Electromagnetism Class 10 Past Paper 2024 PDF

What is Magnetic Effect of a Current ? A current produces a magnetic field around it. This called Magnetic Effect of a Current. Electromagnetism is a branch of Physics, that deals with the electromagnetic force that occurs between electrically charged particles. Why is it called electromagnetism? It...

What is Magnetic Effect of a Current ? A current produces a magnetic field around it. This called Magnetic Effect of a Current. Electromagnetism is a branch of Physics, that deals with the electromagnetic force that occurs between electrically charged particles. Why is it called electromagnetism? It's called the electromagnetic force because it includes the formerly distinct electric force and the magnetic force; magnetic forces and electric forces are really the same fundamental force. The electromagnetic force is one of the four fundamental forces -gravitational, electromagnetic, strong and weak. (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. He used magnetic compass to find the direction. Experiment: AB is a wire lying in the north-south direction and connected to a battery through a rheostat and a taping key. A compass needle is placed just below the wire. Observations: (1) When the key is open i.e., no current passes through the wire, the needle points in the N-S direction with the north pole of needle pointing towards the north direction. In this position, the needle is parallel to the wire. (2) When the key is pressed, a current passes in the wire in the direction from A to B and the north pole (N) of the needle deflects towards the west. (3) When the direction of current in the wire is reversed by reversing the connections at the terminals of the battery, the north pole (N) of the needle deflects towards the east (4) If the compass needle is placed just above the wire, the north pole (N) deflects towards east when the direction of current in wire is from A to B but the needle deflects towards west. REASON for Deflection : 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. Inference: A current (or moving charge) produces a magnetic field around it. This is called the magnetic effect of current. MAGNETIC FIELD AND FIELD LINES DUE TO CURRENT IN A STRAIGHT WIRE When a current is passed through a conducting wire, a magnetic field is produced around it. At a point, the direction of magnetic field will be along the tangent drawn on the magnetic field line at that point. Experiment: Take a sheet of smooth cardboard with a hole at its centre. Place it horizontal and pass a thick copper wire XY vertically through the hole. Connect an ammeter A, battery B, rheostat Rh and a key K between the ends X and Y of the wire as shown in fig. Sprinkle some iron filings on the cardboard and pass an electric current through the wire by inserting the plug in the key K. Gently tap the cardboard. It is observed that the iron filings get arranged along the concentric circles around the wire. From the magnetic field lines pattern, Conclusion: (1) The magnetic field lines form the concentric circles around 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. (3) On increasing current in the wire, the magnetic field lines become denser and the iron filling get gets arranged. RULE TO FIND THE DIRECTION OF MAGNETIC FIELD 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) The magnetic field lines due to current in a loop (of thick wire or circular coil) can be obtained by the following experiment. Experiment: Take a piece of thick wire A, bent in the form of a loop (or coil). It is passed at two points P and Q through a horizontal cardboard C such that the points P and Q form the diameter of the wire or coil. Now sprinkle some iron filings on the cardboard. Connect a battery through a rheostat and a key between the ends X and Y of the loop. Close the key to pass current through the coil and tap the cardboard gently. 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 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, 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 if (i) the strength of current in loop is increased, and (ii) the number of turns in the loop is increased. 6) The magnetic field lines pass through the loop in same direction. The polarity at the two faces of loop depends on the direction of current in the loop. Clock rule (clockwise current-south pole and anticlockwise current-north pole) Looking at the face of the loop, if the current in wire around that face is in anticlockwise direction, the face has the forth polarity, while if the current at that face is in clockwise direction, the face has the south polarity. 