Class 11 Important Questions for Physics - Magnetic Effect of Electric Current

1. INFOMATICA ACADEMY CONTACT: 9821131002/9029004242 Magnetic Effect of Electric Current ❖Illustrative Examples 1)Find the magnitude of magnetic induction at a distance of 4 cm due to a long straight wire carrying a current of 10A. (t o = 4n x 10-7 Wb/Am) Data : a = 4 cm = 4 m, I = 10A, Wb/Am. (Ans. B = 5 10-5 Wb/m2) 2)A circular coil of 8 tums and mean radius 10cm carries a current of 1 ampere. Find the magnitude of magnetic induction at its centre.( Wb/Am) Data: n=8, I=1A, r = 10cm= 10-1m, Wb/Am. (Ans. B = 5.027 10-5 Wb/m2) 3)A conductor of length 2A cm carrying a current of 5 A is placed in uniform magnetic induction of 4 T making an angle of 300 with the direction of the field. Find the force acting on the conductor. Data : I =5A, 8=4T, , I = 20 cm = 0.2 m (Ans. F = 2N) 4)Two long and straight conductors, placed parallel to each other are separated by 10 cm, carrying currents of 2A and 4A respectively in opposite direction. Find the force per unit length exerted by each conductor on the other. . Data : a = 10cm = 0.1 m, I1 = 2A, I2 = 4A, . (Ans. 16 10-5 N/m) 5)A particle carrying a charge of 200 C moves at an angle of 300 to a uniform magnetic field of induction 5 Wb/m2 with a speed of 2 m/s. Calculate the force acting on the particle. Data : q = 200 C = 200 , = 300 , sin 300= 0.5 , B = 5 , v = 2 m/s. (Ans. F = 10-3N) 6)A circular coil having 50 turns each of radius 0.05m carries a current of 1A. Find the magnitude of magnetic induction at a point 0.2m, from its centre on its axis. ( ). Data: n = 50, a = 0.05m, I = 1A, x = 0.2m, F.Y.J.C. Science Mag. Effect of Electric Current

2. INFOMATICA ACADEMY CONTACT: 9821131002/9029004242 (Ans. B = 0.8964 10-5 Wb/m2) ❖Theory Questions 1)State right hand rule, for magnetic field due to current carrying wire. 2)Define magnetic induction. State its SI unit. 3)State Biot-Savert’s law and write it in vector form. 4)Obtain an expression for the magnetic induction at the centre of a circular coil carrying a current. 5)State the formula for magnetic induction at a point due to a straight current carrying conductor. 6)Derive an expression for the magnetic induction at a point on the axis of a circular coil carrying a current. 7)State Fleming's is left hand rule. 8)Derive an expression for force experienced by a current carrying conductor placed in uniform magnetic field. Discuss when will be the force maximum and minimum. 9)Derive an expression for the force per unit length between two parallel conductors carrying current and hence define ampere. 10)Define the SI unit of current. 11)Derive an expression for torque on a rectangular current loop in a uniform magnetic field. 12)Compare the properties of electric field and magnetic field. 13)Define I tesla. 14)Write a short note on Oersted's experiment. 15)Explain the concept of polarity of a current carrying circular coil. OR Explain the equivalence between current carrying circular coil and magnetic dipole. 16)Whether the source of magnetic field is analogous to the source of electric field? 17)State the formula for the force, which is acted upon in the uniform magnetic field when a charge moves with certain velocity. 18)A magnetic field that varies in magnitude from point to point but has a constant direction (east to west) is set up in a chamber. A charged particle enters the chamber and travels undeflected along a straight path with constant speed. What can you say about the intial velocity of the particle? 19)A charged particle enters an environment of a strong and non- uniform magnetic field varying from point both in magnitude and direction and comes out of it F.Y.J.C. Science Mag. Effect of Electric Current

