Short Answer TypeDescribe briefly with the help of a circuit diagram, how the flow of current carries in a p-n-p transistor is regulated with emitter-base junction forward biased and base-collector junction reverse biased.
In the given block diagram of a receiver, identify the boxes labelled as X and Y and write their functions.
You are given three lenses L1, L2 and L3 each of focal length 20 cm. An object is kept at 40 cm in front of L1, as shown. The final real image is formed at the focus ‘I’ of L3. Find the separations between L1, L2 and L3.
Define the terms (i) ‘cut-off voltage’ and (ii) ‘threshold frequency’ in relation to the phenomenon of photoelectric effect.
Using Einstein’s photoelectric equation shows how the cut-off voltage and threshold frequency for a given photosensitive material can be determined with the help of a suitable plot/graph.Mention three ‘different modes of propagation used in communication system. Explain with the help of a diagram how long distance communication can be achieved by ionospheric reflection of radio waves.
Draw a plot of potential energy of a pair of nucleons as a function of their separations. Mark the regions where the nuclear force is (i) attractive and (ii) repulsive. Write any two characteristic features of nuclear forces.
In a Geiger–Marsden experiment, calculate the distance of closest approach to the nucleus of Z =80, when an a-particle of 8 MeV energy impinges on it before it comes momentarily to rest and reverses its direction.
How will the distance of closest approach be affected when the kinetic energy of the a-particle is doubled?
The ground state energy of hydrogen atom is – 13.6 eV. If an electron makes a transition from an energy level – 0.85 eV to –3.4 eV, calculate the wavelength of the spectral line emitted. To which series of hydrogen spectrum does this wavelength belong?
Long Answer Type(a)
Write the expression for the force 
, acting on a charged particle of charge ‘q’, moving with a velocity in the presence of both electric field E and magnetic field B. Obtain the condition under which the particle moves un deflected through the fields.
(b)
A rectangular loop of size l × b carrying a steady current I is placed in a uniform magnetic field. Prove that the torque 
acting on the loop is given by 
is the magnetic moment of the loop.
a) Electric field on the particle is given by, 
Magnetic force on the particle is given by, 
Total force will be, 
When the charged particle moves perpendicular to both electric and magnetic field, F = 0
b) Consider a loop PQRS of length l, breadth b suspended in a uniform magnetic field.
Length of the loop, PQ= RS = l
Breadth of the loop, QR = SP = b
At any instance, plane of loop makes angle with the direction of magnetic field B
Suppose that the forces on sides PQ, QR, RS and SP are 
respectively.
The resultant of forces F2 and F4 is zero.
The side PQ and RS of current loop are perpendicular to the magnetic field, therefore the magnitude of each of forces F1 and F3 is,
F = I l B sin 90o = I l B
Moment of couple or torque is given by,
(Magnitude of one Force F) x perpendicular distance = (BIl). (b sin 
) = I (lb) B sin 
But, lb is the area of the loop = A.
But, lb is the area of the loop = A.
In vector form, 
Magnetic dipole moment of rectangular current loop is given by, M = NIA
Direction of torque is perpendicular to the direction of magnetic field.
Switch
