Moving Charges and Magnetism (17 Nov) - Live Session - NEET 2020 Contact Number: 9667591930 / 8527521718

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A circular coil of radius R carries an electric current. The magnetic field due to the coil at a point on the axis of the coil located at a distance r from the centre of the coil, such that r>>R varies as

1. 1/r

2. 1/r^{3/2}

3. 1/r^{2}

4. 1/r^{3}

The magnetic field due to a straight conductor of uniform cross-section of radius a and carrying a steady current is represented by :

1.

2.

3.

4.

Two parallel beams of positrons moving in the same direction will :

1. repel each other

2. will not interact with each other

3. attract each other

4. be deflected normal to the plane containing the two beams

A proton and an $\mathrm{\alpha}$-particle, moving with the same velocity, enter a uniform magnetic field, acting normal to the plane of their motion. The ratio of the radii of the circular paths described by the proton and $\mathrm{\alpha}$-particle is :

1. 1:2

2. 1:4

3. 4:1

4. 1:16

Circular loop of a wire and a long straight wire carry currents I_{c} and I_{e}, respectively as shown in figure. Assuming that these are placed in the same plane, the magnetic fields will be zero at the centre of the loop when the separation H is :

1. $\frac{{\mathrm{I}}_{\mathrm{e}}\mathrm{R}}{{\mathrm{I}}_{\mathrm{c}}\mathrm{\pi}}$

2. $\frac{{\mathrm{I}}_{c}\mathrm{R}}{{\mathrm{I}}_{e}\mathrm{\pi}}$

3. $\frac{{\mathrm{\pi I}}_{\mathrm{c}}}{{\mathrm{I}}_{\mathrm{e}}\mathrm{R}}$

4. $\frac{{\mathrm{I}}_{\mathrm{e}}\mathrm{\pi}}{{\mathrm{I}}_{\mathrm{c}}\mathrm{R}}$

What si the magnetic field at a distance R from a coil radius r carrying current I?

1. $\frac{{\mathrm{\mu}}_{0}{\mathrm{IR}}^{2}}{2{(\mathrm{R}}^{}}$^{2}^{2}

2. $\frac{{\mathrm{\mu}}_{0}{\mathrm{Ir}}^{2}}{2{(\mathrm{R}}^{}}$^{2}^{2}

3. $\frac{{\mathrm{\mu}}_{0}\mathrm{I}}{2\mathrm{r}}$

4. $\frac{{\mathrm{\mu}}_{0}\mathrm{l}}{2\mathrm{R}}$

A long straight wire of radius $\mathrm{\alpha}$ carries a steady current i. The current is uniformaly distributed across its cross section. The ratio of the magnetic field at a/2 and 2a is

1. 1/2

2. 1/4

3. 4

4. 1

In a mass spectrometer used for measuring the masses of ions, the ions are initially accelerated by an electric potential V and then made to decribe semicircular paths of radius R using a magnetic field B. If V and B are kept constant, the ratio $\left(\frac{\mathrm{charge}\mathrm{on}\mathrm{the}\mathrm{ion}}{\mathrm{mass}\mathrm{of}\mathrm{the}\mathrm{ion}}\right)$ will be proportional to

1. $\frac{1}{\mathrm{R}}$

2. $\frac{1}{{\mathrm{R}}^{2}}$

3. ${\mathrm{R}}^{2}$

4. R

Two concentric coils each of radius equal to 2 $\mathrm{\pi}$ cm are placed at right angles to each other. 3 ampere and 4 ampere are the currents flowing in each coil respectively. The magnetic induction in Weber/${\mathrm{m}}^{2}$ at the centre of the coils will be $({\mathrm{\mu}}_{0}=4\mathrm{\pi}\mathrm{x}{10}^{-7}\mathrm{Wb}/\mathrm{A}.\mathrm{m})$

1. ${10}^{-5}$

2. $12\mathrm{x}{10}^{-5}$

3. $7\mathrm{x}{10}^{-5}$

4. $5\mathrm{x}{10}^{-5}$

The magnetic field due to a square loop of side a carrying a current I at its centre is

1. $\frac{{\mathrm{\mu}}_{0}\mathrm{i}}{2\mathrm{a}}$

2. $\frac{{\mathrm{\mu}}_{0}\mathrm{i}}{\sqrt{2}\mathrm{\pi a}}$

3. $\frac{{\mathrm{\mu}}_{0}\mathrm{i}}{2\mathrm{\pi a}}$

4. $\sqrt{2}\frac{{\mathrm{\mu}}_{0}\mathrm{i}}{\mathrm{\pi a}}$

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 anf the magnetic field is halved the resulting path would have a radius