3. INFOMATICA ACADEMY following a complicated trajectory. Would its final speed equal the initial speed if it CONTACT: 9821131002/9029004242 suffered no collisions with the environment. 20)Define Lorentz force and derive the expression for the Lorentz force on a charge by a magnetic field. 21)An electron travelling west to east enters a chamber having a uniform electrostatic field in north to south direction. specify the direction in which a uniform magnetic, field should be set up to prevent, the electron from dal from its straight line path. 22)Statethe condition under which torque acting on a rectangular current loop is zero. 23)State why torque due to circular loop is maximum. ❖Textbook Problems 1)A straight wire carries a current of 5A. Calculate the magnitude of magnetic induction at a point of from 10 cm away from the conducting wire. ( Wb/Am) (Ans. : Wb/m2) 2)A current of l0A passes through a coil having 5 turns and produces a magnetic field at the centre of the coil having magnitude T. Calculate diameter of the coil. ( Wb/Am) (Ans. : 1.257m) 3)A straight current carrying conductor, 30cm long carries a current of 5A, lt is placed in a uniform magnetic field of induction 0.2 T, with its length making an angle of 600 with the direction of field. Find the force acting on the conductor. (Ans. : 0.2598 N) 4)Two long parallel current carrying conductors are 0.4rn apart in air and carrying currents 5 A and 10 A respectively. Calculate the force per metre on each wire, if the currents are in the same direction. ( Wb/Am) (Ans. : If currents are in the same direction, the force of attraction and if currents are in the opposite direction, the force of repulsion ) 5)A particle carrying a charge moves with velocity m/s, at right angle to a uniform field of magnetic induction 0.005 T. Find the value of the charge of the particle if it experiences a force of N. (Ans. : 1.333 C) 6)Calculate the magnetic field of induction, due to circular coil of 400 turns and radius 0.05m carrying a current of 5A, at a point on the axis of the coil at a distance 0.1 m.( Wb/Am) (Ans. : 2.249 T) F.Y.J.C. Science Mag. Effect of Electric Current

4. INFOMATICA ACADEMY 7)A conductor of length 25 cm is placed a) parallel b) perpendicular c) inclined at an CONTACT: 9821131002/9029004242 angle 300 to a uniform field of magnetic induction 2T. If a charge of IC passes through it in 5 s, calculate the force experienced by the conductor. (Ans. : Zero, 0.1 N, 0.05 N) 8)A proton from cosmic rays enters the earth's magnetic field in a direction perpendicular to the field. If the velocity of proton is m/s and B =1.6 wb/m2, find the force exerted on the proton by the magnetic field. (Charge on a proton (e) = 1.6 C) ( Ans. : 5.12 N) 9)A circular loop has a radius l0cm and it is carrying a current of 0.1 A. Calculate its magnetic moment. ( Ans. : 3.14 Am2) ❖Practice problems ➢Type-I : Problems based on Biot–Savart’s law 1)A current of 5 A is flowing from south to north in a wire kept along north-south direction. Find the magnetic field due to 1 cm piece of wire at a point 2 m north east from the piece of wire. (Ans. The magnetic field due to wire at the point is 8.8410-10T) 2)A long straight wire carries current of 35 A. What is the magnitude of the field B at a point 20 cm from the wire? (Ans. The magnitude of field at point 20 cm is 3.5 10-5 T.) 3)A long straight conductor, carrying a current of 12 A, is kept in air. Find the distance from the conductor at which the magnetic induction is 4 10-5 Wb/m2. (Ans. The distance from the conductor is 6 cm.) 4)A long straight wire in the horizontal plane carries a current of50 A in north-south direction. Give the magnitude and direction of B at a point 2.5 m east of the wire. (Ans. The magnitude of B is 4 10-6 T and direction is vertically downward.) 5)A horizontal overhead power line carries a current of 90 A in an east to west direction. What is the magnitude and direction of magnetic field due to the current 1.5 m below the line? (Ans. The magnitude of magnetic field is 1.2 10-5 T. It acts in southward direction) 6)At what distance from a long straight wire carrying a current of 12 A will the magnetic field be equal to 3 10-5 Wb m2. (Ans. 8 10-2 m ) 7)A long wire carries a current of 5 A. Find the position of a point where it produces a magnetic field of 0.36 10-4 Wb/m2. (Ans. 2.78 cm) F.Y.J.C. Science Mag. Effect of Electric Current

