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All questions of Magnetism for JAMB Exam

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Two concentric coils carry the same current in opposite directions. The diameter of the inner coil is half that of the outer coil. If the magnetic field produced by the outer coil at the common centre are 1 T, the net field at the centre is
  • a)
    4T
  • b)
    2T
  • c)
    1T
  • d)
    3T
Correct answer is option 'C'. Can you explain this answer?

Krishna Iyer answered

The magnetic field produced by a current-carrying coil at its center is given by the formula,
B = μ0 * (N*I/R),
where,
B is the magnetic field,
μ0 is the permeability of free space,
N is the number of turns in the coil,
I is the current through the coil, and
R is the radius of the coil.
In this case, both the coils carry the same current but in opposite directions. So, the fields produced by them will be in opposite directions. Also, the diameter of the inner coil is half that of the outer coil. Thus, the radius of the inner coil will be half that of the outer coil.
Therefore, the field at the center due to the inner coil will be double that due to the outer coil (because the magnetic field is inversely proportional to the radius).
Since the fields are in opposite directions, the net field at the center will be the difference between the two fields. That is, 2B (due to the inner coil) - B (due to the outer coil) = B.
So, if the field due to the outer coil is 1 T (Tesla), the net field at the center will also be 1 T.
Hence, the correct answer is 3. 1T.

Ampere’s circuital law is analogous to a law in electrostatics.That law is
  • a)
    Gauss’s law
  • b)
    Kirchhoff’s law
  • c)
    Faraday’s law
  • d)
    Coulomb’s law
Correct answer is option 'A'. Can you explain this answer?

Vijay Bansal answered
Ampere's law is similar to Guass's law in electrostatics. 

Guass's law corelates surface integral of electric field E with charge q over a surface. 

Ampere's law corelates the line integral of magnetic field B with current over a closed path.

Which combination of magnetic field lines and poles shows two magnets repelling each other?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Dr Manju Sen answered
  • The discovery that one particular pole of a magnet orients northward, whereas the other pole orients southward allowed people to identify the north and south poles of any magnet.
  • It was then noticed that the north poles of two different magnets repel each other, and likewise for the south poles. Conversely, the north pole of one magnet attracts the south pole of other magnets.
  • This situation is analogous to that of electric charge, where like charges repel and unlike charges attract. In magnets, we simply replace the charge with a pole: Like poles repel and unlike poles attract.

If a long hollow copper pipe carries a direct current, the magnetic field associated with the current will be
  • a)
    only outside the pipe
  • b)
    only inside the pipe
  • c)
    neither outside nor inside the pipe
  • d)
    both inside and outside the pipe
Correct answer is option 'A'. Can you explain this answer?

Hansa Sharma answered
For a loop inside the hollow portion of the conductor, the current enclosed is zero until the loop is on the surface or inside the material of the conductor.
∴Binside=0
For any loop outside, current enclosed by the amperian loop is not zero, hence the magnetic field is non-zero.
 

Wire of length l, carries a steady current I. It is bent first to form a circular coil of one turn. The same wire of same length is now bent more sharply to give two loops of smaller radius the magnetic field at the centre caused by the same current is
  • a)
    one third of its initial value
  • b)
    nine times of its initial value
  • c)
    four times of its initial value
  • d)
    unaltered
Correct answer is option 'C'. Can you explain this answer?

Om Desai answered
Let the radii be r1​ and r2​ respectively.
Since there are two turns of radius r2​, r1​=2r2​
Magnetic field B at the centre of  the coil of radius r1​ B1​=​μo​i/2r1​=​μo​i​/4r2
Magnetic field B at the center of the coil of radius r2​ B2​=2×​μo​i​/2r2
∴ B2/B1 =(2× μo​i/2r2​)/(μo​i /4r2​)​ ​​=4
Hence the answer is option C, four times its initial value.
 

 Ratio of magnetic force Fm and electric force Fe acting on a charge getting undeflected through the field is
  • a)
    E/B
  • b)
    B/E
  • c)
    1
  • d)
    0
Correct answer is option 'C'. Can you explain this answer?

