so a strong permanent magnet is made like an electromagnet using the piece of steel, instead of soft iron. Difference between Electromagnet and a Permanent Magnet: Electromagnet 1. It is made of soft iron. 2. It produces the magnetic field so long as current flows in its coil. 3. The magnetic field strength can be changed. Permanent Magnet 1. It is made of steel. 2. It produces a permanent magnetic field. 3. The magnetic field strength cannot be changed. ADVANTAGES OF AN ELECTRO MAGNET OVER A PERMANENT MAGNET An electromagnet has the following advantages over a permanent magnet: (1) An electromagnet can produce a strong magnetic field. (2) The strength of the magnetic field of an electromagnet can easily be changed by changing the current (or the number of turns) in its solenoid. (3) The polarity of the electromagnet or the direction of the field produced by it can be reversed by reversing the direction of current in its solenoid. USES OF ELECTROMAGNET Electromagnets are mainly used for the following purposes: (1)For lifting and transporting heavy iron scrap, girders, plates, etc. particularly when it is not convenient to take the help of human labour. Electromagnets are used to lift as much as 20,000 kg of iron in a single lift. To unload the iron objects at the desired place, the current in the electromagnet is switched off so that the electromagnet gets demagnetised and the iron objects get detached. (2) For loading the furnaces with iron. (3) For separating the iron pieces from debris and ores, where iron exists as impurities (e.g., for separating iron from the crushed copper ore in copper mines). (4) For removing the pieces of iron from wounds. (5) In scientific research, to study the magnetic properties of a substance in a magnetic field. (6) In several electrical devices such as electric bell, telegraph, electric tram, electric motor, relay, microphone, loud speaker, etc. Use of electromagnet in an electric bell: An electric bell is one of the most commonly used application of an electromagnet. Note: If an a.c. source is used in place of the battery, the core of electromagnet will get magnetised, but the polarity at its ends will change. Since attraction of armature does not depend on the polarity of the electromagnet, so the bell will still ring on pressing the switch K. B) FORCE ON A CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD AND ITS APPLICATION IN D.C. MOTOR FORCE ON A CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD: Lorentz found that a charge moving in a magnetic field, in a direction other than the direction of magnetic field, experiences a force. It is called the Lorentz force. Magnitude of force: Experimentally it is found that the magnitude of force acting on a current carrying wire placed in a magnetic field in the direction perpendicular to its length, depends on the following three factors: (a) The force F is directly proportional to the current I flowing in the wire (b) The force F is directly proportional to the strength of magnetic field B. (c) The force F is directly proportional to the length 1 of the wire (within the magnetic field) Combining the eqns. (i), (ii) and (iii), F = K IBI where K is a constant, whose value depends on the choice of the unit. In S.I. units, the unit of B is such that K = 1 Then F = IBl Unit of magnetic field: B= From the above equation, so the S.I Unit of magnetic field is or . It is also named as Tesla (symbol T) or Weber / meter2 (symbol Wb m-2). Fleming s left hand rule for the direction of force: Fleming s left hand rule : Stretch the forefinger, central finger and the thumb of your left hand mutually perpendicular to each other as shown in Fig. 10.23. If the forefinger indicates the direction of magnetic field and the central finger indicates the direction of current, then the thumb will indicate the direction of motion of conductor (i.e., force on conductor). Simple D.C. Motor: An electric motor is a device which converts the electrical energy into mechanical energy. C) ELECTROMAGNETIC INDUCTION AND ITS APPLICATIONS TO A.C. GENERATOR Electromagnetic Induction: Whenever there is a change in the number of magnetic field lines linked with a conductor, an electromotive force (e.m.f.) is developed between the ends of the conductor which lasts as long as there is a change in the number of magnetic field lines through the conductor. This phenomenon is called the electromagnetic induction. Demonstration of the phenomenon of electromagnetic induction: Conclusions: (1) A current flows in the coil only when there is a relative motion between the coil and the magnet. (2) The direction of current is reversed if the direction of motion (or polarity of the magnet) is reversed. (3) The current in the coil is increased by (i) the rapid motion of magnet (or coil), (ii) the use of a strong magnet, and (iii) increasing the area of cross section of coil and by increasing the number of turns in the coil. Flemmi2ng s right hand rule: Stretch the thumb, central finger and forefinger of your right hand mutually perpendicular to each other as shown in Fig. If the forefinger indicates the direction of magnetic field and the thumb indicates the direction of motion of the conductor, then the central finger will indicate the direction of induced current.
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