11 NCERT PHYSICS SUBJECTIVE QUESTION

Q1. What is Doppler's effect in sound waves?  Give an example.  What is the cause of Doppler's effect? Solution When a source of sound is approaching or is being approached by an observer, the pitch of the sound appears to be higher than its normal, and when it is receding or being receded by the observer, the pitch of the sound appears to fall below the normal.  This apparent change in the pitch due to the relative motion between the source and the observer (and the medium) is called Doppler's effect. When a whistling engine approaches an observer who is standing on the platform, he notices a rise in pitch, and fall in pitch when the engine recedes from him. It can be explained as follows.  When the whistling engine is approaching, the engine is following the waves already sent to the observer.  This results in crowding up of the waves.  The waves reach the observer in quicker succession and the pitch or the frequency, therefore, appears to rise....

Q1. What are coupled oscillations? Solution A system of two bodies connected by a spring so that both can oscillate in a straight line along the length of the string is known as a coupled oscillator.  The oscillations produced by a coupled oscillator are called coupled oscillations. Q2. Why are soldiers ordered to move out of step while crossing a bridge? Solution Every oscillator has its own frequency of oscillation.  When frequency of external force becomes equal to natural frequency of oscillations, resonance takes place and amplitude of vibration becomes maximum.  Bridge can be considered as stretched string.  If soldiers march in steps and frequency of marching steps is the same as that of frequency of bridge, then bridge will be set into resonant vibrations and hence collapse.  That is why soldiers are asked to break their steps. Q3. How earthquakes some times cause disaster? Solution When the waves produced during earthquake have...

Q1. What is meant by the term ' Molar specific heat ' of a gas?  The molar specific heat of hydrogen in the temperature range of about 250 K to 750 K is about (5/2) R.  At lower temperatures the value of molar specific heat of hydrogen decreases to the value typical of monoatomic gases (3/2) R while at higher temperatures, it tends to the value (7/2) R.  Explain. Solution Molar specific heat capacity of a gas refers to the amount of energy required for 1 mole of a substance to raise its temperature by 1 K.  In the temperature beyond 70 kelvin, rotational motion of H 2 gas starts.  So at 250 K < T < 750, the number of degrees of freedom becomes five -2 rotational and 3 translational Therefore,  C v =f/2 R becomes, C v = 5/2 R.  For lower temperatures only translational degrees of freedom will exist and no rotational freedom. Therefore,  C v = 3/2 R. Q2. What do you mean by an ideal gas? Solution An ideal gas or a perfect ga...

Q1. Discuss the application of the first law of thermodynamics to Isothermal process. Solution Isothermal process :  Since temperature remains constant in an isothermal process, dU = 0 From the first law of thermodynamics, dQ = dU + dW Therefore, dQ = dW = PdV (a)  Therefore, if a gas is to expand isothermally, then an amount of heat equivalent to the work done by the gas will have to be supplied to it from the external source. (b)  During isothermal compression, dV will be negative.  This implies that during an isothermal compression, the amount of heat equivalent to the work done on the gas will have to be removed from it. (c)  During isothermal compression or expansion, there is no change in the internal energy of the gas.   Q2. What thermodynamic variable is defined by (a)  Zeroth Law? (b)  First Law? Solution (a) The thermodynamic variable defined by the zeroth law is temperature. (b) The thermodynamic variable ...

Q1. One thermometer is placed half metre above the tip of the fire and one thermometer is placed half metre at the side of the fire. Would the readings of the two thermometers be same or different and why? Solution The thermometer above the fire would show higher temperature as compared to the thermometer placed at the side of the fire because above the fire, heat is received due to radiation as well as convection whereas at the side of fire, heat is received only due to radiation. Q2. Define triple point of water. Why is it unique? Solution It is the temperature at which the three phases of water, namely, ice, liquid water and water vapour are equally stable and coexistent. The triple point is suitable because it is unique, i.e., it occurs at one single temperature = 273.15 K and one single pressure of about 0.46 cm of the Hg column. This point is unique because any temperature or pressure higher or lower will change the phase of one or more states of water. Q3. W...

Q1. If mustard oil and water are dropped out from a tube separately, the mustard oil comes out later than water. Why? Solution If mustard oil and water are dropped out from a tube separately, the mustard oil comes out later than water because the coefficient of viscosity of mustard oil is very large as compared to the coefficient of viscosity of water. Hence, water rushes out quickly than mustard oil. Q2. What is the weight of the body, when it falls with terminal velocity through a viscous medium? Solution When a body falling through a viscous medium attains terminal velocity, the apparent weight of the body is balanced by the viscous force due to the medium. Therefore, the weight of a body falling with terminal velocity is zero. Q3. Bernoulli's theorem is a consequence of which principle? Solution Bernoulli's theorem is a consequence of the principle of conservation of energy applied to fluids in a motion. Q4. A plane is in level flight at const...