CBSE Class 12 Physics Moving Charges And Magnetism Assignment

1. A narrow stream of protons and deuterons, having the same momentum values, enter a region of uniform magnetic field directed perpendicular to their common direction of motion. What would be the ratio of the radii of the circular paths described by the protons and deuterons?

2. An electron of kinetic energy 10keV moves perpendicular to the direction of a uniform magnetic field of 0.8 mT. Calculate the time period of rotation of the electron in the magnetic field.

3. Show that two long parallel conductors carrying equal currents in the same direction attract each other.

4. State Biot-Savart’s law. Use it to obtain the magnetic field at the centre of a circular coil of radius ‘a’ and carrying current I.

5. Find the ratio of magnitudes of the magnetic field of a current carrying circular coil of radius ‘a’ at its centre and at an axial point for which the value of ‘d’, the distance of axial point from the centre of the circle is √3a.

6. How can a large field be produced in a solenoid?

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

8. Write expression for Lorentz magnetic force. Hence define ‘ampere’.

9. What is the importance of a radial field and how is it produced?

10. Why is the core of moving coil galvanometer made of soft iron?

 Important Questions for NCERT Class 12 Physics Moving Charges And Magnetism

Question. A metallic rod of mass per unit length 0.5 kg m^–1 is lying horizontally on a smooth inclined plane which makes an angle of 30° with the horizontal. The rod is not allowed to slide down by flowing a current through it when a magnetic field of induction 0.25 T is acting on it in the vertical direction. The current flowing in the rod to keep it stationary is

(a) 7.14 A
(b) 5.98 A
(c) 14.76 A
(d) 11.32 A

Answer :  D

Question. A long straight wire carries a certain current and produces a magnetic field 2 × 10^–4 Wb m^–2 at a perpendicular distance of 5 cm from the wire.
An electron situated at 5 cm from the wire moves with a velocity 10⁷ m/s towards the wire along perpendicular to it. The force experienced by the
electron will be (charge on electron 1.6 × 10^–19 C)
(a) 3.2 N
(b) 3.2 × 10^–16 N
(c) 1.6 × 10^–16 N
(d) zero

Answer :  B

Question. A long solenoid carrying a current produces a magnetic field B along its axis. If the current is doubled and the number of turns per cm is halved, the new value of the magnetic field is
(a) B/2
(b) B
(c) 2 B
(d) 4 B

Answer :  B

Question. Two long parallel wires are at a distance of 1 metre.
Both of them carry one ampere of current. The force of attraction per unit length between the two wires is
(a) 5 × 10–8 N/m
(b) 2 × 10–8 N/m
(c) 2 × 10–7 N/m
(d) 10–7 N/m 

Answer :  C

Question. When a charged particle moving with velocity v is subjected to a magnetic field of induction B , the force on it is non-zero. This implies that
(a) angle between is either zero or 180°
(b) angle between is necessarily 90°
(c) angle between can have any value other than 90°
(d) angle between can have any value other than zero and 180°.

Answer :  D

Question. A straight wire of length 0.5 metre and carrying a current of 1.2 ampere is placed in uniform magnetic field of induction 2 tesla. The magnetic field is perpendicular to the length of the wire. The force on the wire is
(a) 2.4 N
(b) 1.2 N
(c) 3.0 N
(d) 2.0 N.

Answer :  B

Question. Ionized hydrogen atoms and a-particles with same momenta enters perpendicular to a constant magnetic field, B. The ratio of their radii of their paths rH : ra will be
(a) 1 : 4
(b) 2 : 1
(c) 1 : 2
(d) 4 : 1

Answer :  B

Question. If a long hollow copper pipe carries a current, then produced magnetic field will be
(a) both inside and outside the pipe
(b) outside the pipe only
(c) inside the pipe only
(d) neither inside nor outside the pipe.

Answer :  B

Question. The magnetic field at a distance r from a long wire carrying current i is 0.4 tesla. The magnetic field at a distance 2r is
(a) 0.2 tesla
(b) 0.8 tesla
(c) 0.1 tesla
(d) 1.6 tesla 

Answer :  A

Question. A proton and an alpha particle both enter a region of uniform magnetic field B, moving at right angles to the field B. If the radius of circular orbits for both the particles is equal and the kinetic energy acquired by proton is 1 MeV, the energy acquired by the alpha particle will be
(a) 1.5 MeV
(b) 1 MeV
(c) 4 MeV
(d) 0.5 MeV

Answer :  B 

Question. A proton carrying 1 MeV kinetic energy is moving in a circular path of radius R in uniform magnetic field. What should be the energy of an a-particle to describe a circle of same radius in the same field?
(a) 2 MeV
(b) 1 MeV
(c) 0.5 MeV
(d) 4 MeV 

Answer :  B

Question. Under the influence of a uniform magnetic field, a charged particle moves with a constant speed v in a circle of radius R. The time period of rotation of the particle
(a) depends on R and not on v
(b) is independent of both v and R
(c) depends on both v and R
(d) depends on v and not on R.

