PHYSICS DICTIONARY

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Babinet’s Compensator

It is a device used for producing circular & elliptically polarized light and for their detection.

Back EMF

It is the electromagnetic force in an inductive circuit which acts in such a direction so as to oppose any change of current in the circuit.

Background

Term generally used in nuclear physics. The background radiation refers to the energetic particles reaching earth surface mainly due to cosmic rays comprising neutrons, muons, neutrinos, gamma etc.

Baking

Process in which materials meant for vacuum application are subjected to heat condition to reduce outgassing rate.

Ballistic Galvanometer

A moving coil galvanometer, in which coil has high inertia that indicates presence of an electric charge by single impulse imparted to coil by small instantaneous current, the quantity of electricity that passes being proportional to deflection of coil.

Ballistic Pendulum

A physical pendulum consisting of a large mass suspended from a rod; when it is stuck by a projectile, its displacement is used to measure the projection’s velocity.

Ballistics

Science of mechanics that deals with behavior and effects of projectiles, especially bullets, rockets etc.

Balmer Series

The spectrum of wavelength falling in visible region due to transition of electrons from higher orbits to second orbit is called Balmer series.

Band Spectrum

This spectrum is due to transition of electrons combined with rotatory, translatory and vibration effects of molecules. Hot gases in molecular state produce band spectrum.  It is also called molecular spectra. It consists of bright bands of different colors over dark background. Each band consists of closely spaced lines. The spacing between two bands and also width of the band depends on nature of compound. At very high temperature, the band spectrum changes to line spectrum as the molecules split in to atoms.     

 

Band Theory

Theory which aims at classifying materials as conductors, insulators, semiconductors based on the distribution of electron energy states. In solids, due to proximity of atoms, each distinct atomic state splits into series of closely packed electron states called as electron energy band. There are three types of electron band structures possible at 0 K as per this theory.

Band Width

Term used in amplifier. It is the band of frequencies over which the amplification gain remains constant.

Bar

It is a unit of pressure.

Barns

Unit used for nuclear scattering interactions. It is used to represent the measure of probability of interaction between small particles. The value of one barn is 10^-28 m² and is approximate crossection area of Uranium nucleus.   

Barometer

Instrument invented by Evangelista Torricelli to measure atmospheric pressure and hence for assisting in forecasting weather. 

Bartlett Force

It is type of nuclear force in which there is exchange of spin coordinates but not position coordinates between nucleons.

Baryons

They are a type of Fermions which are heavier than mesons.  They constitute the two nucleons with anti particles & Hyperons. 

or

Fermions whose mass is at least as great as mass of Proton and which can interact strongly are called Baryons.

Battery

A battery is an electrochemical cell which consists of an anode, a cathode and an electrolyte. It is used to provide a static potential for power or release electrical charge when needed.

BCC

It is a crystal structure equivalent to two interpenetrating simple cubic cells. The total number of atoms in unit cell is two. The coordination number is eight.

Beat Frequency

Phenomenon which can be heard when two sound waves of different frequency approach human ear; constructive and destructive interference leads to alternation of soft and loud sound. "The beat frequency equals absolute value of the difference in frequency of the two waves."

PHYSICS DICTIONARY

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Aberration
Defect in images formed by optical system arrangement.

Aberration of Starlight
The phenomenon of apparent displacement of star in the sky due to finite speed of light and motion of earth in its orbit about the sun is known as aberration of starlight.

Abrasive
A hard and wear resistant material that is used to wear, grind or cut away other material.


Abscissa
The horizontal coordinate of a point in a plane Cartesian coordinate system obtained by measuring parallel to the X-axis is called as abscissa.


Absolute Error
The difference between true value and measured value is called as absolute error.


Absolute Humidity
Absolute humidity denotes the amount of humidity in air regardless of the saturation level, expressed as the total mass of water molecules per air volume.


Absolute Permeability
Constant of proportionality between magnetic density and magnetic field strength of a material put in uniform magnetic field.


Absolute permittivity
Permittivity of vacuum is called absolute permittivity and its value is 8.85 x 10-12 F/m.


