Physics Solved Past Paper 2nd year 2013 Karachi Board

SECTION ‘A’ (MULTIPLE CHOICE QUESTIONS)

xv) This was the first experimental verification of Einstein’s mass energy relation:
* Oeutron-inducedreaction
* Protons-induced reaction
* Gama induced reaction
* none of these

xvi) Meter bridge is used to measure:
* electric current
* voltage
* resistance
* capacitance

xvii) The sum of all the energies of the molecules in a substance is called:
* Heat energy
* Kinetic energy
* Temperature
* Potential energy

SECTION ‘B’ (SHORT – ANSWER QUESTIONS

2.i) Derive an expression for the Force on a conductor carrying current, placed in a uniform magnetic field.
Ans: See on Page # 234.

ii) With the help of a diagram, explain the working of a Full wave rectifier.
FULL WAVE RECTIFIER:
WORKING: In a full wave rectification, current flows through the load in the same direction for both half cycles of input A.C. voltage.
Two diodes Of and 02 can produce full wave rectification using a centre-tap transformer. A.C. Voltage of opposite polarity are applied simultaneously 1..) 01 and O2• In the next half cycle D1 does not conduct ‘but O2 conducts. hence unidirectional voltage is obtained as output

Note: A full wave rectification with four diodes can be produced by using a “bridge circuit” without using centre-tapped secondary. It
provides greater D.C. output voltage.

iii) How is the Chain reaction controlled in a nuclear reactor?
CHAIN REACTION: The emission of three neutrons (on the average) in the fission of a U-235 nudeus permits a self-sustaining chain reaction.
Consider a piece of high enrichec” urianium i.e. the uranium containing more U-235 atoms which is only 0.71% in natural uranium.

Then any stray neutron will prouce fission in one of the atoms of uranium, releasing 2 -to 3 neutrons. It is said that multiplication factor (K). is greater than- 1. If conditions are correct, the-neut-ons will produce fission in other uranium atoms in a rapid geometric progression. Hence a self maintained chain reaction will build -up, It will- result in an, Explosive vidence.
The minimum quantity-of fission material that is capable of producing self-sustaining chain reaction is called “critical mass’. A sub-critical piece of U-235 will not produce chain reaction.
Control Nuclear Reactor: Boron rods are used to control the reactor

iv) An Alternating current generator operates at 79Hz. The area of the coil is 500 cm2. Calculate the number of turns in the. coil
when a magnetic field of induction 0.06 web/m2 produces a maximum potential difference of 149 volts.

v) A hydrogen atom-in the grpund_state get excited by absorbing _a photon of 12.15 eV~ Findttie quantum number of this state. vi) Pair annihilation occurred due to a head-on-collision of an electron and a positron ‘having the same kinetic energy, producing pair of photons each having energy ‘of 2.5 MeV: What were their kinetic energies before collision? Given
moc2 = 0.511 MeV

vii) The number of atoms per gram of aaRa226is 2.666 x 1021 and it decays with a half life of 1622 years; find the Activity and
Decay constant of the sample:

xiii) How is the resistance of a metallic conductor affected by variation of temperature?
As the temperature of a metallic conductor rises, the amplitude of the vibration of the atoms in the lattice increases. This in turn, increases the probability of their collision with free electrons. This impedes the drift of the electrons. Hence the resistance of the conductor increases.

xiv) Give the construction and working of Wilson cloud chamber.
See on Page # 240.

xv) A voltmeter measuring upto 200 volts has a total resistance of 20,000 ohms. What additional series resistance must be connected to it to increase its range to 600 volts?

SECTION ‘C’ (DETAILED – ANSWER QUESTIONS)

3.a) Give the two statements of the Second law of Thermodynamics. On the basis of the First law of Thermodynamics, explain Isobaric process and derive its equation.
SECOND LAW OF THERMODYNAMICS
(1) Clausiu’s Statement: It is impossible to cause heat to flow from a
cold body to a hot body without the expenditure of work. This refrigerator statement depicts that no cyclic process can transfer heat from a colder place to a hotter place with no input of mechanical work.
(2) Kelvin’s statementl: It is impossible to construct a “perpetual motion machine” (PMM) of the second kind, i.e. it is impossible to device an engine, woiking in cycles continuosly, which device heat from a source and performs an equivalent amount of work without rejecting any heat to a body at a’ lower temperature {sink}. This engine statement depicts that.no cyclic process can convert heat completely into work ..
This statement shows that two bodies at different temperature are essential for the working of a heat engine. This fact seems to be a law of nature.
ISOBARIC PROCESS: A process that takes place at constant pressure is called an “isobaric process”
Consider a gas cylinder with insulated walls and pistons. Let the piston be moveable and frictionless.
If heat is absorbed by the gas, it expands. The internal pressure increases and the gas molecules push the piston up to a distance such that the internal pressure is equal to the extemal pressure.
The change of state takes place from PI Vl, Tl to PI V2, T2.
If is the distance moved by the piston and if A is the area of cross section of the piston, then, work done by the system

Conclusions:

(i) The heat absorbed at constant pressure is 6.Qp =~U + P6.V (called enthapy, 6.H)
(ii) The internal energy of a system may be changed, n two ways (a)
by doing work and (b) by transferring heat.
(iii) The work is given by the area unde, the graph on a PV – diagram.

b) What is Photo-electric effect? Explain its important results. Also derive Einstein’s photo-electric equation.
Ans: See on Page # 244.

4.a) What is a Compound capacitor? Derive an expression for its capacitance, if the space between the plates is partially filled with a slab of dielectric.
COMPOUND CAPACITOR: Capacitor consisting of two parallel plates partially filled with some dielectric and remaining with free space is called compound capacitor.
CAPACITANCE OF COMPOUND CAPACITOR: Consider a capacitor consisting of two parallel plates at a distance “d” from each other. Let the area of plates be “A” capacitance-is given by C = Eo A —– (1)

Now insert a dielectric of thickness “1” between the plates, Now this capacitor becomes the equivalent capacitor of series combination of two. capacitors. One capacitor with free space of thickness (d – t) and other with dielectric of thickness (t) using eq (1) capaictance of capacitor with free space becomes.

Capacitance of capacitor with dielectric of thickness (t) becomes

ELECTROSTATIC POTENTIAL ENERGY: The absolute electrostatic potential energy of electron in the orbit is given as E.P.E =-qV where “q” is the charge of electron and “V” is potential of that particular orbit in which the electron is revolving and it is given .by

Negative sign shown that energy is stored in the electron.
Total Energy: The total energy of the electron in a particular orbit is

5.a) State Ampere’s law. Derive the expression for magnetic field Of induction inside a long solenoid.
Ans: See on Page # 251.

b) Give the construction and working of a Moving coil galvanometer. Prove that the deflection is directly proportional to the current in radial magnetic field.
Ans: See on Page # 247

Posted on January 4, 2016 in 2nd Year 2013 Karachi Board Past Papers