5. INFOMATICA ACADEMY 8)A wire placed along the north-south direction carries a current of B A from south to CONTACT: 9821131002/9029004242 north. Find the magnetic field due to a I cm piece of wire at a point 200 cm north- east from the piece. (Ans. 1.4 10-9 T) 9)An infinitely long straight wire carries a current of one ampere. The magnetic field it produces is balanced by the earth's horizontal magnetic field of 0.36 10-4 T. Find the shortest distance of this point from the wire. (Ans. 5.5 10-3m) ➢Type-II : Problems based on magnetic induction at the centre of a circular coil carrying current. 10)A current of 15 A passes through a coil of3 turns and produces a field of 0.654 10-4 T at the centre of the coil. Calculate the radius of the coil. (Ans. The radius of the coil is 0.432 m.) 11)Calculate the magnetic induction at the centre of a coil of radius 20 cm when a current of 40 mA flows through 120 turns of the coil. (Ans. The magnetic induction at the centre is 1.507 10-5 wb/m2.) 12)Two coils. of wire P having 50 turns and 10 cm radius and Q having 200 turns and 15 cm radius are connected in series. Compare the magnetic inductions at their centres when a current is passed through them. (Ans. The ratio of magnetic inductions at their centres is .) 13)Two flat circular coils have a common centre, but their planes are at right angles to each other. The inner coil has 150 turns and a radius of cm. The outer coil has 400 turns and a radius of 2 cm. Find the magnitude of the resultant magnetic induction at the common centre of the coils when a current of 200 mA is sent through each of them. (Ans. The magnitude of resultant magnetic held 10-3 wb/m2.) 14)Find the magnetic induction at the centre of a circular coil of radius 0.1 m having 50 turns and carrying a current of 500 mA. (Ans. 5 10-5 Wb/m2) 15)A circular coil carries a current of 2 A and has an average radius of 8 cm. If the magnetic induction at the centre of the coil is 3.14 10 4 Wb/m2. Find the number of turns. (Ans. 20) 16)Two coils, P having 100 tums and 10 cm radius and Q having 500 turns and 20 cm radius are connected in series. Compare the values of magnetic induction at their centres when a current is passed through them. (Ans. BP : BQ = 2 :5) F.Y.J.C. Science Mag. Effect of Electric Current

6. INFOMATICA ACADEMY 17)What current should be passed through a flat circular coil of 20 cm diameter and CONTACT: 9821131002/9029004242 having 100 closely wound tums to produce a magnetic induction of 3.142 10-4 Wb/m2 at its centre? (Ans. 0.5A) 18)A flat circular coil of insulated wire of 56 turns, each of average radius 5.5 cm has a current of 3 A passing through it. What is magnetic induction at the centre of the coil? (Ans. 19.2 10-4 Wb/m2) 19)Magnetic induction at the centre of a circular current carrying coil of wire is 10-4 Wb/m2. The number of turns in the coil is 50 and the average radius of the coil is 2 cm. Find the current in the coil. (Ans. 0.0637 A) ➢Type-III: Problems based on magnetic induction at a point along the axis of a coil carrying current. 20)Calculate the magnetic field due to a circular coil of 500 turns and of diameter 0.1 m, carrying a current of 7 A, at a point on the axis of the coil at a distance of 0.12 m (Ans. The magnetic field due to the circular coil is 2.5 10-3 T.) 21)The magnetic field due to a current-carrying circular loop of radius 12 cm at its centre is 0.50 10-4 T. Find the magnetic field due to this loop at a point on the axis at a distance of 5.0 cm from, the centres. (Ans. 3.9 10-5T) 22)A circular coil having 50 turns each of radius 0.05 m carries a current of 1 A. Find the magnitude of magnetic induction at a point 0.2 m from its centre on its axis. (Ans. 0.8964 10-19) ➢Type-IV : Problems based on force acting on a conductor carrying current in a magnetic field. 23)An electron moving with a velocity of 5.0 107ms-1 enters a magnetic field of 1.0 Wb m-2 at an angle of 300. Calculate the force on the electron. (Ans. The force on the electron is 4.0 10-12 N.) 24)In XOY plane, a magnetic field of 4 T is applied along the position Y-axis. A proton moves at 3 106 m/s making an angle of 600 with X-axis. Calculate the magnetic force on the proton. What will be the force, if the proton moves along +Y axis? (Ans. Force on proton is 9.6 10-13 N along +Z direction. When proton moves along +Y axis, force on it is zero.) 25)A wire carrying a current 20 A and 1 m in length is placed in a magnetic field of 0.4 Wb/m2. What is the force acting on the wire if it is placed at 450 to the field? (Ans. The force acting on the wire is 5.656 N ) F.Y.J.C. Science Mag. Effect of Electric Current