Lavanya Menon answered
if the particle is undeflected, then the magnetic force equals the electric force:
q x v x B=q x E
B is the magnitude of the magnetic field
E is the magnitude of the electric field
v is the magnitude of the velocity of the particle.
If the forces are equal then:
v=E/B
For undeflected v=1
E/B=1

A particle of charge 1.6 x 10-19 C and mass 1.8 x 10-27 kg is moving around the path of radius 2 x 104 m with velocity 2.4 x 106 m/s. The magnetic field necessary is (in Wb/m²)​
  • a)
    13.5 x 10-6
  • b)
    135 x 10-6
  • c)
    0.135 x 10
  • d)
    1.35 x 10-6
Correct answer is option 'D'. Can you explain this answer?

Sreemoyee Choudhury answered
Explanation:

When an electron is projected in a uniform electric field and a uniform magnetic field, both pointing in the same direction as the electron's velocity, the following happens:

1. Electric field:

The electric field exerts a force on the electron in the direction of the field. Since the electron is negatively charged, it experiences a force opposite to the direction of the electric field. Therefore, the electric field does not affect the direction of the electron's motion.

2. Magnetic field:

The magnetic field exerts a force on the electron perpendicular to both the field direction and the electron's velocity. The force is given by the Lorentz force equation:

F = q(v x B)

where F is the force, q is the charge of the electron, v is its velocity, and B is the magnetic field.

In this case, the force is directed inward, towards the center of the circular path. The magnitude of the force is given by:

|F| = qvB

where |F| is the magnitude of the force.

Since the force is perpendicular to the velocity, it causes the electron to move in a circular path around the magnetic field lines. The radius of the path is given by:

r = mv/qB

where r is the radius of the path, m is the mass of the electron, and v is its velocity.

3. Combined effect:

Since the electric field does not affect the direction of the electron's motion, the only effect is due to the magnetic field. As the electron moves in a circular path, it loses kinetic energy due to the work done by the magnetic force. Therefore, its velocity decreases in magnitude.

Hence, the correct option is D- The electron velocity will decrease in magnitude.

Which of the following statements about earth's magnetism is correct
  • a)
    The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south-west to the north-east.
  • b)
    The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south-east to the north-west.
  • c)
    The earth behaves as a magnet with the magnetic field pointing approximately from the geographic east to the west.
  • d)
    The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south to the north.
Correct answer is option 'D'. Can you explain this answer?

Hansa Sharma answered
According to recent researches the magnetic field of earth is considered due to a large bar magnet situated in earth's core.
It is considered that the north pole of this large magnet is situated at the geographical south of earth and vice versa and as the magnetic field due to a bar magnet is from north pole to south pole of the maget thus the earth's magnetic field is considered from geographical south to geographical north which are respectively north and south poles of the bar magnet.

A circular coil of radius r carries current I. The magnetic field at its center is B. at what distance from the center on the axis of the coil magnetic field will be B/8
  • a)
    √3R
  • b)
    √2R
  • c)
    2R
  • d)
    3R
Correct answer is option 'A'. Can you explain this answer?

Shilpa Saha answered
As you know that magnetic field at point on the axis of current carrying ring is 


where x is the point on the axis of ring, R is the radius of ring , i is the current carrying on ring and N is the number of turns .



This is possible only when x = +-√3R 
Hence, √3R distance from the centre magnetic field is equal to magnetic field at centre .

Infinite number of straight wires each carrying current I are equally placed as shown in the figure. Adjacent wires have current in opposite direction. Net magnetic field at point P is
                                                                    
  • a)
     
  • b)
     
  • c)
     
  • d)
    Zero
Correct answer is option 'B'. Can you explain this answer?

