Physics 2006 Set 3 Sol

Physics 2005 Set 3 Close

Subjective Test

These are additional set 3 questions.

Question 1 ( 1.0 marks)

Name an appropriate communication channel needed to send a signal of band-width 100 kHz over adistance of 8 km. (1)

Solution:

Coaxial cable can be used to send a signal of band-width 100 kHz over a distance of 8 km.


Give the direction in which the induced current flows in the coil mounted on an insulating stand whena bar magnet is quickly moved along the axis of the coil from one side to the other as shown in thefigure. (1)

Solution:

(i) When the South Pole of the bar magnet is moved towards the coil, the current flows clockwise asseen from the right hand side.

(ii) When the North Pole of the bar magnet is moved away from the coil on the side, current flowsanticlockwise as seen from the right hand side.


Does the stopping potential in photoelectric emission depend upon

(i) the intensity of the incident radiation in a photocell?

(ii) the frequency of the incident radiation? (1)

Solution:

(i) The stopping potential does not depend on the intensity of the incident radiation in a photocell.

(ii) The stopping potential depends on the frequency of incident radiation.


An electron and a proton, moving parallel to each other in the same direction with equal momenta,enter into a uniform magnetic field which is at right angles to their velocities. Trace their trajectoriesin the magnetic field. (1)

Solution:

The electron as well as the proton will trace a circular path having same radius, but opposite indirection as shown in the figure below:

2/3/2011 Subjective Test Paper - Physics - Meritn…

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Define the term ‘dielectric constant’ of a medium. (1)

Solution:

Dielectric constant of a medium is the ratio of the force between two charges placed at a certaindistance apart in air to the force between two charges placed at a certain distance apart in themedium.


Figure shows a bar magnet M falling under gravity through an air cored coil C. Plot a graph showingvariation of induced e.m.f (E) with time (t). What does the area enclosed by the E − t curve depict?(1)

Solution:

The graph given below shows the variation of induced emf (E) with time (t):

The area enclosed by the E-t curve represents the work done per unit induced current in the circuit.


In a metre bridge, the balance point is found to be at 39.5 cm from the end A, when the resistor Y isof 12.5 ohm. Determine the resistance of X. Why are the connections between resistors in a metrebridge made of thick copper strips? What happens if the galvanometer and cell are interchanged atthe balance point of the bridge? Would the galvanometer show any current? (3)

Solution:

Resistance, Y = 12.5 Ω

Length of AD = 39.5 cm

Length of DB = 100 − 39.5 cm = 60.5 cm

At the balance point:



The resistance X is 8.16 Ω.

When the galvanometer and the cell are interchanged, the balance point will not be affected. Thegalvanometer will not show any current.


Define the terms ‘resistivity’ and ‘conductivity’ and state their S.I. units. Draw a graph showing thevariation of resistivity with temperature for a typical semiconductor. (3)

OR

State the principle of potentiometer. With the help of circuit diagram, describe a method to find theinternal resistance of a primary cell. (3)

Solution:

Resistivity of a material is the resistance offered by a unit cube of the material, when the currentflows perpendicularly through its faces.

S.I unit of resistivity is Ohm metre (Ωm).

Conductivity is the reciprocal of resistivity.

S.I unit of conductivity is Ω−1m−1.

The graph plotted below shows the variation of resistivity with temperature for a typicalsemiconductor.

OR

The connections are made as follows:

The internal resistance (r) of the cell E is to be determined. A resistance R is connected across thecell through a key K.

The key K′ is closed and K is kept open. The balance point is found out.

Let l1 be the balancing length in this case.

Then,

E ∝ l1 …. (i)

A resistance R is introduced in the resistance box R and the key K is kept closed.

Let l2 be the balancing length in this case.

When the circuit is closed, the potential difference across the cells falls to .

Then,



Dividing equation (i) by equation (ii), we obtain

The internal resistance of the cell is given by equation (iii).


Explain, with the help of a circuit diagram, how the thickness of depletion layer in p-n junction diodechanges when it is forward biased. In the following circuits, which one of the two diodes is forwardbiased and which is reverse biased?

Solution:

A p-n junction can be forward biased by applying a voltage such that its n-side is connected to thenegative terminal and p-side is connected to the positive terminal.

The applied voltage V is opposite to the junction barrier potential. The effective barrier potential is VB−V and the energy carrier across the junction decreases. The flow of majority charge carriers acrossthe junction increases and ultimately the width of the junction decreases.

(i) p-n junction is forward biased.

(ii) p-n junction is reverse biased.



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Physics 2006 Set 3 Sol