7. INFOMATICA ACADEMY 26)A straight conductor 2 m long carrying a current of 15 A is kept at right angles to a CONTACT: 9821131002/9029004242 uniform magnetic field of induction 5 10-3 Wb/m2. What is the force acting upon it? (Ans. The force acting on the conductor is 0.15 N.) 27)What is the magnitude of magnetic force per unit length on a wire carrying a current of 8 A and making an angle of 300 with the direction of a uniform magnetic field of 0.15 T? (Ans. The magnitude of magnetic force per unit length is 0.6 N/m.) 28)A straight conductor carrying a current of 2.45 A, floats horizontally in air, in a uniform magnetic field of induction 4 10-4 Wb/m2. The field is horizontal and at right angles to the length of the conductor. If the conductor has linear density (mass per unit length) of 0.1 g/m, find the value of ‘g' at the place. (Ans. The value of 'g' is 9.8 m/s2.) 29)A particle carrying a charge of 6.4 10-19 C moves at angle of 300 with a uniform magnetic field of induction 6 10-5 Wb/m2. Find the value of the force acting on the particle if its velocity is 1.8 104 m/s. (Ans. 3.456 10-19N ) 30)A 10 cm long portion of a straight wire carrying a current of 10 A placed in a magnetic field of 0.1 T making an angle of 530 with the wire. What magnetic force does the wire experience? (Ans. 0.08 N) 31)An -particle moves at right angle to a uniform magnetic field of 2 mT. lf it experiences a magnetic force of 2 10-15 N, find its speed (Ans. 3.125 106 m/s) 32)A current carrying wire, 20 cm long is situated in a uniform magnetic field of induction 4 10-5 Wb/m2 with its length inclined at 30" to the direction of the field. If the wire experiences a sideward force of 2 10-6 N, find the current in the wire. (Ans. 0.5 A) 33)An particle is projected with a velocity of 5 104 m/s in a uniform magnetic field in a direction making an angle of 300 with the direction of the field. If the magnetic induction is 10-2 Wb/m2 and the lateral force acting on the particle is 8 10-17 N, find the charge on the -particle. (Ans. 3.2 10-19 C) 34)A proton moving with a speed of 5 106 m/s is subjected to a magnetic field of 0.4 T inclined at 300 to the direction of velocity of the proton. What is the acceleration of the proton? [Mass of the proton : 1.6 10 27 kg.] (Ans. 1014m/s2) 35)A current of 2.5 A flows downwards through a vertical conductor of length 1 m, if the horizontal component of the earth's magnetic induction at the place is 4.2 105 Wb/m2, find the magnitude and the direction of the force acting on the conductor. (Ans. 1.05 10-4 N/m towards east) F.Y.J.C. Science Mag. Effect of Electric Current

8. INFOMATICA ACADEMY 36)Find the force acting on a straight conductor 5 m long and carrying a current of 5 A CONTACT: 9821131002/9029004242 when kept inclined at an angle of 300 to a uniform magnetic field of induction 10-3 Wb/m2. (Ans. 1.25 10-2 N) ➢Type-V: Problems based on force between two infinitely long current carrying parallel conductors. 37)Two infinitely long, parallel and straight conductors each carrying a current of 5 A are separated by a short distance in air. If each conductor experiences a force of 10-4 N/m, find the distance between them. (Ans. The distance between the two conductors is 5 cm.) 38)Two wires 10 m long and 12 cm apart, carry the same current. If the wires exert a force of 1.5 10-2 N on each other. Find the current through each wire. (Ans. The current through each wire is 30 A) 39)Two parallel wires are 2 cm apart and carry currents of 30 A and 40 A in the same direction. Find the force they exert on each other, if the length of each is 10 cm. (Ans. The force exerted on each wire by the other is 12 10-4 N.) 40)Calculate the force per unit length on a long straight wire carrying the current of 4 A due to a parallel wire carrying 6 A current. Distance between the wires is 3 cm. (Ans. The force per unit length of the wire is1.6 10-4 N/m) 41)The wires which connect the battery of an automobile to its starting motor carry a current of 300 A (for a short time). What is the force per unit length between the wires if they are 70 cm long and 1.5 cm apart? Is the force attractive or repulsive? (Ans. Force per unit length is 1.2 N/m and is repulsive.) 42)Two very long straight conductors kept parallel to each other at a distance of 5 cm apart carry equal currents in opposite directions. If they repel each other with a force of 2.5 10-5 N/m, calculate the magnitude of the current in each conductor. (Ans. 2.5 A) 43)Find the force per unit length between two long straight conductors kept at 0.2 m apart and carrying currents of 4 A and 6 A respectively. (Ans. 24 10-6 N/m) 44)Two straight wires are kept parallel to each other and are 5 cm apart. One carries a current of 2 A in one direction and the other a current of 3 A in the opposite direction. Find the force one exerts upon the other per unit length. (Ans. 2.4 10-5 N/m Repulsive) 45)Two straight wires, each 10 cm long, are parallel to one another and each caries a current of 40 A. What force is experienced by either wire when separated by a distance of 20 cm currents flowing in the F.Y.J.C. Science Mag. Effect of Electric Current

