Structure of Atom II Flashcards
what is an electromagnetiv radition/wave or radiant energy
James Maxwell (1870) was the first to give a
comprehensive explanation about the
interaction between the charged bodies and
the behaviour of electrical and magnetic fields
on macroscopic level. He suggested that when
electrically charged particle moves under
accelaration, alternating electrical and
magnetic fields are produced and
transmitted. These fields are transmitted in
the forms of waves called electromagnetic
waves or electromagnetic radiation.
The transfer of energy by electromagnetic waves is called electromagnetic radiation.
what are the characs of em waves
(i) The oscillating electric and magnetic
fields produced by oscillating charged
particles are perpendicular to each other
and both are perpendicular to the
direction of propagation of the wave.
(ii) These components( electric and magnetic field components) have the same wavelength, frequency, speed and amplitude, but they vibrate in two mutually perpendicular planes.
(iii) Unlike sound waves or water waves,
electromagnetic waves do not require
medium and can move in vacuum.
(iv) It is now well established that there are
many types of electromagnetic
radiations, which differ from one another
in wavelength (or frequency). These
constitute what is called
electromagnetic spectrum
(v) different kinds of unit are used to represent em radiations
different components of em spectrum
Some
examples are: radio frequency region
around 106 Hz, used for broadcasting;
microwave region around 1010 Hz used
for radar; infrared region around 1013 Hz
used for heating; ultraviolet region
around 1016Hz a component of sun’s
radiation. The small portion around 1015
Hz, is what is ordinarily called visible
light. It is only this part which our eyes
can see (or detect). Special instruments
are required to detect non-visible
radiation
define:
(i) wavelength
(ii) frquency
(iii) velocity
(iv) aplitude
(v) wave number
(i) dist b/w two consecutive crests/troughs
si unit: m
(ii) no of waves passing through a pointin 1 second
si unit: Hz( hertz) or second inverse(s⁻¹) [unit given by heinrich hertz]
(iii) linear distance travelled by a wave in 1 sec
m/s-1
(iv)the height of a crest or depth of trough, represented by a
(v) (ν ). It is defined as the number of wavelengths
per unit length. Its units are reciprocal of
wavelength unit, i.e., m–1. However commonly
used unit is cm–1 (not SI unit).
drawbacks of wave nature of radiatins
(i) the nature of emission of radiation from
hot bodies (black -body radiation)
(ii) ejection of electrons from metal surface
when radiation strikes it (photoelectric
effect)
(iii) variation of heat capacity of solids as a
function of temperature
(iv) line spectra of atoms with special
reference to hydrogen.
these could not be explained
give an example of black body radiation wrt iron rod
When solids are heated they emit
radiation over a wide range of wavelengths.
For example, when an iron rod is heated in a
furnace, it first turns to dull red and then
progressively becomes more and more red as
the temperature increases. As this is heated
further, the radiation emitted becomes
white and then becomes blue as the
temperature becomes very high. In terms of frequency, it means that the frequency of
emitted radiation goes from a lower frequency
to a higher frequency as the temperature
increases. The red colour lies in the lower
frequency region while blue colour belongs to
the higher frequency region of the
electromagnetic spectrum
what is a black body
The ideal body,
which emits and absorbs radiations of all
frequencies, is called a black body and the
radiation emitted by such a body is called
black body radiation
why is black body a violation em wave theory
acc to em wave theory, energy is absrobed or emitted continuously. Hence energy is directly proportional to inetsntity( square of amplitude). so acc to em theory, when a body is heated the radiation give n by ti may vary in intensity but should have the same frequency. but this was not the case as the different colours emitted have different frequency
wavelength-inetensity graph explanation
main points of max plancks quantum theory
(i) energy is not absrobed or emitted continuously but in a discontinuous manner in the form of small packets of emergy called quantum ( quanta plu.). for light each quantum is a photon
(ii) The energy of each quantum is direclty proportional to frquency.
(iii) The total amount of energy will always be an integral number of whole number product of no of quanta,
E= nhv
how was photoelectric effect discoered
In 1887, H. Hertz performed a very interesting
experiment in which electrons (or electric
current) were ejected when certain metals (for
example potassium, rubidium, caesium etc.)
Light of a particular frequency strikes
a clean metal surface inside a vacuum
chamber. Electrons are ejected from the
metal and are counted by a detector that
measures their kinetic energy.
explainthe main postulates of photelectric effect
The electrons are ejected from the metal
surface as soon as the beam of light
strikes the surface, i.e., there is no time
lag between the striking of light beam
and the ejection of electrons from the
metal surface.
(ii) The number of electrons ejected is
proportional to the intensity or
brightness of light.
(iii) For each metal, there is a characteristic
minimum frequency,ν0
(also known as
threshold frequency) below which
photoelectric effect is not observed. At a
frequency ν >ν
0
, the ejected electrons
come out with certain kinetic energy.
The kinetic energies of these electrons
increase with the increase of frequency
of the light used.
(iv) whena light of any fequency falls on the metal, the entire energy of the light is given to the e- immediately
important application of photoelectric effect
automatic sensored door opener. there is a streem of light is interruppted by person and hence it opens
explain bb radiation wrt placnks quantum theory
when the metal is being heated, the energy increases continously. hence the frequency of the radiation emitted by it also increases from lower frequency to higher freqeucny. red has lower frequency(high wavelength), then yellow , blue so on.