Answer :  B 

Question. An electron moving with kinetic energy 6.6X10^-14 J enters in a magnetic field of 4X10^-3T at right angles to it. The radius of its circular path will be nearest to : 
(a) 100 cm
(b) 75 cm
(c) 25 cm
(d) 50 cm

Answer :  D

Question. A charged particle enters a magnetic field H with its initial velocity making an angle of 45° with H.
Then the path of the particle will be: 
(a) circle
(b) helical
(c) a straight line
(d) a circle

Answer :  B

Question. A particle of mass m, charge Q and kinetic energy T enters in a transverse uniform magnetic field of induction  B. After 3 seconds the kinetic energy of the particle will be

(a) T
(b) 4T
(c) 3T
(d) 2T 

Answer :  A

Question. A charge having e/m equal to 108 C/kg and with velocity 3 × 105 m/s enters into a uniform magnetic field B = 0.3 tesla at an angle 30° with direction of field. The radius of curvature will be
(a) 0.01 cm
(b) 0.5 cm
(c) 1 cm
(d) 2 cm 

Answer :  D

Question. A positively charged particle moving due East enters a region of uniform magnetic field directed vertically upwards. This particle will
(a) move in a circular path with a decreased speed
(b) move in a circular path with a uniform speed
(c) get deflected in vertically upward direction
(d) move in circular path with an increased speed.

Answer :  B

Question. A galvanometer of resistance, G, is shunted by a resistance S ohm. To keep the main current in the circuit unchanged, the resistance to be put in series with the galvanometer is
(a) G /(S +G)
(b) S2 /(S +G)
(c) SG /(S +G)
(d) G2/ (S +G )

Answer :  D

Question. Magnetic field intensity at the centre of the coil of 50 turns, radius 0.5 m and carrying a current of 2 A, is
(a) 3 × 10^–5 T
(b) 1.25 × 10^–4 T
(c) 0.5 × 10^–5 T
(d) 4 × 10⁶ T 

Answer :  B

Question. A coil of one turn is made of a wire of certain length and then from the same length a coil of two turns is made. If the same current is passed in both the cases, then the ratio of the magnetic inductions at their centres will be
(a) 4 : 1
(b) 1 : 4
(c) 2 : 1
(d) 1 : 2

Answer :  B

Question. A galvanometer has a coil of resistance 100 ohm and gives a full scale deflection for 30 mA current. If it is work as a voltmeter of 30 volt range, the resistance required to be added will be
(a) 900 W
(b) 1800 W
(c) 500 W
(d) 1000 W. 

Answer :  A

Question. A 10 eV electron is circulating in a plane at right angles to a uniform field at magnetic induction 10–4 Wb/m² (= 1.0 gauss), the orbital radius of electron is
(a) 11 cm
(b) 18 cm
(c) 12 cm
(d) 16 cm 

Answer :  A

Question. A uniform magnetic field acts right angles to the direction of motion of electrons. As a result, the electron moves in a circular path of radius 2 cm. If the speed of electrons is doubled, then the radius of, the circular path will be
(a) 2.0 cm
(b) 0.5 cm
(c) 4.0 cm
(d) 1.0 cm 

Answer :  C

Question. A deuteron of kinetic energy 50 keV is describing a circular orbit of radius 0.5 metre in a plane perpendicular to magnetic field B. The kinetic energy of the proton that describes a circular orbit of radius 0.5 metre in the same plane with the same B is
(a) 25 keV
(b) 50 keV
(c) 200 keV
(d) 100 keV

Answer :  D

Question. A beam of electron passes undeflected through mutually perpendicular electric and magnetic fields.
If the electric field is switched off, and the same magnetic field is maintained, the electrons move
(a) in a circular orbit
(b) along a parabolic path
(c) along a straight line
(d) in an elliptical orbit.

Answer :  A

Question. A beam of electrons is moving with constant velocity in a region having electric and magnetic fields of strength 20 V m^–1 and 0.5 T at right angles to the direction of motion of the electrons. What is the velocity of the electrons?
(a) 8 m s^–1
(b) 5.5 m s^–1
(c) 20 m s^–1
(d) 40 m s^–1

Answer :  D

Question. A long wire carrying a steady current is bent into a circular loop of one turn. The magnetic field at the centre of the loop is B. It is then bent into a circular coil of n turns. The magnetic field at the centre of this coil of n turns will be
(a) nB
(b) n2B
(c) 2nB
(d) 2n2B.

