✓ Board-verified
Physics · 2026 · 3 marks
CBSE 2026 · Region 3 · Set 1 · Q23
(a) A circular coil of 30 turns and radius 8.0 cm carrying a current of 6 A is suspended vertically in a uniform horizontal magnetic field of 1.0 T.…
A circular coil of $\displaystyle 30$ turns and radius $\displaystyle 8.0$ cm carrying a current of $\displaystyle 6$ A is suspended vertically in a uniform horizontal magnetic field of $\displaystyle 1.0$ T . The field lines make an angle of $\displaystyle 30^{\circ}$ with the plane of the coil. Calculate the magnitude of the external torque that must be applied to prevent the coil from turning. What would happen if the circular coil is replaced by a planar coil of irregular shape that encloses the same area, keeping other parameters unchanged ?
An alpha particle (mass $\displaystyle 6.4 \times 10^{-27} \mathrm{~kg}$ and charge $\displaystyle 3.2 \times 10^{-19} \mathrm{C}$ ) having $\displaystyle 8$⋅$\displaystyle 0$ MeV energy, enters a region of a uniform magnetic field of $\displaystyle 0 \cdot 5 \mathrm{~T}$. If the field is directed perpendicular to the velocity of the particle, find the radius of the circular path described by the particle. Mention the condition under which the particle in this region (i) describes a helical path, and (ii) goes straight undeviated.
Marking-scheme solution
$\displaystyle \tau=\mathrm{NIBA} \sin \theta$
$\displaystyle =30 \times 6 \times 1 \times 3.14 \times\left(8 \times 10^{-2}\right)^{2} \times \sin 60^{\circ}$
$\displaystyle =90 \times 64 \times 3.14 \times 1.73 \times 10^{-4}$
$\displaystyle =3.13 \mathrm{~N} \mathrm{~m}$
Torque will remain the same.
$\displaystyle K.E=\dfrac{q^{2} B^{2} r^{2}}{2 m}$
$\displaystyle r=\dfrac{\sqrt{2 \times K.E. \times m}}{q B}$
$\displaystyle =\dfrac{\sqrt{2 \times 8 \times 1.6 \times 10^{-19} \times 10^{6} \times 6.4 \times 10^{-27}}}{3.2 \times 10^{-19} \times 0.5}$
$\displaystyle =\dfrac{12.8 \times 10^{-20}}{1.6 \times 10^{-19}}$
$\displaystyle =8 \times 10^{-1} \mathrm{~m}=80 \mathrm{~cm}$
(i)
Particle enters the magnetic field at an angle $\displaystyle 0^{\circ}<\theta<90^{\circ}$ (Alternatively: if a component of velocity is parallel or antiparallel to the magnetic field and another component is perpendicular to it)
(ii)
Particle moves parallel or antiparallel to the magnetic field.
All Moving Charges and Magnetism questions2026 question paperNCERT solutions: Moving Charges and MagnetismPractice Moving Charges and Magnetism →
More from Moving Charges and Magnetism
- A 2.0 cm segment of wire, carrying 5.0 A current in positive y-direction lies along y -axis, as shown in the figure. The magnetic field at a point (3…2024
- Two thin long parallel wires A and B are separated by a distance r and carry current I each in the same direction. The force per unit length exerted…2023
- Derive an expression for the magnetic field B, due to a circular coil of N turns, each of radius r carrying current I, at a distance ' x ' from the…2026
- Assertion: A proton and an electron enter a uniform magnetic field B with the same momentum p such that p is perpendicular to B. They describe…2024
- Beams of electrons and protons move parallel to each other in the same direction. They2023
- (i) A particle of mass m and charge q is moving with a velocity v in a magnetic field B as shown in the figure. Show that it follows a helical path.…2024
- Figure shows a narrow beam of electrons entering with a velocity of 3 × 10^7 m / s, symmetrically through the space between two parallel horizontal…2026
- A particle of mass m and charge q moving with a uniform velocity v=v_0 x i+v_0 y j enters a region with a magnetic field B=B_0 j. After some time, an…2023
CBSE Class 12 Physics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.