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. Experiment: Take a cardboard having two slits PQ and P'Q' parallel to each other, at a small separation and parallel to its length. Wind a uniform spiral of an insulated thick copper wire through the two slits such that the axis of spiral is in the plane of the cardboard. Connect a battery through a rheostat and a key between the ends X and Y of the solenoid. Sprinkle some iron filings on the cardboard and pass current through the solenoid by closing the key. Gently tap the cardboard. The iron filings on the cardboard get arranged in a definite pattern as shown by the dotted lines representing the pattern of magnetic field lines due to the current carrying solenoid. 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. (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 soft iron increases the strength of magnetic field of the solenoid as soft iron has a high magnetic permeability. (5) 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). 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. (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: To construct an I-shaped electromagnet, a thin insulated copper wire XY is wound in form of a solenoid around a straight soft iron bar. The bar remains magnetised so long the electric current flows through the solenoid, but it gets demagnetised as soon as the current is switched off thus it is a temporary magnet. Use: A bar shaped electromagnet is commonly used in relay. Construction of U Shaped (or horse shoe) electromagnet: To construct a horse-shoe electromagnet, a thin insulated copper wire XY is spirally wound on the two arms of a U-shaped soft iron core, such that the winding as seen from the ends, is in opposite sense on the two arms. Between the ends X and Y of the wire, a battery through an ammeter, rheostat and a key, is connected. We get a very strong magnetic field in the gap between the two poles. The magnetic field in the gap vanishes as soon as current in the circuit is switched off. Thus, it is a temporary magnet. Use: Horse shoe magnets are used in gadgets like d.c. motor, a.c. generator, electric bell, 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: (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 Permanent Magnet 1. It is made of soft iron. 1. It is made of steel. 2. It produces the 2. It produces a magnetic field so long as permanent magnetic current flows in its coil. field. 3. The magnetic field 3. The magnetic field strength can be changed. 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. (2) For loading the furnaces with iron. (3) For separating the iron pieces from debris and ores, where iron exists as impurities (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. 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. Since charge in motion constitutes a current, therefore a conductor carrying moving charges (or current) placed in a magnetic field, in direction other than the direction of magnetic field, also experiences a force and can produce motion in the conductor. Magnitude of force: 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 L of the wire. F = K IBL 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= 𝐅 From the above equation, so the S.I Unit of magnetic 𝐈𝐱𝐋 field is or 𝐧𝐞𝐰𝐭𝐨𝐧. It is also named (symbol T) or 𝐚𝐦𝐩𝐞𝐫𝐞 𝐱 as Tesla 𝐦𝐞𝐭𝐫𝐞 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. 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. Principle: A d.c. motor works on the principle that on passing electric current through a conductor placed normally in a magnetic field, a force acts on the conductor as a result of which the conductor begins to move and thus mechanical energy (or work) is obtained. Construction and its main parts : The main parts of an electric motor are : (1) the armature coil ABCD mounted on an maxle, (2) the split parts S₁ and S₂ of a ring (i.e., a copper slip ring being divided in two parts S₁ and S₂) which is known as split ring commutator, (3) a pair of carbon (or copper) brushes B₁ and B2, (4) a horse-shoe electromagnet NS, and (5) a d.c. source (i.e., battery). 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: Experiment: Wind an insulated copper wire in form of a spiral on a paper (or wooden) cylinder so as to form a coil in the form of a solenoid. Connect a centre zero galvanometer G between the two ends of the solenoid. Then place a magnet NS at some distance, along the axis of solenoid as shown in Fig. Observations: It is observed that: (1) When the magnet is stationary (v = 0), there is no deflection in the galvanometer and its pointer reads zero [Fig. (a)]. (2) When the magnet with its north pole facing the solenoid is moved towards it, the galvanometer shows a deflection towards right showing that a current flows in the solenoid in the direction B to A as shown in Fig. (b). (3) As the motion of magnet is stopped, the pointer of galvanometer comes to zero position [Fig.(c)]. This shows that the current in solenoid flows as long as the magnet is in motion. (4) If the magnet is moved away from the solenoid, the pointer of galvanometer deflects towards left [Fig.