Physics [2 Mark Question Answer]

Advantage: A very small amount of nuclear fuel (such as uranium-235) can produce a tremendous amount of energy.

Disadvantage: The waste obtained from nuclear plants causes environmental
pollution.

Background radiations are the radioactive radiations to which we all are exposed even in the absence of an actual visible radioactive source. It total does is not very large, so it oes not cause any serious biological damage to us.

Two sources of background radiations are:
(i) Potassium present in our body.
(ii) Cosmic rays coming from outer space.

Radium is a radioactive substance. It gives out radioactive radiations which show fluorescence in zinc sulphide. Zinc sulphide is a fluorescent material, so it glows in dark.

α and β are positive and negative charged particles respectively, therefore these are deflected in electric or magnetic field whereas γ radiations are not charged particles so does not deflect.

β-rays are a stream of electrons emitted by the nucleus of an atom and γ-rays are electromagnetic waves of high frequency given off by nuclei.

Free electrons leave the metal surface only if sufficient energy is supplied to the metal. This emission of electrons from metal surface is called the electron emission.

This reaction is not allowed because even though charge (atomic number) is conserved in it, the mass number [= A on the L H.S. and equal to (A + 4)] on the R.H.S. is not conserved.

(i) Number of neutrons = Mass number – Atomic number
= 20 – 9 = 11
(ii) Number of protons = Atomic number = 9.

All the above radiations are not emitted in a single decay. There will be either an α emission or a β emission, which may be accompanied by γ-emission.

Radioactivity is a nuclear phenomenon. It is the process of spontaneous emission of α or β and γ radiations from the nuclei of atoms during their decay.
Example: uranium, radium.

The process of self, spontaneous and random emission of a or P and y radiations from the nucleus of elements of atomic number higher than 82, is called radioactivity.

In a CRT, the external P.D. refers to the electrical signal. The P.D. controls the deflection of electrons beam which strikes in the desired fashion direction and converts the electrical signal to visual signal.

(i) Decrease work function.
(ii) Coating the cathode with a layer of alkaline earth metallic oxides (or) hydrides.

Fluorescent screen helps to obtain images on the screen by converting electrical signals to light (or) visual signals by ionizing radiations to light waves.

Electrons in outer orbits are weakly bound with the nucleus. In solids these weakly bound electrons leave their individual atom and become a part of it. These electrons are known as free electrons.

Purpose of covering cathode by thorium and carbon is to reduce its work function, decrease its melting point.

By using materials of low work functions, the emission of electrons from such a surface takes place at relatively low temperature (at 1700°C in instead of 2500°C approximately).

(i) The substance must have high melting point.
(ii) The work function of the substance should be low so that electrons may be emitted even at low temperature.

As nucleus is positively charged it strongly attracts the negative charged electrons. The electron orbit close to the nucleus are tightly bound by strong attractive force of nucleus. These electrons are known as bound electrons.

No. of isotopes of an element have the same atomic number but they differ in their atomic masses, and hence in such isotopes, the number of neutrons will be different.

Atomic mass = Number of protons + Number of neutronsor 235 = 92 + Number of neutrons
∴ Number of neutrons = 235 - 92 = 143

Uses:
(a) In the field of medical science (X-rays photography used in surgery)
(b) In scientific work (to study crystal structure).

The wavelength of radiation B depends upon:
(a) The potential difference applied between cathode and anode.
(b) The material of the target.

The properties of radiation B differ from those of visible light are:
(a) These radiations can penetrate through the substances which are opaque to visible light.
(b) They destroy living cells.