Answer :  B

Question. A uniform electric field and a uniform magnetic field are acting along the same direction in a certain region. If an electron is projected in the region such that its velocity is pointed along the direction of fields, then the electron
"(a) will turn towards right of direction of motion
(b) speed will decrease
(c) speed will increase
(d) will turn towards left of direction of motion.

Answer :  B

Question. The magnetic force acting on a charged particle of charge –2 mC in a magnetic field of 2 T acting in y direction, when the particle velocity is
(2^i+ 3^j)×10⁶ ms⁻¹ is
(a) 4 N in z direction
(b) 8 N in y direction
(c) 8 N in z direction
(d) 8 N in –z direction. 

Answer :  D

Question. An electron moves in a circular orbit with a uniform speed v. It produces a magnetic field B at the centre of the circle. The radius of the circle is proportional to
(a) √B / v
(b) B/v
(c) √v / B
(d) v/B

Answer :  C

Question. The magnetic field due to a square loop of side a carrying a current I at its centre is 
(a) μ₀i / 2a
(b) μ₀i / √2Πa
(c) μ₀i / 2Πa
(d) √2μ0i /Πa

Answer :  D

Question. Electron of mass m and charge q is travelling with a speed along a circular path of radius r at right angles to a uniform magnetic field of intensity B. If the speed of the electron is doubled and the magnetic field is halved the resulting path would have a radius 
(a) 2 r
(b) 4 r
(c) r /4
(d) r/2

Answer :  B

Question. A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B′, at radial distances a/2 and 2a respectively, from the axis of the wire is
(a) 1
(b) 4
(c) 1 /4
(d) 1/2

Answer :  A

Question. A straight wire of diameter 0.5 mm carrying a current of 1 A is replaced by the another wire of 1 mm diameter carrying the same current. The strength of the magnetic field far away is
(a) one-quarter of the earlier value
(b) one-half of the earlier value
(c) twice the earlier value
(d) same as the earlier value. 

Answer :  D

Question. The magnetic induction at a point P which is at the distance of 4 cm from a long current carrying wire is 10–3 T. The field of induction at a distance 12 cm from the current will be
(a) 3.33 × 10^–4 T
(b) 1.11 × 10^–4 T
(c) 33 × 10^–3 T
(d) 9 × 10^–3 T

Answer :  A

Question. A long solenoid of 50 cm length having 100 turns carries a current of 2.5 A. The magnetic field at the centre of the solenoid is (m0 = 4p × 10–7 T m A–1)
(a) 6.28 × 10–4 T
(b) 3.14 × 10–4 T
(c) 6.28 × 10–5 T
(d) 3.14 × 10–5 T 

Answer :  A

Question. Two toroids 1 and 2 have total number of turns 200 and 100 respectively with average radii 40 cm and 20 cm respectively. If they carry same current i, the radio of the magnetic fields along the two loops is
(a) 1 : 1
(b) 4 : 1
(c) 2 : 1
(d) 1 : 2

Answer :  A

Question. A charged particle moves through a magnetic field in a direction perpendicular to it. Then the
(a) speed of the particle remains unchanged
(b) direction of the particle remains unchanged
(c) acceleration remains unchanged
(d) velocity remains unchanged. 

Answer :  A

Question. Magnetic field intensity at the centre of the coil of 50 turns, radius 0.5 m and carrying a current of 2 A, is
(a) 3 × 10^–5 T
(b) 1.25 × 10^–4 T
(c) 0.5 × 10^–5 T
(d) 4 × 10⁶ T 

Answer  B

Question. A coil of one turn is made of a wire of certain length and then from the same length a coil of two turns is made. If the same current is passed in both the cases, then the ratio of the magnetic inductions at their centres will be
(a) 4 : 1
(b) 1 : 4
(c) 2 : 1
(d) 1 : 2

Answer B

Question. A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B′, at radial distances a/2 and 2a respectively, from the axis of the wire is
(a) 1
(b) 4
(c) 1/4
(d) 1/2

Answer A

Question. The magnetic field at centre, P will be
(a) μ0 /4Π
(b) μ0 /Π
(c) μ0 /2Π
(d) 4μ0Π

Answer C

Question. A straight wire of diameter 0.5 mm carrying a current of 1 A is replaced by the another wire of 1 mm diameter carrying the same current. The strength of the magnetic field far away is
(a) one-quarter of the earlier value
(b) one-half of the earlier value
(c) twice the earlier value
(d) same as the earlier value. 

Answer D

Question. If a long hollow copper pipe carries a current, then produced magnetic field will be
(a) both inside and outside the pipe
(b) outside the pipe only
(c) inside the pipe only
(d) neither inside nor outside the pipe. 
 