9. INFOMATICA ACADEMY i. same direction and? CONTACT: 9821131002/9029004242 ii. opposite direction? (Ans. i. Attractive : 1610-5 N ii. Repulsive 16 10-5 N) 46)Two long parallel wires are 0.4 m apart in air and carry currents 5 A and 12 A respectively. Calculate the force on each metre of the wire. if the currents are in i. the same direction ii. the opposite direction. (Ans. i. Attractive : 310-5 N/m ii. Repulsive 310-5 N/m ) ➢Type-VI : Problems based on torque on a current loop in magnetic field. 47)What is the maximum torque on a rectangular coil of area 5 cm 12 cm of 600 turns, when carrying a current of 10-s A in a magnetic field of 0.10 T? (Ans. The maximum torque on the rectangular coil is 3.6 10-6 Nm) 48)A square coil of side 10 cm consists of 20 turns and carries a current of 12 A. The coil is suspended vertically and the normal to the plane of the coil makes an angle of 300 with the direction of a uniform horizontal magnetic field of magnitude 0.80 T. What is the magnitude of torque experienced by the coil? (Ans. The magnitude of torque experienced by the coil is 0.96 Nm) 49)Given a uniform magnetic field of 100 gauss in an east to west direction and 44 cm long wire with a current carrying capacity of at the most 10 A. What is the magnitude of the maximum torque? (Ans. The magnitude of maximum torque is 1.54 10-3 Nm) 50)Calculate the torque on a 100 tum rectangular coil of length 40 cm and breadth 20 cm carrying current of 10 A, when placed making an angle of 30" with a magnetic field of 5 T. (Ans. 200 Nm) 51)A rectangular coil of dimensions 4 cm x 10 cm consists of 50 turns of wire and caries a current of 25 mA. (i) Calculate the magnitude ol its magnetic moment. (ii) If a uniform magnetic field of magnitude 0.35 T is applied parallel to the plane of coil, what is the magnitude of the torque acting of the coil? (Ans. 5 10-3 Am2 , 1.75 10-3 Nm) ➢Type-VII : Miscellaneous 52)A circular coil of radius 0.4 m and having 500 turns, carries a current of 200 mA. Find the magnitude of the magnetic induction at a point on its axis at a distance of i.0.3 m ii. 100 m from its centre (Ans. The magnitude of magnetic induction at a point at distance of: i. 0.3 m is 8.04 10-5 Wb/m2. Ii. 100 m is 1.005 10-11 Wb/m2) F.Y.J.C. Science Mag. Effect of Electric Current

