Tag: physics

Questions Related to physics

Choose the correct answer from the alternatives given.
Radiations of intensity $0.5\ W/m^2$ are striking a metal plate. The pressure on the plate is then

laws of heat transfer heat and thermodynamics physics

  1. $0.166 \, \times \, 10^{-8} \, N \, m^{-2}$

  2. $0.332 \, \times \, 10^{-8} \, N \, m^{-2}$

  3. $0.111 \, \times \, 10^{-8} \, N \, m^{-2}$

  4. $0.083 \, \times \, 10^{-8} \, N \, m^{-2}$

Show answer
Correct Option: A
Explanation:

Given: The intensity of the incident radiations is $0.5\ W/m^2$.


To find: The radiation pressure on the plate.

The radiation pressure experienced by the plate is given as;
$P = \dfrac{I}{c}\\Rightarrow \dfrac{0.5}{3 \times 10^8}\\Rightarrow  0.166 \times 10^{-8} N m^{-2}$

Option $(A)$ is correct.

A pan filled with hot food cools from $94^oC$ to $86^oC$ in$2$ minutes when the room temperature is at $20^oC$. The time taken to cool it from $71^oC$ to $69^oC$ is 

laws of heat transfer heat and thermodynamics physics

  1. $12\,s$

  2. $22\,s$

  3. $32\,s$

  4. $42\,s$

Show answer
Correct Option: D
Explanation:

For approximate calculation of the time taken,

$\cfrac { { T } _{ i }-{ T } _{ f } }{ \Delta t } =k\left[ \cfrac { { T } _{ i }+{ T } _{ f } }{ 2 } -{ T } _{ o } \right] $
where,
${ T } _{ o }\longrightarrow $room temperature
$T _{i} \longrightarrow$initial temperature
$T _{f} \longrightarrow$final temperature
$\Delta t \longrightarrow$time taken
$k \longrightarrow$constant
$\Longrightarrow \cfrac { 94-86 }{ 2 } =k\left[ \cfrac { 94+86 }{ 2 } -20 \right] \ \Longrightarrow 4=k[90-20]=k[70]\ \therefore k=\cfrac { 4 }{ 70 } \ \Longrightarrow \cfrac { 71-69 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ \cfrac { 71+69 }{ 2 } -20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \left[ 70-20 \right] \ \Longrightarrow \cfrac { 2 }{ \Delta t } =\cfrac { 4 }{ 70 } \times 50\ \therefore \Delta t=\cfrac { 70 }{ 2\times 50 } =0.7min=42sec$

Five kilomoles of oxygen is heated at constant pressure. The temperature of the oxygen gas is increased from 295 K to 305 K. If the molar heat capacity of oxygen at  constant pressure is 6.994 kcal/kmole K. The amount of heat absorbed is in kcal,

laws of heat transfer heat and thermodynamics physics

  1. 249.7

  2. 44

  3. 349.7

  4. 539.7

Show answer
Correct Option: C

Water is used to cool radiators of engines, because:

laws of heat transfer heat and thermodynamics physics

  1. Of its lower density

  2. It is easily available

  3. It is cheap

  4. It has high specific heat

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Correct Option: D

Star A emits radiation of maximum intensity at a wavelength of $5000 \mathring{A}$ and it has temperature $ 1227^oC $. If star B has temperature $ 2727^oC $ , then the maximum intensity would be observed at 

laws of heat transfer heat and thermodynamics physics

  1. $ 4000 \mathring{A} $

  2. $ 2250 \mathring{A} $

  3. $ 3000 \mathring{A} $

  4. $ 2500 \mathring{A} $

Show answer
Correct Option: D
Explanation:

$\begin{array}{l} \dfrac { { \lambda { m^{ 1 } } } }{ { \lambda m } } =\dfrac { T }{ { { T^{ 1 } } } }  \ \dfrac { { \lambda { m^{ 1 } } } }{ { 5000A } } =\dfrac { { 1227+273k } }{ { 2727+273k } }  \ \lambda { m^{ 1 } }=2500A \end{array}$

A standing wave is represented by an equation $y= 20 sin (50 \pi t )cos (10 \pi x)$. The frequency of the wave is

physics stationary waves formation of stationary waves stationary waves and its graphical representation standing waves in strings from moving to stationary stationary (or standing) waves

  1. 20 Hz

  2. 50 Hz

  3. 25 Hz

  4. 10 Hz

Show answer
Correct Option: C
Explanation:

The equation of a stationary wave is $y= 2A sin \omega t cos K x$. Comparing this equation with the equation given in the problem, we have
$2 \pi f = 50 \implies f = 25 Hz$

The correct option is (c)

Which of the following statement is incorrect during propagation of plane progressive mechanical wave?

physics stationary waves formation of stationary waves stationary waves and its graphical representation standing waves in strings from moving to stationary stationary (or standing) waves

  1. All the particles are vibrating in the same phase.

  2. Amplitude of all the particles is equal.

  3. Particles of the medium executes SHM.

  4. Wave velocity depends upon the nature of the medium.

Show answer
Correct Option: A
Explanation:

During propagation of a plane progressive mechanical wave all the particles are vibrating with different phases.
While all other statement are correct.

A standing wave is given by the equation $x=10 sin 5\pi t cos 3 x$. The amplitude of the wave will be

physics stationary waves formation of stationary waves stationary waves and its graphical representation standing waves in strings from moving to stationary stationary (or standing) waves

  1. a constant at all times

  2. will be increasing as t increases

  3. will be decreasing as t increases

  4. will fluctuate at x increases

Show answer
Correct Option: D
Explanation:

The amplitude of the stationary wave is $10 cos 3 x$ and this fluctuates as x increases

The correct option is (d)

A standing wave is represented by an equation $y= 10 sin 50 \pi t cos 10 \pi x$. The distance between adjacent nodes of the wave is

physics stationary waves formation of stationary waves stationary waves and its graphical representation standing waves in strings from moving to stationary stationary (or standing) waves

  1. 0.5 m

  2. 0.2 m

  3. 0.1 m

  4. 0.3 m

Show answer
Correct Option: C
Explanation:

At nodes, displacement $y=0$

For first node:
$\cos { \left( 10\pi x \right) =0\quad =\cos { \left( \cfrac { \pi  }{ 2 }  \right)  }  } \ \therefore \quad 10\pi x=\cfrac { \pi  }{ 2 } \ x=\cfrac { 1 }{ 20 } $
For second node:
$\cos { \left( 10\pi x \right) =0\quad =\cos { \left( \cfrac { 3\pi  }{ 2 }  \right)  }  } \ \therefore \quad 10\pi x=\cfrac { 3\pi  }{ 2 } \ x=\cfrac { 3 }{ 20 } $
$\therefore$ distance between them $=\cfrac { 3 }{ 20 } -\cfrac { 1 }{ 20 } \ \quad \quad =\cfrac { 1 }{ 10 } =0.1m$

The frequency of a sound wave is 250 Hz and its wavelength is 100 cm. The distance travelled by a sound wave in the time taken to produce 100 waves is ________.

physics stationary waves formation of stationary waves stationary waves and its graphical representation standing waves in strings from moving to stationary stationary (or standing) waves

  1. 100m

  2. 200m

  3. 300m

  4. 400m

Show answer
Correct Option: A
Explanation:

$v = 250 \times 1 = 250  m  s^{-1}$
$t = \displaystyle \frac{100}{250} s = 0.45$
Distance travelled by wave in the time taken to produce 100 waves $=$ v $\times$ time
$= $ 250 $\times$ 0.45
$=112.5m \approx 100m$