Anaya Patel answered
Magnetic field at P is equal to the magnetic field due to AB,CD,EF and so on.
 Magnetic field due to AB is +Z direction.
and CD is negative −Z direction and EP is positive +Zdrection and so on.
BP​= (​μ0​I/4πd1)​ [sin30°−sin(−30°)k]+ ​(μ0​I/4πd2)​[sin30°−sin(−30°)l−k]+ μ0​I /4πd3​ ​[sin30°−sin(−30°)k]+...∞
BP​=2sin30°× (μ0​I​/4π) [(1/d1)​​−(1/d2​)​+(1/d3​)​−(1/d4​)​+(1/d5​)​....∞]k
= μ0​I/4π ​[(1/acos30°)​−(1/2acos30°)​+(1/3acos30°)−....∞]k
= μ0​I/4π(acos30°) ​[1−(1/2)​+(1/3)​−(1/4)​+(1/5)....∞]k
=(2μ0​I/4π√​3​a) ​ln(1+1)k= (μ0​I/4π√3​a) ​ln22k=(μ0​I/4π√3​a) ​ln4k

The force acting on a charge q moving with velocityin a magnetic field is given by
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Neha Sharma answered
The magnetic force on a free moving charge is perpendicular to both the velocity of the charge and the magnetic field with direction given by the right hand rule . The force is given by the charge times the vector product of velocity and magnetic field.

What kinds of materials are used for coating magnetic tapes?
  • a)
    Diamagnetic Materials
  • b)
    Ferrites
  • c)
    Electromagnet
  • d)
    Paramagnetic materials
Correct answer is option 'B'. Can you explain this answer?

Divey Sethi answered
Ceramics are used for coating magnetic tapes in a cassette player or for building memory stores in a modern computer. Ceramics are specially treated barium-iron oxides and are also called ferrites.

A neutron, a proton, an electron and an a-particle enters a uniform magnetic field with equal velocities. The field is directed along the inward normal to the plane of the paper. Which of these tracks followed are by a - particle.
  • a)
    A
  • b)
    B
  • c)
    C
  • d)
    D
Correct answer is option 'B'. Can you explain this answer?

Om Desai answered
Path C is undeviated. Therefore, it is of neutron's path. From Fleming's left-hand rule magnitude force on positive charge will be leftwards and on negative charge is rightwards. Therefore, tack D is of electron. Among A and B one of proton and other of α−particle ≤.
Further, r=mv/Bq or r∝m/q
Since, (m/q)>(m/q)P
∴rα>rP
or track B is of α particle.
 

Two long thin, parallel conductors carrying equal currents in the same direction are fixed parallel to the x–axis, one passing through y = a and the other through y = –a. The resultant magnetic field due to the two conductors at any point is B. Which of the following are correct ?
                                           
  • a)
     B = 0 for all points on the x-axis
  • b)
    At all points on the y-axis, excluding the origin, B has only a z-component.
  • c)
    At all point of the z-axis, excluding the origin, B has only a y-component.
  • d)
    B cannot have an x-component.
Correct answer is option 'A,B,C'. Can you explain this answer?

Preeti Khanna answered
On x-axis
A.  B=(μ0I/2πa)- (μ0I/2πa)=0
B.  On y axis say at (y, 0, 0)
B=[-μ0I/2π(a+y)]k + [μ0I/2π(a-y)]k
So except at origin, B has only z-components.
B cannot have x-component as B is perpendicular to direction of I. 
 

Which of the following laws give the direction of induced e.m.f
  • a)
    Faraday’s Law
  • b)
    Ampere’s Theorem
  • c)
    Biot Savart Law
  • d)
    Lenz’s Law
Correct answer is option 'D'. Can you explain this answer?

Knowledge Hub answered
Lenz’s law is used for determining the direction of induced current.
Lenz’s law of electromagnetic induction states that the direction of induced current in a given magnetic field is such that it opposes the induced change by changing the magnetic field.
Following is the formula of Lenz’s law:
ϵ=−N (∂ϕB/∂t)
Where,
  • ε is the induced emf
  • ∂ΦB is the change in magnetic flux
  • N is the number of turns in the coil
Lenz’s law finds application in electromagnetic braking and in electric generators

A Charge is fired through a magnetic field. The magnetic force acting on it is maximum when the angle between the direction of motion and magnetic field is
  • a)
    π
  • b)
    zero
  • c)
    π/2
  • d)
    π/4
Correct answer is option 'C'. Can you explain this answer?