1. 2r

2. 4r

3. $\frac{\mathrm{r}}{4}$

4. $\frac{\mathrm{r}}{2}$

Electron moves at right angles to a magnetic field of 1.5 x 10${}^{-2}$ tesla with speed of 6 x 10${}^{7}$ m/s. If the specific charge of the electron is 1.7 x 10${}^{11}$ C/kg. The radius of circular path will be

1. 3.31 cm

2. 4.31 cm

3. 1.31 cm

4. 2.35 cm

An electron beam passes through a magnetic field of 2 x 10${}^{-3}$ Wb/m${}^{2}$ and an electric field of 1.0 x 10${}^{4}$ V/m both acting simultaneously. The path of electron remains undeviated. The speed of electron if the electric field is removed, and the radius of electron path will be respectively

1. 10 x 10${}^{6}$ m/s, 2.43 cm

2. 2.5 x 10${}^{6}$ m/s, 0.43 cm

3. 5 x 10${}^{6}$ m/s, 1.43 cm

4. none of these

A charged particle is released from rest in a region of uniform and magnetic fields which are parallel to each other. The particle will move on a:

1. straight line

2. circle

3. helix

4. cycloid

Four wires, each of length 2.0 m, are bent into four loops P, Q, R and S and then suspended in a uniform magnetic field. if the same current is passed each other, then the torque will be maximum on the loop

1. P

2. Q

3. R

4. S

A square coil of side a carries a current I. The magnetic field at the centre of the coil is

1. $\frac{{\mathrm{\mu}}_{\mathrm{o}}\mathrm{I}}{\mathrm{a\pi}}$

2. $\frac{\sqrt{2}{\mathrm{\mu}}_{0}\mathrm{I}}{\mathrm{a\pi}}$

3. $\frac{{\mathrm{\mu}}_{0}\mathrm{I}}{\sqrt{2}\mathrm{a\pi}}$

4. $\frac{2\sqrt{2}{\mathrm{\mu}}_{0}\mathrm{I}}{\mathrm{a\pi}}$

A charged particle moves through a magnetic field in a direction perpendicular to it. Then the

1. velocity remains unchanged

2. speed of the particle remains unchanged

3. directoin of the particle remains unchanged

4. acceleration remains unchanged

Wires 1 and 2 carrying currents ${\mathrm{i}}_{1}$ and ${\mathrm{i}}_{2}$ respectively are inclined at an angle $\mathrm{\theta}$ to each other. What is the force on a small element d/ of wire 2 at a distance of r wire 1 (as shown in figure) due to the magnetic field of wire 1?

1. $\frac{{\mathrm{\mu}}_{0}}{2\mathrm{\pi r}}{i}_{1}{i}_{2}dl\mathrm{tan}\theta $

2. $\frac{{\mathrm{\mu}}_{0}}{2\mathrm{\pi r}}{i}_{1}{i}_{2}dl\mathrm{sin}\theta $

3. $\frac{{\mathrm{\mu}}_{0}}{2\mathrm{\pi r}}{i}_{1}{i}_{2}dl\mathrm{cos}\theta $

4. $\frac{{\mathrm{\mu}}_{0}}{4\mathrm{\pi r}}{i}_{1}{i}_{2}dl\mathrm{sin}\theta $

If we double the radius of a coil keeping the current through it unchanged, then the magnetic field at any point at a large distance from the centre becomes approximately

1. double

2. three times

3. four times

4. one-fourth

A portion of a conductive wire is bent in the form of a semicircle of radius r as shown nelow in fig. At the centre of semicircle, the magnetic induction will be

1. zero

2. infinite

3. $\frac{{\mathrm{\mu}}_{0}}{4\mathrm{\pi}}.\frac{\mathrm{\pi i}}{r}gauss$

4. $\frac{{\mathrm{\mu}}_{0}}{4\mathrm{\pi}}.\frac{\mathrm{\pi i}}{r}\mathrm{tesla}$

A coil of circular cross-section having 1000 turns and 4 cm^{2} face area is placed with its axis parallel to a magnetic field which decreases by 10^{-2} Wb m^{-2} in 0.01 s. The emf induced in the coil is :

1. 400 mV

2. 200 mV

3. 4 mV

4. 0.4 mV

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