Electromagnetism Class 10 ICSE, Schemes and Mind Maps of Physics

Download Electromagnetism Class 10 ICSE and more Schemes and Mind Maps Physics in PDF only on Docsity! Electro-Magnetism Oersted’s Experiment on the Magnetic Effect of Electric Current  The following observations were made by Hans Oersted:  The experiment suggests that a current-carrying wire produces a magnetic field around it. This is called the magnetic effect of electric current. Magnetic Field and Field Lines due to a Current in a Straight Conductor  The magnetic field lines form concentric circles around the wire with their plane perpendicular to the straight wire and with their centres lying on the wire.  When the direction of current in the wire is reversed, it is seen that the pattern of iron filings does not change, but the direction of deflection of the compass needle gets reversed.  On increasing the 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 so, it is effective up to a larger distance. Rule to Find the Direction of Magnetic Field  The direction of magnetic field is given by the right-hand thumb rule.  Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then, your fingers will encircle the conductor in the direction of the field lines of the magnetic field. Similarities between a current-carrying solenoid and a bar magnet  The magnetic field lines of a current-carrying solenoid are similar to the magnetic field lines of a bar magnet.  A current-carrying solenoid when suspended freely will set itself in the north–south direction exactly in the same manner as a bar magnet.  A current-carrying solenoid acquires the attractive property of a magnet. When iron filings are brought near the solenoid, it attracts them when current flows through the solenoid. Dissimilarities between a current-carrying solenoid and a bar magnet  The magnetic field strength due to a solenoid can be altered by altering the current in it, while the magnetic field strength of a bar magnet cannot be changed.  The direction of the magnetic field due to a solenoid can be reversed by reversing the direction of the current in it, but the direction of the magnetic field of a bar magnet cannot be changed. Electromagnet  An electromagnet is a temporary strong magnet made of a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet. (a) I-shaped electromagnet (or bar magnet)  An I-shaped electromagnet is constructed by winding a thin insulated copper wire in the form of a solenoid around a straight soft iron bar. The ends of the wire are connected to a battery through an ammeter, rheostat and key.  When current is passed through the winding of a solenoid by closing the key, one end of the bar becomes the South Pole (S) because the current at this face is clockwise, while the other end at which the current is anticlockwise becomes the North Pole (N).  The soft iron bar acquires magnetic properties only when an electric current flows through the solenoid and loses magnetic properties when the current is switched off; thus, it is a temporary magnet. Such magnets are commonly used in a relay. (b) U-shaped (or horseshoe) electromagnet  To construct a horseshoe electromagnet, a thin insulated copper wire is spirally wound on the arms of a U-shaped soft iron core, such that the winding on the two arms as seen from the ends is in the opposite sense.  When current is passed through the winding by closing the key, one end of the arm becomes the South Pole (S) as the current at this face is clockwise, and the other end of the arm becomes the North Pole (N) as the current at this face is anticlockwise.  Thus, we get a very strong magnetic field in the gap between the two poles. The magnetic field in the gap vanishes as the current in the circuit is switched off. It is also a temporary magnet. Such magnets are used in a DC motor, AC generator etc. Ways of increasing the magnetic field of an electromagnet  The magnetic field of an electromagnet (I- or U-shaped) can be increased by the following two ways:  By increasing the number of turns of winding in the solenoid  By increasing the current through the solenoid Uses of Electromagnets Electromagnets are mainly used for the following purposes:  To lift and transport large masses of iron scrap, girders, plates etc.  To load furnaces with iron.  To separate magnetic substances such as iron from debris and raw materials.  To remove pieces of iron from wounds.  In several electrical devices such as electric bell, telegraph, electric tram, electric motor, relay, microphone, loud speaker etc.  In scientific research, to study the magnetic properties of a substance in a magnetic field. Permanent Magnet  A permanent magnet is a naturally occurring magnet.  A strong permanent magnet is made of steel instead of soft iron.  These magnets are used in electric meters (e.g. galvanometer, ammeter, voltmeter) and in a magnetic compass etc. Electromagnet Permanent magnet It produces a magnetic field as long as the current flows through its coils. It produces a permanent magnetic field. It is made of soft iron. It is made of steel. The magnetic field strength can be changed. The magnetic field strength cannot be changed. The polarity of an electromagnet can be reversed. The polarity of an electromagnet cannot be reversed. It can be easily demagnetised by switching off the current. It cannot be easily demagnetised.  An electromagnet has the following advantages over a permanent magnet:  An electromagnet can produce a strong magnetic field.  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.  The polarity of the electromagnet can be changed by reversing the direction of current in its solenoid. Use of electromagnet in an electric bell An electric bell is one of the most commonly used applications of an electromagnet.  The electric bell and its main parts are shown in the figure below: Electromagnetic Induction  Whenever there is a change in the number of magnetic field lines associated with a conductor, an electromotive force (emf) is developed between the ends of the conductor which lasts as long as the change is taking place. This phenomenon is called electromagnetic induction. Conclusion  A current flows in the coil only when there is a relative motion between the coil and the magnet due to which the galvanometer connected with the coil shows deflection.  The direction of deflection in a galvanometer is reversed if the direction of motion (or polarity of the magnet) is reversed.  The current in the coil is increased i. By the rapid motion of the magnet (or coil) ii. By using a strong magnet iii. By increasing the area and the number of turns in the coil Faraday’s Laws of Electromagnetic Induction i. Whenever there is a change in the magnetic flux linked with a coil, an emf is induced. The induced emf lasts as long as there is a change in the magnetic flux linked with the coil. ii. The magnitude of the emf induced is directly proportional to the rate of change of the magnetic flux linked with the coil. When the rate of change of the magnetic flux remains uniform, a steady emf is induced. Factors Affecting the Magnitude of Induced EMF The magnitude of induced emf is equal to the rate of change of magnetic flux, i.e. Change in magnetic flux Induced e.m.f. Time in which the magnetic flux changes  Thus, for a given coil and magnet, emf depends on the following two factors: (i) Change in the magnetic flux (ii) Time in which the magnetic flux changes Direction of Induced EMF The direction of induced emf (and hence the direction of induced current) can be obtained by any of the following two rules:  Fleming's right-hand rule: Stretch the thumb, middle finger and forefinger of your right hand mutually perpendicular to each other. If the forefinger indicates the direction of the magnetic field and the thumb indicates the direction of the motion of the conductor, then the middle finger will indicate the direction of the induced current.  Lenz's law: The direction of induced emf (or induced current) always tends to oppose the cause which produces it. AC Generator  An AC generator is a device which converts mechanical energy into electrical energy using the principle of electromagnetic induction.  In a generator, a coil is rotated in a magnetic field. Due to rotation, the magnetic flux linked with the coil changes and therefore an emf is induced between the ends of the coil. Thus, a generator acts like a source of current if an external circuit containing a load is connected between the ends of its coil.  The figure below represents the emf induced between the ends of the coil with respect to the position of the coil in the magnetic field when seen along the axis of rotation from the position of slip rings. Frequency of Alternating Current  In one complete rotation of the coil, we get one cycle of alternating emf in the external circuit.  The alternating emf thus produced has a frequency which is equal to the frequency of rotation of the coil.  If the coil makes n rotations per second, then the magnitude of induced emf is given as 0e e sin2 nt  and the current is expressed as 0i i sin2 nt 