(d)] showing that the current in solenoid flows again, but now in direction A to B which is opposite to that shown in Fig.(b). Current becomes zero as soon as the magnet stops. (5) If the same action (movement) of magnet is done rapidly, the deflection in the galvanometer is more than before although the direction of deflection remains same. It shows that now more current flows. (6) If the magnet is brought towards the solenoid by keeping its south pole towards it, the pointer of galvanometer deflects towards left [Fig. (e)] showing that the current in solenoid flows in direction A to B which is opposite to that shown in Fig. (b). If the magnet is rapidly moved back and forth (oscillated) in this position, a current alternating in direction flows through the coil. 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. Faraday's Explanation Electromagnetic induction is the phenomenon in which an e.m.f. is induced in the coil if there is a change in the magnetic flux linked with the coil. FARADAY'S LAWS OF ELECTRO MAGNETIC INDUCTION On the basis of the above experimental observations, Faraday formulated the following two laws of electromagnetic induction: (1) Whenever there is a change in magnetic the flux linked with a coil, an e.m.f. is induced. The e.m.f. induced lasts so long the magnetic flux linked with the coil changes. (2) The magnitude of e.m.f. induced is directly proportional to the rate of change of gain magnetic flux linked with the coil. If magnetic flux changes at a constant rate, a steady e.m.f. is produced. Flemming’s right hand rule: Stretch the thumb, central finger and forefinger of your right hand mutually perpendicular to each other. 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. Lenz's law: According to Lenz's law, the direction of induced e.m.f. (or induced current) is such that it opposes the cause which produces it. A.C. GENERATOR An a.c. generator is a device which converts the mechanical energy into the electrical energy using the principle of electromagnetic induction. Principle: 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 e.m.f. is induced between the ends of the coil.  DISTINCTION BETWEEN D.C. AND A.C. Direct Current (d.c.) Alternating Current (a.c) 1. It is the current of constant 1. It is the current of magnitude. magnitude varying perodically with time. 2. It flows in one direction in 2. It reverses its direction the circuit. periodically while flowing in a circuit. 3. It is obtained from a cell (or 3. It is obtained from an a.c. battery). generator and mains. Advantage of a.c. over d.c. (1) It is cheaper and easy to generate a.c. than d.c. (2) The efficiency of an a.c. generator is higher than that of a d.c. generator. (3) It is easy to change a.c. into d.c. (4) The magnitude of a.c. voltage can easily be increased or decreased by the use of step up or step down transformer. (5) a.c. can be transmitted over a long distance without much loss in energy in the line wires. DISTINCTION BETWEEN A.C. GENERATOR AND D.C. MOTOR A.C. Generator D.C. Motor 1. A generator is a device 1. A d.c. motor is a device which converts the which converts the electrical mechanical energy into the energy into the mechanical electrical energy. energy. 2. A generator works on the 2. A d.c. motor works on the principle of electro- magnetic principle of force acting on a induction. current carrying conductor placed in a magnetic field. 3. In a generator, the coil is 3. In a d.c. motor, the current rotated in a magnetic field so from a d.c. source flows in the as to produce an electric coil placed in a magnetic field current. due to which the coil rotates. 4. A generator makes use of 4. A d.c. motor makes use of two separate coaxial slip two parts of a slip ring (i.e., rings. split rings) which act as a commutator. Similarities between an A.C. generator and a D.C. motor (1) Both in an a.c. generator and d.c. motor, a coil rotates in a magnetic field between the pole pieces of a powerful electromagnet. (2) Both in an a.c. generator and d.c. motor, there is a transformation of energy from one form to the other form. TRANSFORMER In our daily life, we use various electrical appliances which require working voltage different from the voltage of the mains (i.e., 220 V) e.g. a door bell needs 6 V, while the picture tube in a television needs nearly 10,000 V. Transformer is a device by which the magnitude of an alternating e.m.f. can be increased or decreased. Note: A transformer does not affect the frequency of the alternating voltage. The frequency remains unchanged (= 50 Hz). Principle: A transformer works on the principle of electromagnetic induction and makes use of two coils having different number of turns. The two coils are named as: (1) the primary coil, and (2) the secondary coil. Note: A transformer cannot be used with the direct current (d.c.) source because if current in primary coil is constant (i.e., d.c.), then the magnetic field due to primary coil remains