Answer B

Question. The magnetic field at a distance r from a long wire carrying current i is 0.4 tesla. The magnetic field at a distance 2r is
(a) 0.2 tesla
(b) 0.8 tesla
(c) 0.1 tesla
(d) 1.6 tesla 

Answer A

Question. The magnetic induction at a point P which is at the distance of 4 cm from a long current carrying wire is 10–3 T. The field of induction at a distance 12 cm from the current will be
(a) 3.33 × 10^–4 T
(b) 1.11 × 10^–4 T
(c) 33 × 10^–3 T
(d) 9 × 10^–3 T

Answer A

Question. A long solenoid of 50 cm length having 100 turns carries a current of 2.5 A. The magnetic field at the centre of the solenoid is (m0 = 4p × 10–7 T m A^–1)
(a) 6.28 × 10^–4 T
(b) 3.14 × 10^–4 T
(c) 6.28 × 10^–5 T
(d) 3.14 × 10^–5 T 

Answer A

Question. Two toroids 1 and 2 have total number of turns 200 and 100 respectively with average radii 40 cm and 20 cm respectively. If they carry same current i, the radio of the magnetic fields along the two loops is
(a) 1 : 1
(b) 4 : 1
(c) 2 : 1
(d) 1 : 2

Answer A

Question. A long solenoid carrying a current produces a magnetic field B along its axis. If the current is doubled and the number of turns per cm is halved, the new value of the magnetic field is
(a) B/2
(b) B
(c) 2 B
(d) 4 B 

Answer B 

Question. A square loop, carrying a steady current I, is placed in a horizontal plane near a long straight conductor carrying a steady current I1 at a distance d from the conductor as
shown in figure. The loop will experience  
(a) a net attractive force towards the conductor
(b) a net repulsive force away from the conductor
(c) a net torque acting upward perpendicular to the horizontal plane
(d) a net torque acting downward normal to the horizontal plane.

Answer A 

Question. Two long parallel wires are at a distance of 1 metre.
Both of them carry one ampere of current. The force of attraction per unit length between the two wires is
(a) 5 × 10^–8 N/m
(b) 2 × 10^–8 N/m
(c) 2 × 10^–7 N/m
(d) 10^–7 N/m 

Answer C

Question. Two parallel wires in free space are 10 cm apart and each carries a current of 10 A in the same direction. The force exerted by one wire on the other, per metre length is
(a) 2 × 10^–4 N, repulsive
(b) 2 × 10^–7N, repulsive
(c) 2 × 10^–4 N, attractive
(d) 2 × 10^–7N, attractive.

Answer C

Question. A rectangular coil of length 0.12 m and width 0.1 m having 50 turns of wire is suspended vertically in a uniform magnetic field of strength 0.2 Weber/m2. The coil carries a current of 2 A. If the plane of the coil is inclined at an angle of 30° with the direction of the field, the torque required to keep the coil in stable equilibrium will be
(a) 0.24 N m
(b) 0.12 N m
(c) 0.15 N m
(d) 0.20 N m 

Answer D

Question. A current loop in a magnetic field
(a) can be in equilibrium in two orientations, both the equilibrium states are unstable.
(b) can be in equilibrium in two orientations, one stable while the other is unstable.
(c) experiences a torque whether the field is uniform or non uniform in all orientations.
(d) can be in equilibrium in one orientation.

Answer B 

Question. A square current carrying loop is suspended in a uniform magnetic field acting in the plane of the loop. If the force on one arm of the loop is ^→F the net force on the remaining three arms of the loop is
(a) 3^→F
(b) -^→F
(c) −3^→F
(d) ^→F

Answer B

Question. A charged particle (charge q) is moving in a circle of radius R with uniform speed v. The associatedmagnetic moment m is given by
(a) qvR2
(b) qvR2/2
(c) qvR
(d) qvR/2

Answer D

Question. A circular loop of area 0.01 m2 carrying a current of 10 A, is held perpendicular to a magnetic field of intensity 0.1 T. The torque acting on the loop is
(a) 0.001 N m
(b) 0.8 N m
(c) zero
(d) 0.01 N m.

Answer C

Question. A coil carrying electric current is placed in uniform magnetic field
(a) torque is formed
(b) e.m.f is induced
(c) both (a) and (b) are correct
(d) none of these

Answer A

Question. A current carrying coil is subjected to a uniform magnetic field. The coil will orient so that its plane becomes
(a) inclined at 45° to the magnetic field
(b) inclined at any arbitrary angle to the magnetic field
(c) parallel to the magnetic field
(d) perpendicular to magnetic field.

Answer D

Question. Current sensitivity of a moving coil galvanometer is 5 div/mA and its voltage sensitivity (angular deflection per unit voltage applied) is 20 div/V. The resistance of the galvanometer is
(a) 40 Ω
(b) 25 Ω
(c) 250 Ω
(d) 500 Ω

Answer C