# NCERT Solutions for class 12th Physics Chapter 4 Moving Charges and Magnetism

#### Exercise

Question 1. A circular coil of wire consisting of 100 turns, each of radius 8.0 cm carries a cur rent of 0.40 A. What is the magnitude of the magnetic field B at the centre of the coil?

Question 2. A long straight wire carries a current of 35 A. What is the magnitude of the field B at a point 20 cm from the wire?

Question 3. A long straight wire in the horizontal plane carries a current of 50 A in north to south direction. Give the magnitude and direction of B at a point 2.5 m east of the wire.

Question 4. A horizontal overhead power line carries a current of 90 A in east to west direction. What is the magnitude and direction of the magnetic field due to the current 1.5 m below the line?

Question 5. What is the magnitude of magnetic force per unit length on a wire carrying a current of 8 A and making an angle of 30° with the direction of a uniform magnetic field of 0.15 T?

Question 6. A 3.0 cm wire carrying a current of 10 A is placed inside a solenoid perpendicular to its axis. The magnetic field inside the solenoid is given to be 0.27 T. What is the magnetic force on the wire?

Question 7. Two long and parallel straight wires A and B carrying currents of 8.0 A and 5.0 A in the same direction are separated by a distance of 4.0 cm. Estimate the force on a 10 cm section of wire A.

Question 8. A closely wound solenoid 80 cm long has 5 layers of windings of 400 turns each. The diameter of the solenoid is 1.8 cm. If the current carried is 8.0 A, estimate the magnitude of B inside the solenoid near its centre.

Question 9. 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 30° with the direction of a uniform horizontal magnetic field of magnitude 0.80 T. What is the magnitude of torque experienced by the coil?

Question 10. Two moving coil galvanometers, M1 and M2 have the following particulars:

Note: Refer Chapter at a Glance (18)

Question 11. In a chamber, a uniform magnetic field of 6.5 G (1 G = 10-4 T) is maintained. An electron is shot into the field with a speed of 4.8 x 106 m s-1 normal to the field. Explain why the path of the electron is a circle. Determine the radius of the circular orbit. (e = 1.6 x 10-19 C, me= 9.1 x 10-31 kg)

Question 12. In question 4.11 obtain the frequency of revolution of the electron in its circular orbit. Does the answer depend on the speed of the electron?

Question 13. (a) A circular coil of 30 turns and radius 8.0 cm carrying a current of 6.0 A is suspended vertically in a uniform horizontal magnetic field of magnitude 1.0 T. The field lines make an angle of 60° with the normal of the coil. Calculate the magnitude of the counter torque that must be applied to prevent the coil from turning.

(b) Would your answer change, if the circular coil in (a) were replaced by a planar coil of some irregular shape that encloses the same area? (All other particulars are also unaltered.)

Question 14. Two concentric circular coils X and Y of radii 16 cm and 10 cm, respectively, lie in the same vertical plane containing the north to south direction. Coil X has 20 turns and carries a current of 16 A; coil Y has 25 turns and carries a current of 18 A. The sense of the current in X is anticlockwise, and clockwise in Y, for an observer looking at the coils facing west. Give the magnitude and direction of the net magnetic field due to the coils at their centre.

Question 15. A magnetic field of 100 G (1 G = 10-4 T) is required which is uniform in a region of linear dimension about 10 cm and area of cross-section about 10-3 m2. The maximum current carrying capacity ofa given coil of wire is 15 A and the number of turns per unit length that can be wound round a core is at most 1000 turns m-1. Suggest some appropriate design particulars of a solenoid for the required purpose. Assume the core is not ferromagnetic.

Question 16. For a circular coil of radius R and N turns carrying current I, the magnitude of the magnetic field at a point on its axis at a distance x from its centre is given by,

Question 17. A toroid has a core (non-ferromagnetic) of inner radius 25 cm and outer radius 26 cm, around which 3500 turns of a wire are wound. If the current in the wire is 11 A, what is the magnetic field (a) outside the toroid,
(b) inside the core of the toroid, and (c) in the empty space surrounded by the toroid.

Question 18. Answer the following questions:

• (a) 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 initial velocity of the particle?
• (b) A charged particle enters an environment of a strong and non­ uniform magnetic field varying from point to point both in magnitude and direction, and comes out of it following a complicated trajectory. Would its final speed equal the initial speed if it suffered no collisions with the environment?
• (c) 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 deflecting from its straight-line path.

