Physics Ncert Solutions Class 12th

(a) Consider circuit in Fig 3.10. How much energy is absorbed by electrons from the initial state of no current (ignore thermal motion) to the state of drift     velocity? 

(b) Electrons give up energy at the rate of RI² per second to the thermal energy. What time scale would one associate with energy in problem (a)? n = no of electron/volume = 10²⁹/m3, length of circuit = 10 cm, cross-section = A = (1mm)²

In an experiment with a potentiometer, VB = 10 V. R is adjusted to be  50 Ω (figure). A student wanting to measure voltage E₁of a battery (approx. 8 V) finds no null point possible. He then diminishes R to 10 Ω  and is able to locate the null point on the-last (4th) segment of the potentiometer. Find the resistance of the potentiometer wire and potential drop per unit length across the wire in the second case.

A room AC runs for 5 a day at a voltage of 220 V. The wiring of the room consists of Cu of 1 mm radius and a length of 10 m. Power consumption per day is 10 commercial units. What fraction of it goes in the joule heating in wires? What would happen if the wiring is made of aluminium of the same dimensions?

A room AC runs for 5 a day at a voltage of 220 V. The wiring of the room consists of Cu of 1 mm radius and a length of 10 m. Power consumption per day is 10 commercial units. What fraction of it goes in the joule heating in wires? What would happen if the wiring is made of aluminium of the same dimensions?

A positive charge Qis uniformly distributed along a circular ring of radius R. A small test charge q is placed at the centre of the ring as shown in figure. Then

(a) If q>0 and is displaced away from the centre in the plane of the ring it will be pushed back towards the centre

(b) If q<0 and is displaced away from the centre in the plane of the ring, it will never return to the centre an will continue moving till it hits the ring

(c) If q<0 , it will perform SHM for small displacement along the axis.

(d) Q at the centre of the ring is in an unstable equilibrium within the plalne of the ring for q>O

A positive charge Q is uniformly distributed along a circular ring of radius R. A small test charge q is placed at the centre of the ring figure. Then,

(a) If q > 0 and is displaced away from the centre in the plane of the ring, it will be pushed back towards the centre

(b) If q < 0 and is displaced away from the centre in the plane of the ring, it will never return to the centre and will continue moving till it hits the ring

(c) If q < 0, it will perform SHM for small displacement along the axis

(d) Q at the centre of the ring is in an unstable equilibrium within the plane of the ring for q > 0

Refer to the arrangement of arrangement of charges in figure and a gaussian surface of radius R with Q charge at the center. Then 

(a) Total flux through the surface of the sphere is $\frac{-Q}{\mathit{\epsilon }0}$ .

(b) Field on the surface of the sphere of the sphere is -Q/4 $\pi \epsilon$ ₀R².

(c) Flux through the surface of sphere due to 5Q is zero.

(d) Field on the surface of sphere due to -2Q is same everywhere

Consider a region inside which there are various types of charges but the total charge is zero. At points outside the region,
(a) The electric field is necessarily zero
(b) The electric field is due to the dipole moment of the charge distribution only
(c) The dominant electric field is ∝ 1/r³, for large r, where r is the distance from an origin in this regions r
(d) The work done to move a charged particle along a closed path, away from the region, will be zero

The electric field at a point is
(a) Always continuous
(b) Continuous if there is no charge at that point
(c) Discontinuous only if there is a negative charge at that point (d) discontinuous if there is a charge at that point

Multiple Choice Questions

(One or More than One Correct Answer Type)

If Ф_s E . dS = 0 over a surface, then

(a) The electric field inside the surface and on it is zero
(b) The electric field inside the surface is necessarily uniform
(c) The number of flux lines entering the surface must be equal tothe number of flux lines leaving it
(d) All charges must necessarily be outside the surface