10. INFOMATICA ACADEMY 53)A circular loop has a radius 12 carrying a current of 0.5 A magnetic moment. CONTACT: 9821131002/9029004242 cm and it is Calculate its magnetic moment. (Ans. 2.3 10-2 Am2) 54)A current-carrying circular coil of 100 turns and radius 5 cm produces a magnetic field of 6.0 10 t T at its centre. Find the current in the coil. (Ans.48 mA) 55)Find the magnetic induction of the centre of a circular coil of wire of 50 turns and of average radius 6.284 cm when a current of 0.3 A is passed through the coil. (Ans. 1.5 10-4 Wb/m2) 56)Two long, straight, parallel conductors, carrying currents in opposite directions. They are separated by a distance of I cm in air. The current in one conductor is twice that in the other. If each conductor repels the other with a force of 2.5 10-6 N/m, find the values of current in each of them. (Ans. 0.25 AA, 0.5 A) 57)A circular coil of 50 tums carries a current of 250 mA. The radius of the coil is 2 cm. Determine the field due to the coil at its centre and on its axis when the distance from the centre is equal to the radius of the coil. (Ans. 12.5 10-5 Wb/m2 , 4.42 10-5 Wb/m2) ❖Multiple choice Questions 1)A moving charge will produce (A)no field. (C) a small electric field (B)a magnetic field. (D) infinite electric field. 2)Two points, A and B, are at distance aA and aB from a long, straight, current carrying conductor. If , the magnetic inductions at the two points are related by (A) (B) (C) (D) 3)The current in wire is directed towards east and the wire is placed in magnetic field directed towards north. The direction of the force on this wire is . (A)due south (C) vertically upwards (B)due east (D) due south east 4)A charge + q is projected with a velocity in a magnetic field of induction . The force acting on the charge will be maximum, when the angle between the directions and is (A)0° (B) 30° (C) 90° (D)450 F.Y.J.C. Science Mag. Effect of Electric Current

11. INFOMATICA ACADEMY 5)How many number of turns of circular coil of radius 12.56 cm and current 1 A will CONTACT: 9821131002/9029004242 produce the magnetic induction of 5 10-5 T at its centre ? (A)5 (B)10 (C) 15 (D) 20 6)Magnetic effect of current was discovered by (A)Faraday (C) Ampere (B)Oersted (D) Joule 7)Two circular coils are made of two identical wires of the same length. If the number of turns of the two coils are 4 and 2, then the ratio of magnetic inductions at the centres will be (A)2:1 (B)2:3 (C)1 : 1 (D)3 : 2 8)Magnetic field due to a straight wire at a distance ‘r’ from the wire will be given by (A) (B) (C) (D) 9)The magnetic field of 5 10-5 Wb/mf was produced at a point due to a current 2 A passing through a straight wire. The distance of point from the wire is (A)0.08 cm (B) 0.8 cm (C) 8 cm (D) 80 cm 10)Two circular coils 1 and 2 have both their radii and number of turns in the ratio1:2. If the currents in them are in the ratio 2:1, the magnetic inductions at the centres of the coils are in the ratio (A)1:1 (B)2:1 (C) 1:2 (D)1 : 8 11)The magnetic induction at the centre of circular coil for double turns and half radius will be (A)B/4 (B) 2B (C) B/2 (D) 4B 12)The magnetic induction at the centre of a circular current carrying coil of radius ‘a’ will be (A) (B) (C) (D) Zero 13)Two long parallel wires are at a distance of 1 m. If both of them carry one ampere of current, then the force of attraction per unit length between the two wires is (A)2 10-7 N/m (C) 5 10-8 N/m (B)2 10-8 N/m (D) 5 10-7 N/m 14)A conductor of length 5m and carrying a current of 2A is kept inclined at 30° to a uniform magnetic field of induction 0.4 T. The force acting on the conductor will be (A)1 N (B) 2 N (C) 3 (D) 0.5 N 15)The radius and number of turns of a circular coil are double. The magnetic induction at its centre will be . F.Y.J.C. Science Mag. Effect of Electric Current

12. INFOMATICA ACADEMY (A)one fourth CONTACT: 9821131002/9029004242 (C) same (B)four times (D) double 16)An electric charge ‘e’ moves with a constant velocity ‘v’ parallel to the lines of force of a uniform magnetic field B, the force experienced by the charge is (A)Bev (B) ev/B (C) zero (D) e/Bv 17)Two parallel wires, each of length 0.5 m, are lying at a distance of 1 m. If the current flowing in each wire is 1 ampere then the force between them will be (A)107 N (B)10-7N (C) 5N (D) 4 10-7N 18)A charge of 1 C is moving in a magnetic field of 0.5 tesla with a velocity of 10m/sec. Force experienced by the charge is (A)5 N (B) 11 N (C) 0.5 N (D) 15 N 19)Magnetic field is not associated with (A)a change in uniform motion. (B)an accelerated charge. (C)a deaccelerated charge. (D)a stationary charge. 20)If the current in a circular coil is clockwise, the pole produced on its front face will be (A)N-pole. (C) Either N-pole or S-pole. (B)S-pole. (D) Neither N-pole nor S-pole. 21)Which of the following is the unit of magnetic induction? (A)T (B) Wb/m2 (C) N/Am (D) All of these 22)The strength of the magnetic field at a point distance r near a long straight current carrying wire is B. The field at a distance r/2 will be (A)2B (B) B/4 (C) 4B (D) B/2 23)On passing electric current in two long straight conductors in mutually opposite direction, the magnetic force acting between them will be . (A)Repulsive (C) Zero (B)Attractive (D) neither attractive nor repulsive 24)A straight wire of diameter 0.5 mm carrying a current of 1 A is replaced by another wire of 1 mm diameter and carrying the same current. The strength of magnetic field far away is (A)twice the earlier value. (C) half of the earlier value. (B)quarter of its earlier value (D) unchanged. 25)The magnitude of magnetic induction at a point due to small element is given by (A)Biot-Savart’s law. (C)Ampere’s law. F.Y.J.C. Science Mag. Effect of Electric Current

