Science (English Medium) कक्षा १२ - CBSE Question Bank Solutions

A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. Find the magnifying power of the telescope for viewing distant objects when

1. the telescope is in normal adjustment,
2. the final image is formed at the least distance of distinct vision.

Draw a ray diagram of Astronomical Telescope for the final image formed at infinity

The Electric flux through the surface

(i)	(ii)
(iii)	(iv)

Five charges q₁, q₂, q₃, q₄, and q₅ are fixed at their positions as shown in figure. S is a Gaussian surface. The Gauss’s law is given by `oint_s E.ds = q/ε_0`

Which of the following statements is correct?

If `oint_s` E.dS = 0 over a surface, then ______.

1. the electric field inside the surface and on it is zero.
2. the electric field inside the surface is necessarily uniform.
3. the number of flux lines entering the surface must be equal to the number of flux lines leaving it.
4. all charges must necessarily be outside the surface.

If there were only one type of charge in the universe, then ______.

1. `oint_s` E.dS ≠ 0 on any surface.
2. `oint_s` E.dS = 0 if the charge is outside the surface.
3. `oint_s` E.dS could not be defined.
4. `oint_s` E.dS = `q/ε_0` if charges of magnitude q were inside the surface.

Consider a region inside which there are various types of charges but the total charge is zero. At points outside the region

1. the electric field is necessarily zero.
2. the electric field is due to the dipole moment of the charge distribution only.
3. the dominant electric field is `∞ 1/r^3`, for large r, where r is the distance from a origin in this region.
4. the work done to move a charged particle along a closed path, away from the region, will be zero.

Refer to the arrangement of charges in figure and a Gaussian surface of radius R with Q at the centre. Then

1. total flux through the surface of the sphere is `(-Q)/ε_0`.
2. field on the surface of the sphere is `(-Q)/(4 piε_0 R^2)`.
3. flux through the surface of sphere due to 5Q is zero.
4. field on the surface of sphere due to –2Q is same everywhere.

An arbitrary surface encloses a dipole. What is the electric flux through this surface?

If the total charge enclosed by a surface is zero, does it imply that the elecric field everywhere on the surface is zero? Conversely, if the electric field everywhere on a surface is zero, does it imply that net charge inside is zero.

In 1959 Lyttleton and Bondi suggested that the expansion of the Universe could be explained if matter carried a net charge. Suppose that the Universe is made up of hydrogen atoms with a number density N, which is maintained a constant. Let the charge on the proton be: e_p = – (1 + y)e where e is the electronic charge.

1. Find the critical value of y such that expansion may start.
2. Show that the velocity of expansion is proportional to the distance from the centre.

Kirchhoff’s junction rule is a reflection of ______.

1. conservation of current density vector.
2. conservation of charge.
3. the fact that the momentum with which a charged particle approaches a junction is unchanged (as a vector) as the charged particle leaves the junction.
4. the fact that there is no accumulation of charges at a junction.

In a meter bridge the point D is a neutral point (Figure).

1. The meter bridge can have no other neutral point for this set of resistances.
2. When the jockey contacts a point on meter wire left of D, current flows to B from the wire.
3. When the jockey contacts a point on the meter wire to the right of D, current flows from B to the wire through galvanometer.
4. When R is increased, the neutral point shifts to left.

What are the advantages of the null-point method in a Wheatstone bridge? What additional measurements would be required to calculate `R_(unknown)` by any other method?

What is the advantage of using thick metallic strips to join wires in a potentiometer?

Why are alloys used for making standard resistance coils?

Power P is to be delivered to a device via transmission cables having resistance RC. If V is the voltage across R and I the current through it, find the power wasted and how can it be reduced.

The circuit in figure shows two cells connected in opposition to each other. Cell E₁ is of emf 6V and internal resistance 2Ω; the cell E₂ is of emf 4V and internal resistance 8Ω. Find the potential difference between the points A and B.

Two cells of voltage 10V and 2V and internal resistances 10Ω and 5Ω respectively, are connected in parallel with the positive end of 10V battery connected to negative pole of 2V battery (Figure). Find the effective voltage and effective resistance of the combination.

A rectangular conducting loop consists of two wires on two opposite sides of length l joined together by rods of length d. The wires are each of the same material but with cross-sections differing by a factor of 2. The thicker wire has a resistance R and the rods are of low resistance, which in turn are connected to a constant voltage source V₀. The loop is placed in uniform a magnetic field B at 45° to its plane. Find τ, the torque exerted by the magnetic field on the loop about an axis through the centres of rods.