CBSE Delhi-Set-3-2014

Drift velocity (v_d):

It is defined as the average velocity with which free electrons get drifted towards the positive end of the conductor under the influence of an external electric field applied. The drift velocity of electrons is of the order of 10^-4ms^-1.

The relationship of drift velocity with current is given by

Where, I = current following through the conductor,

n = number of density of free electrons,

A = area of cross section of the conductor and

e = charge of an electron.

Variation of current versus voltage for GaAs

The strong non-linear variation between current and voltage occurs in a material of GaAs. After a certain voltage, the current decreases as voltage increases, i.e., the effective resistance is negative.

Here, the convex lens is in contact with a plane mirror and an axial point object is placed at O at a distance OC = 20 cm.

As the image I of the object coincides with O, the rays refracted first from the lens and then reflected by the plane mirror must retrace their path. This can be possible, when the refracted rays fall normally on the mirror, i.e., the refracted rays from a beam parallel to principal axis of the lens. Hence, the object O must be at the focus of the convex lens.

Block diagram of a generalized communication system:

The element labelled 'X' in the figure is called 'channel'. The function of channel is to connect the transmitter and receiver. The signal from the transmitter is carried to the receiver by the communicating channel. A channel may be in the form of wires or wireless.

A glass bob is non-conducting, while a metallic bob is conducting. Due to the non-conducting nature of the glass bob, it will only experience the Earth's gravitational pull and hence will reach earlier.
As metallic bob has conducting nature, eddy current is induced in it when falls through the magnetic field of the Earth. By Lenz's law, the current induced is such that it opposes the motion of the metallic bob. The metallic bob will experience a force in the upward direction which will slow down its speed to some extent; hence, metallic bob will reach the Earth after the glass bob.

Equipotential surface is that surface at every point of which electric potential is same.

Imagine two points A₁ and A₂ on one surface. If V₁ and V₂ be the potentials at points A₁ and A₂ respectively then,

V₁ = V₂ or V₁ – V₂ = 0.

As the potential difference between any two points is the work done in taking a unit positive charge from one point to other is zero, hence it can be concluded that no work will be done if charge is moved from one point to other along an equipotential surface. This is only possible if line of force is perpendicular to such surface.

Cyclotron Principle: In cyclotron, an oscillating electric field is used to accelerate positively charged particle to high energy in presence of strong magnetic field.

Construction and working: It consists of two hollow metallic cylindrical chambers shaped like the letter D, hence these chambers called as Dees ie D₁ and D₂. The D’s are separated slightly along their diameters and these Dees are enclosed in an evacuated chamber. A very high frequency and high voltage oscillator is connected to these Dees. A source of positive ions is placed between the poles of electromagnet.

The positive ion is getting attracted towards negatively charged D due to electrostatic attraction, when the alternating current shifts from positive terminal to the negative terminal, the positive ions moves in the upper D. The ions follow a circular path and as they gain energy, their rotational radius increases and the ion spiral out from the Dees. The deflector plate helps to take the ion out and the ions are allowed to hit a target or a nucleus to bring about a nuclear reaction. Square wave electric fields are used to accelerate the charged particles in a cyclotron.