Torque

Q. At a point A on the earths surface, the angle of dip is $\delta =+25°$. At a point B on the earths surface, the angle of dip is$\delta =-25°$. We can interpret that

1. A and B are both located in the northern hemisphere.
2. A is located in the southern hemisphere and B is located in the northern hemisphere.
3. A is located in the northern hemisphere and B is located in the southern hemisphere.
4. A and B are both located in the southern hemisphere.

Q. To increase the current sensitivity of a moving coil galvanometer by $50$ %, its resistance is increased so that the new resistance becomes twice its initial resistance. By what factor does the voltage sensitivity change?

1. decreases by $75$ %
2. increases by $75$ %
3. decreases by $25$ %
4. increases by $25$ %

Q.

What is magnetic flux density and its unit?

Q. An electron moving in a circular orbit of radius $r$ makes $n$ rotations per second. The magnetic field produced at the centre has magnitude

1. zero
2. $\frac{{\mu }_{0}ne}{2\pi r}$
3. $\frac{{\mu }_{0}ne}{2r}$
4. $\frac{{\mu }_0{n}^{2}e}{2r}$

Q. Derive the expression for the torque acting on an electric dipole, when it is held in a uniform electric field. Identify the orientation of the dipole in the electric field, in which it attains a stable equilibrium.

Q.

Why does the value of the dip increase as we move from the equator to pole?

Q. A conducting wire is in the shape of a regular hexagon, which is inscribed inside an imaginary circle of radius R as shown in the figure. A current I flows through the wire. The magnitude of the magnetic field at the centre of the circle is

1. $\frac{\sqrt{3}{\mu }_{0}I}{2\pi R}$
2. $\frac{{\mu }_{0}I}{2\sqrt{3}\pi R}$
3. $\frac{\sqrt{3}{\mu }_{0}I}{\pi R}$
4. $\frac{3{\mu }_{0}I}{2\pi R}$

Q. A magnet is suspended in such a way that it oscillates in the horizontal plane it makes 20 oscillations per minute at a place where dip angle is 30 degree and 15 oscillation per minute at a place where dip angle is 60 degree ratio of total earth`s magnetic at the two places is

Q. A square coil of side 10 cm consists of 20 turns and carries a currentof 12 A. The coil is suspended vertically and the normal to the planeof the coil makes an angle of 30º with the direction of a uniformhorizontal magnetic field of magnitude 0.80 T. What is the magnitude of torque experienced by the coil?

Q. Two short magnets apply a force F on each other. If the separation between them is halved then. What is the new force between them.

Q. A conducting circular loop of resistance $20\Omega$ and cross-sectional area $20\times {10}^{-2}{m}^2$ is placed perpendicular to a spatially uniform magnetic field B, which varies with time t as $B=2\sin \left(50\pi t\right)T$. Find the net charge flowing through the loop in $20ms$ starting from $t=0$.

1. 0.5 $C$
2. 0.2 $C$
3. $0$ $C$
4. 0.14 $C$

Q.

Two identical coils have a common centre and their planes are at right angles to each other and carry equal currents. If the magnitude of the induction field at the centre due to one of the coil is ‘B’. then the resultant magnetic induction field due to combination is

1. B
2. 2B
3. $\sqrt{2}B$
4. $\frac{B}{\sqrt{2}}$

Q. A thin rectangular magnet suspended freely has a period of oscillation equal to $T$. Now it is broken into two equal halves (each having half of the original length) and one piece is made to oscillate freely in the same field. If its period of oscillation is $T^{\prime }$, the ratio $\frac{T^{\prime }}{T}$ is

1. $\frac{1}{2\sqrt{2}}$
2. $\frac{1}{2}$
3. $4$
4. $\frac{1}{4}$

Q. A conducting rod rotates with a constant angular velocity w about the axis which passes through point O and perpendicular to its length. A uniform magnetic field B exists parallel to the axis of the rotation. Then potential difference between the two ends of the rod is : (1) 6Bwl^2 (2) Bwl^2 (3) 10Bwl^2 (4) Zero

Q. (a) A circular coil of 30 turns and radius 8.0 cm carrying a currentof 6.0 A is suspended vertically in a uniform horizontal magneticfield of magnitude 1.0 T. The field lines make an angle of 60°with the normal of the coil. Calculate the magnitude of thecounter torque that must be applied to prevent the coil fromturning. (b) Would your answer change, if the circular coil in (a) were replacedby a planar coil of some irregular shape that encloses the samearea? (All other particulars are also unaltered.)

