Next in the series is MODERN PHYSICS
By modern Physics we mean that Dual Nature, Atoms and Nuclei
For any competition this chapter is very important as it fetches about 10% by studying very less. Actually this topic has got very few equations to learn. We will take all the topics in sequence.
What was the need for this chapter ? Actually the photoelectric effect of light could not be explained by wave nature of LIGHT, so it is considered a particle. The ONLY aspect one has to understand the basis of particle nature is ONE electron in this effect will collide with only one electron.
First we take Dual Nature, in which we presume that electron has nature of particle as well as wave. As a particle we have its mass given as 9.1 × 10–31 kg and charge as 1.6 × 10 –19 C. But certain aspects can be proved only taken it as wave.
In that case KE = ½ mv2 and momentum = mv with energy as ½(pv)
While in case of photon rest mass is zero and it has momentum of p.
Work Function: W The minimum energy required by electron to reach the surface. Now the wave length l0, whose energy equals to work function is termed as threshold wave length. Similarly the threshold frequency is the frequency, corresponding to which energy equals to work function.
Second point to know is the stopping potential V0. It is reverse potential which will stop the photoelectric current completely. In other words it is a measure of kinetic energy of photoelectron emitted.
Then we have principle of conservation of energy depicted by Einstein equation. in various forms.
Graph of frequency Vs Stopping Potential can get value of Plank constant and threshold frequency.
Intensity in particle nature is number of photons per unit area per second, while in wave nature, it is proportional to square of Amplitude. Hence increasing the intensity increases the number of photoelectrons emitted. Thus increasing the saturation current I = venA
On the other hand if frequency is increased, individual energy of photon is increased, but BOT number of photons, in such a case as expected, stopping potential increases and NOT the saturation current.
By Davisson Germer method, the scientist actually calculate the hypothetical or calculated wave length, with actual. Both were found to be very closed so it is said that de Broglie Hypothesis is valid.
Next is Atoms, we have studies Dalton, Thompson, Rutherford and their drawbacks, here Bohr gave hypothesis.
In the Ruther ford experiment we can calculate the distance of closest approach.
Logic for this that at large distance we have only Kinetic energy, and at closest point we have only potential energy
LYMAN When electron jumps from n1 = 1 to n2 = 2,3,4.. we get spectral lines called Lyman series. They lie in the UV region of EM spectrum
BALMER When electron jumps from n1 = 2 to n2 = 3,4,5 we get Balmer series. This series lies in the Visible region of EM spectrum
Similarly we have Paschen, Bracket and Pfund series all of these lie in the infrared region. In these series n1 starts from 3,4and 5 and subsequently n2 are also starting from 4, 6 and 6.
Range of main series
Lyman: 1216 – 912 A0
Balmer: 6563 –3646 A0
Paschen: 18751 –8220 A0
Obviously visible radiations fall only in Balmer series.
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NUCLEUS
When the nucleus is big enough! It contains proton and neutron, Neutron is neutral but proton is positively charged, now as the number of protons increase the electrical repulsion increase. So why does heavy nucleus stays stable!
Answer is existence of strong nuclear forces which are :
Always attractive and also independent of charge!
These are short ranged forces.
So more number of neutrons, more attractive force and hence more stability, But still if the number of protons increase beyond the size of nucleus and number of neutrons, we can have unstable nucleus. Which breaks into smaller nucleus. This process is called fission, whereby an unstable nucleus breaks into two or more stable nucleus.
Similarly joining of two small nucleus to form large nucleus is called the fusion, which is responsible for the energy of SUN.
In decay on nucleus, it is first order chemistry reaction, also half life is similarly defined.