13. INFOMATICA ACADEMY (B)Oersted's law. CONTACT: 9821131002/9029004242 (D)Faraday’s law. 26)In S.I. system, permeability has the unit (A)Weber metre-1 ampere-1 (C) Weber metre-1 ampere-2 (B)Weber metre ampere-1 (D) Weber metre ampere 27)The magnetic field dB due to small current element at a distance and element carrying current is (A) (C) (B) (D) 28)A charge q moves in a region, where electric field and magnetic field both exist. Then the force on it is (A) (B) (C) (D) 29)Weber-m-2 is equal to . (A)tesla (B) henry (C) watt (D) dyne 30)A wire carrying current I and other carrying 2I in the same direction produce magnetic field at the mid-point. What will be the field, when 21 wire is switched off? (A)B/2 (B)2B (C) B (D) 4B 31)A constant current is flowing through a straight conductor. The variation of magnetic induction B with distance r of the observation point will be (A) (C) (B) (D) 32)Two long parallel wires are carrying currents 5 A and 12 A respectively. If the force per unit length between these wires is 3 10-5 N m-1, the distance between them should be (A)400 m (B)4 m (C) 0.4 m (D) 0.04 m F.Y.J.C. Science Mag. Effect of Electric Current

14. INFOMATICA ACADEMY 33)A circular loop of radius 10 cm carrying a current of 1 A is placed in a uniform CONTACT: 9821131002/9029004242 magnetic field of 0.1 T. If the magnetic field is perpendicular to the plane of the loop, the force experienced by the loop is (A)0.001 N (B) 0.314 N (C) 0.52 N (D) zero 34)When the charge is moving inside magnetic field only then force experienced varies (A)directly as the strength of the magnetic field only. (B)directly as the velocity of the charge. (C)directly as the strength of the magnetic field as well as the velocity of the charge. (D)inversely as the strength of the magnetic field and directly as the velocity of the charge. 35)The magnetic induction at the centre of a circular coil is 5 10-5 Wb/m2 and having 50 turns, carrying a current of 500 mA. The diameter of the coil is (A) 0.1m (B) 0.2 m (C) 0.02 m (D) 0.01 m 36)In a television picture tube an electron beam is subjected to a transverse magnetic field of 2.5 10-3 Wb/m2. If the speed of electrons in the beam is 3 105 m/s, then the magnitude of the lateral force exerted by the field on an electron is (A) 2.12 10-16 N (C) 1.2 10-16 N (B)11.2 10-16 N (D) 1.2 10-17N 37)A conductor of length 4 m and carrying a current of 2.5 A is kept inclined at (A)3.464 N (C) 4N (B)2N (D)2.5 N ❖Answers to Multiple choice questions 1.(B) 2. (B) 3. (C) 4. (C) 5. (B) 6.(B) 7. (A) 8.(B) 9. (B) 10. (B) 11. (D) 12. (A) 13. (A) 14. (B) 15.(C) 16.(C) 17.(B) 18.(A) 19.(D) 20.(B) 21.(D) 22. (A) 23. (A) 24.(D) 25.(A) 26.(A) 27.(C) 28.(B) 29. (A) 30.(C) 31.(B) 32.(C) 33.(D) 34.(C) 35. (B) 36.(C) 37.(B) F.Y.J.C. Science Mag. Effect of Electric Current