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Electron volt is a unit of potential difference.

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Important Questions on Work, Power and Energy

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

Two sources of light emit X-rays of wavelength

1 nm

and visible light of wavelength

500 nm

, respectively. Both the sources emit light of the same power

200 W

. The ratio of the number density of photons of X-rays to the number density of photons of the visible light of the given wavelengths is:

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

Light is incident normally on a completely absorbing surface with an energy flux of

25 W c m^{- 2}

. If the surface has an area of

25 c m^{2}

, the momentum transferred to the surface in

40 min

time duration will be:

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

Two particles move at right angle to each other. Their de Broglie wavelengths are

λ_{1}

and

λ_{2}

respectively. The particles suffer perfectly inelastic collision. The de Broglie wavelength

λ

of the final particle, is given by:

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

To excite the spectral line of wavelength

4960 \overset{\circ}{A}

of an atom, an excitation energy of

7.7 eV

is required. The ground state energy of the atom is

10.5 eV

. The energies of two levels involved in the emission of

4960 \overset{\circ}{A}

line are (Assume,

h c = 1240 eV nm

, where

h

is Planck's constant and

c

is speed of light)

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

A pulse of light of duration

100 ns

is absorbed completely by a small object initially at rest. Power of the pulse is

30 mW

and the speed of light is

3 \times 10^{8} m s^{- 1}

. The final momentum of the object is

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

A photodiode sensor is used to measure the output of a

300 W

lamp kept

10 m

away The sensor has an opening of

2 cm

in diameter. How many photons enter the sensor if the wavelength of the light is

660 nm

and the exposure time is

100 m s

. (Assume that all the energy of the lamp is given off as light and

h = 6.6 \times 10^{- 34} J s

)

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

2 m W

laser operates at a wavelength of

500 n m

. The number of photons that will be emitted per second is:
[Given Planck's constant

h = 6.6 \times 10^{- 34} J s,

speed of light

c = 3.0 \times 10^{8} m / s

]

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

The momentum of a photon is

2.5 \times 10^{- 29} kg m s^{- 1}

. Its frequency will be

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

160 W

light source is radiating light of wavelength

6200 \overset{°}{A}

uniformly in all directions. The photon flux at a distance of

1.8 m

is of the order of (Planck's constant

= 6.63 \times 10^{- 34} J s)

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

160 W

infrared source is radiating light of wave length

50000 \overset{°}{A}

uniformly in all directions. The photon flux at a distance of

1.8 m

is the order of

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

A monochromatic source of radiation is operating at

100 W

power. The source emits

2 \times 10^{20}

photons per second. The wavelength of light will be

(h = 6.67 \times 10^{- 34} J s)

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

An electron and a photon have the same wavelength of

10^{- 9} m .

If E is the energy of the photon and p is the momentum of the electron, the magnitude of

\frac{E}{p}

in S.I. units is,

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

The wavelength of

1 keV

photon is

1.24 \times 10^{- 9} m .

What is the frequency of

1 MeV

photon?

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

A light beam of intensity

20 W {cm}^{- 2}

is incident normally on a perfectly reflecting surface of sides

25 cm \times 15 cm

. The momentum imparted to the surface by the light per second is

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

The number of photons falling per second on a completely darkened plate to produce a force of

6.62 \times 10^{-5} N

n

. If the wavelength of the light falling is

5 \times 10^{- 7} m,

then

n = \dots \times 10^{22} . (h = 6.62 \times 10^{- 34} J - s)

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

If the energy of photons corresponding to wavelength of

6000 \overset{\circ}{A}

3.2 \times 10^{- 19} J

. The photon energy for wavelength of

4000 \overset{\circ}{A}

will be _____

EASY

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

Given below are two statements:

Statement $I :$ Two photons having equal linear momenta have equal wavelengths.

Statement $II :$ If the wavelength of the photon is decreased, then the momentum and energy of a photon will also decrease.

In the light of the above statements, choose the correct answer from the options given below.

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

1.5 kW

(kilo-watt) laser beam of wavelength

6400 \overset{\circ}{A}

is used to levitate a thin aluminium disk of same area as the cross-section of the beam. The laser light is reflected by the aluminium disk without any absorption. The mass of the foil is close to,

MEDIUM

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

Some energy levels of a molecule are shown in the figure. The ratio of the wavelengths

r = \frac{λ_{1}}{λ_{2}}

, is given by:

Question Image

HARD

Physics>Mechanics>Work, Power and Energy>Various Forms of Energy and Law of Conservation of Energy

A photon of wavelength

λ

is absorbed by an electron confined to a box of length

\sqrt{\frac{35 h λ}{8 m c}}

, As a result, the electron makes a transition from state,

k = 1

to the state,

n .

Subsequently, the electron transits from the state

n

to the state

m

by emitting a photon of wavelength

λ^{'} = 1.85 λ .

Then

Electron volt is a unit of potential difference.

Important Questions on Work, Power and Energy

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