constant and there will be no change in the magnetic flux linked with the secondary coil and hence no e.m.f. will be induced in the secondary coil. Construction: A transformer consists of mainly three parts: (i) primary coil, (ii) secondary coil and (iii) core. The core is made from the thin rectangular laminated sheets of soft iron of T and U shape, placed alternately one above the other and insulated from each other by a paint (or varnish) coating over them. This forms a simple rectangular core. The ratio of number of turns N, in secondary coil to the number of turns N, in primary coil (i.e., N/N) is called the turns ratio which is denoted by the symbol n, i.e. Turns ratio, n = number of turns in secondary coil NS / number of turns in primary coil NP Note: (1) The laminated core prevents the loss of energy due to induced (or eddy) currents in the core. (2) The advantage of using a closed core is that it gives a closed path for the magnetic field lines and therefore almost all the magnetic field lines, caused due to current in the primary coil, remain linked with the secondary coil (i.e., the flux linkage is nearly perfect) and loss of energy is minimum. (3) The core is made of soft iron so that the hysteresis loss of energy in the core gets minimised. Working: When the terminals of the primary coil are connected to the source of alternating e.m.f., a varying current flows through the primary coil. This varying current produces a varying magnetic field in the core of transformer. Thus the magnetic flux linked with the secondary coil vary due to which an e.m.f. is induced in it. The induced e.m.f. varies in the same manner as the applied e.m.f. in the primary coil varies and thus the induced e.m.f. has the same frequency as that of the applied e.m.f For a transformer, e.m.f. across the secondary coil (ES) / e.m.f. across the primary coil (EP) = number of turns in secondary coil (NS)/ number of turns in primary coil (NP) Thus the magnitude of e.m.f. induced in the secondary coil depends on the following two factors: (1) the ratio of the number of turns in the secondary coil to the number of turns in the primary coil (i.e., turns ratio), and (2) the magnitude of the e.m.f. applied in the primary coil. For an ideal transformer, when there is no energy loss, the output power will be equal to the input power. i.e., power in secondary coil = power in primary coil or ESIS = EPIP Types of transformer: There are two types of transformers: (1) step up transformer, and(2) step down transformer. (1) Step up transformer: The transformer used to change a low alternating voltage to a high alternating voltage (of same frequency) is called a step up transformer (i.e., Es >E P ). Turns ratio n > 1. the current in primary coil is more than in the secondary coil (or Ip > Is), so the wire in the primary coil is kept thicker than in the secondary coil. (2) Step down transformer: The transformer used to change a high alternating voltage to a low alternating voltage (of same frequency) is called a step down transformer (Es < Ep). In a step down transformer, the number of turns in the secondary coil are less than the number of turns in the primary coil (Ns Ip) so the wire in secondary coil is kept thicker than in the primary coil. Note: The use of thicker wire in a coil reduces its resistance and therefore reduces the loss of energy as heat in that coil (This energy loss due to heat is known as copper loss). Uses of transformers: In our houses, both type of transformers are used with electrical appliances which operate at a voltage other than the voltage supplied by the mains (e.g., a door bell needs 6 V, while a T.V. requires nearly 10,000 V). (A) Uses of step up transformers (1) In transmission of electric power at the power generating station to step up the voltage. (2) With television. (3) With wireless sets. (4) With X-ray machines to provide a high accelerating voltage. (B) Uses of step down transformers: (1) With electric bells, night electric bulbs, mobile phones, computers, etc. (2) At the power sub-stations to step down the voltage before its distribution to the consumers.  DISTINCTION BETWEEN STEP UP AND STEP DOWN TRANSFORMERS Step Up Transformer Step Down Transformer 1. It increases the a.c. voltage 1. It decreases the a.c. voltage and decreases the current. and increases the current. i.e., ES> EP and IS < IP i.e., ES < EP and IS > IP 2. The turns ratio NS / ND > 1 2. Its turns ratio NS / NP < 1 i.e., it has more number of i.e., it has less number of turns in the secondary coil turns in the secondary coil than in the primary coil. than in the primary coil. 3. The wire of primary coil is 3. The wire of the secondary thicker than that of the coil is thicker than that of the secondary coil. primary coil. Uses: At power generating Uses: At power substations, station, with X-ray machines, with night electric bulbs, television, etc. computers, electric bells, etc.