Q. The period of oscillation of a magnet of a vibration magnetometer is $2.45s$ at one place and $4.9s$ at the other. The ratio of the horizontal component of the earth’s magnetic field at the two places is

1. 1 : 4
2. 4 : 1
3. 1 : 2
4. 2 : 1

Q. What does this mean isotropy of space underlies the law of conservation of angular momentum?

Q. Four wires each of length 2.0 metres are bent into four loops P, Q, R and S and then suspended into uniform magnetic field. Same current is passed in each loop. Which statement is correct?

1. Couple on loop P will be the highest
2. Couple on loop Q will be the highest
3. Couple on loop R will be the highest
4. Couple on loop S will be the highest

Q. A load of mass $M$ is suspended from a steel wire of length $2$ and radius $1.0\text{mm}$ in Searle's apparatus experiment. The increase in length produced in the wire is $4.0\text{mm}$. Now the load is fully immersed in a liquid of relative density $2$. The relative density of the material of load is $8$. The new value of increase in length of the steel wire is:

1. $\text{5.0 mm}$
2. $\text{4.0 mm}$
3. $\text{3.0 mm}$
4. Zero

Q. 10.A rod of length l rotates with a small but uniform angular velocity w about its perpendicular bisector. A uniform magnetic field B exists parallel to the axis of rotation. The potential difference between the two ends of the rod is

Q. Four wires of equal lengths are bent in the shapes of four loops $\text{P},\text{Q},\text{R}$ and $\text{S}$ as shown in the figure. If each loop carries same current $I$ in the same sense, which loop will have highest magnetic moment ?

1. $\text{P}$
2. $\text{Q}$
3. $\text{R}$
4. $\text{S}$

Q.

A rectangular coil of 100 turns has length 5 cm and width 4 cm. It is placed with its plane parallel to a uniform magnetic field and a current of 2 A is sent through the coil. Find the magnitude of the magnetic field B, if the torque acting on the coil is $0.2N{m}^{-1}.$

Q.

The current sensitivity of a moving coil galvanometer can be increased by

1. Increasing the magnetic field of the permanent magnet

2. Increasing the area of the deflecting coil

3. Increasing the number of turns in the coil

4. Increasing the restoring couple of the coil

Q. A charge Q is spread uniformly over an insulated ring of radius ‘R’. If the ring is rotated with angular velocity $‘{\omega }^{\prime }$ with respect to the normal axis, the magnetic moment of the ring is

1. $\frac{Q\omega R^2}{4}$
2. $\frac{1}{2}Q\omega R^2$
3. $\frac{1}{3}Q\omega R^2$
4. $Zero$

Q.

Calculate the total torque acting on the body shown in figure (10-E2) about the point O.

Q.

Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Give reasons.

Q. A current carrying loop is free to turn in a uniform magnetic field. The loop will then come into equilibrium when its plane is inclined at

1. $0^{\circ }$ to the direction of the field
2. ${45}^{\circ }$ to the direction of the field
3. ${90}^{\circ }$ to the direction of the field
4. ${135}^{\circ }$ to the direction of the field

Q. In the given arrangement, the loop is moved with constant velocity $V$ in a uniform magnetic field $B$ in a restricted region of width $a$. The time for which the $\text{emf}$ is induced in the circuit is

1. $\frac{2b}{V}$
2. $\frac{2a}{V}$
3. $\frac{(a+b)}{V}$
4. $\frac{2(a-b)}{V}$

Q.

How do you measure self inductance?

Q. A circular coil of 20 turns and radius 10 cm is placed in a uniformmagnetic field of 0.10 T normal to the plane of the coil. If the currentin the coil is 5.0 A, 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 magneticfield?(The coil is made of copper wire of cross-sectional area 10–5 m2, andthe free electron density in copper is given to be about 1029 m–3.)