Sol. (a) Initial velocity v is either parallel or antiparallel to B.
(b) Yes, because magnetic force can change the direction of v, not its magnitude.
(c) B should be in a vertically downward direction.

Question 19. An electron emitted bya heated cathode and accelerated through a potential difference of 2.0 kV, enters a region with uniform magnetic field of0.15 T. Determine the trajectory of the electron if the field (a) is transverse to its initial velocity, (b) makes an angle of30° with the initial velocity.

Note: When a charged particle is accelerated through a potential difference V, Its kinetic energy is increased by eV.

Question 20. A magnetic field set up using Helmholtz coils (described in question 4.16) is uniform in a small region and has magnitude ofO.75 T. In the same region, a uniform electrostatic field is maintained in a direction normal to the common axis of the coils. A narrow beam of (single species) charged particles all accelerated through 15 kV enters this region in a direction perpendicular to both the axis of the coils and the electrostatic field. If the beam remains undeflected when the electrostatic field is9.0 x 10-sv m-1, make a simple guess as to what the beam contains. Why is the answer not unique?

Sol. Narrow beam of charged particles remains undeflected and is perpendicular to both electric field and magnetic fields which are mutually perpendicular. So, the electric force is balanced by magnetic force.

Here, we can only obtain charge to mass ratio and same ratio can be in Deuterium ions, He++, Li++, so the beam can contain any of these charged particles.

Question 21. A straight horizontal conduction rod of length 0.45 m and mass 60 g is suspended by two vertical wires at its ends. A current of 5.0 A is set up in the rod through the wires.

(a) What magnetic field should be set up normal to the conductor in order that the tension in the wires is zero?
(b) What will be the total tension in the wires if the direction of current is reversed keeping the magnetic field same as before?
[Ignore the mass of the wires.] g = 9.8 m s-2

Question 22. 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?

Note: The parallel wires carrying the current in opposite direction repel each other.if the current is in the same direction, they attract each other.

Question 23. A uniform magnetic field of 1.5 T exists in a cylindrical region of radius
10.0 cm, its direction parallel to the axis along east to west. A wire carrying current of 7.0 A in the north to south direction passes through this region. What is the magnitude and direction of the force on the wire if,

(a) the wire intersects the axis,
(b) the wire is turned from N-S to northeast-northwest direction.
(c) the wire in the N-S direction is lowered from the axis by a distance of 6.0 cm?

Sol. The magnetic field is in the direction east to west and in the cylindrical region of radius 10 cm.

Question 24. A Uniform magnetic field of 3000 G is esablished along the positive Z-direction. A rectangle loop of sides 10 cm and 5 cm carries a current of 12A. What is the torque on the loop in the different cases shown in figure? What is the torque on the loop in the different cases shown in figure? What is the force on each case? Which case corresponds to stable equilibrium?

Question 25. A circular coil of 20 turns and radius 10cm is placed in a uniform magnetic field of 0.10 T normal to the plane of the coil. H the current in the coil is 5.0A, what is the

(a) total torque on the coil,
(b) total force on the coil,
(c) average force on each electron in the coil due to the magnetic field? (The coil is made of copper wire of cross-sectional area 10-5 m2 and the free electron density in copper is given to about 1029 m-3.)

Question 26. A solenoid 60 cm long and and of radius4.0 cm has 3 layers of windings of 300 turns each. A 2.0 cm long wire of mass 2.5 g lies inside the solenoid (near its centre) normal to its axis; both the wire and the axis of the solenoid are in the horizontal plane. The wire is connected through two leads parallel to the axis of the solenoid to an extenal battery which supplies a current of6.0 A in the wire. What value of current [with appropriate sense of circulation] in the windings the solenoid can support the weight of the wire? g =9.8 ms-2•

Note: To support the wire, net force acting on it should be zero.

Question 27. A galvanometer coil has a resistance of 12Q and the metre show full scale
deflection for a current of 3 m A. How will you convert the metre into a voltmeter of range O to 18 V?

Note: Refer Chapter at a Glance (21)

Question 28. A galvanometer coil has a resistance of 15 Q and the metre shows full scale deflection for a current of 4 mA.How will you convert the metre into an ammeter of range O to 6 A?

Note: Refer Chapter at a Glance (20)

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