Krishna Iyer answered
The force will have a magnitude F=qvB sin q, thus it will be maximum if sin q is maximum. Thus, angle between velocity and magnetic field should be 90o or the charge particle moves perpendicular to the velocity vector.

A rectangular loop carrying a current I is situated near a long straight wire such that the wire is parallel to the one of the sides of the loop and is in a plane of the loop. If a steady current I is established in wire as shown in figure, the loop will
  • a)
    move away from the wire or towards right
  • b)
    remain stationary
  • c)
    rotate about an axis parallel to the wire
  • d)
    move towards the wire
Correct answer is option 'D'. Can you explain this answer?

Top Rankers answered
The long straight wire and side AB  carry current in the same direction, hence will attract each other.
The long straight wire and side CD carry current in the opposite direction, hence will repel each other.
Force on side BC  will be equal and opposite to force on side DA.
Since CD  is farther from the wire than AB,  the force of attraction on  AB  will exceed the force of repulsion on CD.
Hence, there will be a net force of attraction on the loop ABCD and it will move towards the wire.

When a charged particle enters a magnetic field in a direction perpendicular to the field, which one of the following does not change?
  • a)
    velocity of the particle.
  • b)
    mass of the particle.
  • c)
    energy of the particle.
  • d)
    momentum of the particle
Correct answer is option 'C'. Can you explain this answer?

Geetika Shah answered
The magnetic force acts perpendicular to the velocity of the particle. This  causes circular motion. In the magnetic field the speed and kinetic energy of the particle remain constant, but the direction is altered at each instant by the perpendicular magnetic force.

A wire of length l, carrying current is bent into a loop and placed with its plane perpendicular to a magnetic field. In which of the following shapes, is the torque acting on the loop maximum?
  • a)
    Rectangle
  • b)
    Circle
  • c)
    Square
  • d)
    Equilateral triangle
Correct answer is option 'B'. Can you explain this answer?

Om Desai answered
The torque on a current loop depends upon the area of the current loop, when the magnetic field is perpendicular to the plane of the loops the torque has its maximum value,
Τ=I A B
We know I and B for all these cases but A depends upon the geometry. The circle has the greatest area so it should provide the greatest torque.

In the magnetic meridian of a certain place, the horizontal component of the earth’s magnetic field is 0.26G and the dip angle is 600. What is the magnetic field of the earth at this location
  • a)
    0.65G
  • b)
    0.62G
  • c)
    0.58G
  • d)
    0.52G
Correct answer is option 'D'. Can you explain this answer?

Om Desai answered
The earth's magnetic field is Be​ and its horizontal and vertical components are He​ and Hv​
cosθ= He​​/Be​
∴cos60o= (​0.26×10−4​/ Be )T
⇒Be​=(​0.26×10−4)/ (½)​=0.52×10−4T=0.52G

A Rowland ring of mean radius 15 cm has 3500 turns of wire wound on a ferromagnetic core of relative permeability 800. What is the magnetic field B in the core for a magnetising current of  12 A ?
  • a)
    4.48 T
  • b)
    5.48 T
  • c)
    44.8 T
  • d)
    48 T
Correct answer is option 'A'. Can you explain this answer?

Sankar Singh answered
Calculation of Magnetic Field B in the Core

Given:

Mean radius of Rowland ring, r = 15 cm = 0.15 m

Number of turns of wire, N = 3500

Relative permeability of ferromagnetic core, μr = 800

Magnetising current, I = 12 A



The magnetic field B in the core of Rowland ring can be calculated using the formula:

B = (μ0 * N * I) / (2 * r)

where, μ0 is the permeability of free space

μ0 = 4π * 10^-7 Tm/A

Substituting the given values in the formula,

B = (4π * 10^-7 Tm/A * 3500 * 12 A) / (2 * 0.15 m)

B = 4.48 T



Therefore, the magnetic field B in the core of Rowland ring for a magnetising current of 12 A is 4.48 T, which is option A.

Chapter doubts & questions for Magnetism - Physics for JAMB 2025 is part of JAMB exam preparation. The chapters have been prepared according to the JAMB exam syllabus. The Chapter doubts & questions, notes, tests & MCQs are made for JAMB 2025 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests here.

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