Physics Grade 11

Physics Past Papers 11th Class Lahore Board

Section A-(MCQs)

i) The stationary waves consists of:                                                  (Mark 1)

A.    Crests and troughs              
B.    Compressions and elongations
C.    Nodes and anti-nodes      
D.    Reflection and rarefaction
Answer:
C.    Nodes and anti-nodes  

Section B-(Short Questions)

Q.2 iii) Name several repetitive phenomenon occurring in nature which could serve as reasonable time standard.                                                 (Marks 2)

Answer:
Any phenomena occurring in nature that repeats itself after exactly same intervals of time can be used as a measure of time. Some of these phenomena are as follows:
1.       Oscillating pendulum.
2.       Rotation of planets in a solar system.
3.       Change of shadow of an object in sun.
4.       Characteristic vibrations of crystals.
5.       Dropping of a certain mass of water or pouring of a specific mass of sand through a definite hole.

Section C-(Long Questions)

Q.7 a) Define and explain interference of sound waves. What are its kinds and conditions?                                                                                             (Marks 5)

Answer:
Interference:
                          The phenomenon produced due to superposition of two waves of same frequency and amplitude and are travelling in same direction is known as interference.
Explanation:
                          If two loud speakers act as wave sources of harmonic sound waves of a fixed frequency then waves are produced by an audio generator. And if we connect a microphone to a CRO then we can see the input signal of these sound waves on the screen.
From screen we could then see that compressions and rarefactions of sound waves are alternately emitted from the loud speakers. There will be two types of lines of screen, continuous circular lines and dotted circular lines. Continuous lines shows compressions, while dotted one shows the rarefactions.
There are two types of interference of waves:
1.       Constructive Interference.
2.       Destructive Interference
Constructive Interference:
                                                 If two compressions or two rarefactions of two waves fall upon each other, then they will reinforce each other and the resultant amplitude is increased. This type of interference is known as constructive interference.
Conditions for Constructive Interference:
“When two waves arrive at the same place at the same time in phase, they reinforce each other and constructive interference occurs”
Such type of interference takes place when crust of one wave fall on the crust of other wave and similarly trough to trough. This can only be possible if the path difference between the superposing of waves at any point must be equal to the integral multiple of their wavelength.
i.e.                          Path Difference = 0, λ, 2λ, 3λ, 4λ, 5λ, …………….
                                    P.D = nλ
Where,                  n = 0, 1, 2, 3, 4, ………………….
and,                        λ = wavelength of the waves
Destructive Interfere:
                                         If compression of one wave falls upon the rarefaction of the otter, they cancel each other’s effect and the resultant amplitude becomes zero. This kind of interference is called destructive interference.
Conditions for Destructive Interference:
“If two waves arrive at the same place at same time but are out of phase (1800), then destructive interference takes place”
Such type of interference takes place when crust of one wave fall on the trough of other wave. This can only be possible if the path difference between the superposing of waves at any point must be equal to odd integral multiple of their wavelength. Given as,
i.e.                          Path Difference = λ/2, 3λ/2, 5λ/2, 7λ/2, …………….
Where,                  n = 0, 1, 2, 3, 4, ………………….
and,                        λ = wavelength of the waves

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