Absolute pressure
When pressure is measured above absolute zero (or complete vacuum), it is called as absolute pressure.


Absolute Temperature
Temperature measured using Kelvin scale when zero is absolute zero.


Absolute Zero
The temperature at which entropy of a system reaches minimum is referred as absolute zero.


Absorbed Fraction
A term used in internal dosimetry. It is the fraction of photon energy (emitted within a specified volume of material) that is absorbed by volume. The absorbed fraction depends on source distribution, photon energy, size, shape and composition of volume.


Absorbing Power
The ratio of amount of radiations absorbed by the body in a certain time to the amount of radiations incident on it in the same time is called absorbing power of body.


Absorptance
Ratio of amount of radiation absorbed by a surface to the amount of radiation incident upon it is called as absorptance. It is measure of ability of an object to absorb radiation.


Absorption Spectrum

Absorption spectrum is the characteristic property of absorbing material. Using this spectrum, one can identify what are the elements present in absorbing material. It is due to absorption of radiation by matter. Absorption is based on Kirchhoff’s law, which states that a substance which emits particular wavelength of radiation when excited also possess the property of absorbing the same wavelength from incident radiation when unexcited. Absorption spectra consist of dark lines over a bright background. When the white light is passed through the gas in atomic state (say sodium vapor), line absorption spectrum is formed. When white light is passed through molecular gas (say iodine vapor), band absorption spectrum is formed.

Absorption
The optical phenomenon where by the energy of a photon of light is assimilated with in a substance, normally by electronic polarization or by an electron excitation event.


Absorptivity
It is fraction of radiant energy falling upon the body which is absorbed or transformed into heat. This ratio varies with character of the surface and the wave length of incident energy.


Abundance
The ratio of the number of atoms of a specific isotope in a mixture of isotopes of an element to the total number of atoms present is called abundance. It is expressed in percentage.


Acceleration
It is a physical quantity which is defined as rate at which velocity of an object change with time.


Acceleration due to Gravity
The acceleration acquired by body due to gravitational pull is known as acceleration due to gravity.


Accelerator
Device used to accelerate charged particles to gain high energies. They are used in medical applications, making of radio isotopes etc.

Stefan's -Boltzmann Law

This law states that the total amount of radiant energy emitted by a black body per second per unit area is directly proportional to the fourth power of its absolute temperature i.e. E∝T⁴ or E=𝜎T⁴ where 𝜎 is called Stefan's constant. It has a value of 5.67 x 10⁻⁸ Wm⁻²K⁻⁴. This law is strictly true only when the medium surrounding the black body is vacuum.

The same law was established later by Boltzmann theoretically from thermodynamical considerations. Hence, this law is known as Stefan-Boltzmann law.

Consider the case of black body 'A' at absolute temperature 'T1' which is surrounded by another black body at absolute temperature 'T2'.

Now, 

Heat lost by black body 'A' is 𝜎T1⁴ 
Amount of heat absorbed by black body 'A' from black body 'B' is  𝜎T2⁴.

Therefore, Net amount of heat emitted by body 'A' per second per unit area is 𝜎(T1⁴ - T2⁴).

This is the form of "Stefans Boltzman Law". 

Note: This law is true only when medium surrounding the body is vacuum.

What is Statistical Mechanics?

When we consider bodies at macroscopic level they consist of uncountable atoms or molecules i.e. about 10²³ atoms/gm.mole. In such cases we cannot predict the result of interactions between atoms with the help of ordinary classical laws of motion.

Statistical Mechanics is the branch of Science which establishes the interpretation of macroscopic behaviour of system in terms of its microscopic properties.

The main theme is that it doesn't deal with motion of each particle but it takes into account the average or most probable properties of system without going into interior details of characteristics of its constituents.

The larger is the number of particles in the physical system considered, the more nearly correct are the statistical predictions. The smaller is the no. of particles (no. of degrees of freedom) in a mechanical system, the methods of mechanical system cease to have meaning.

Before the advent of quantum theory Maxwell, Boltzmann, Gibbs etc applied statistical methods making the use of classical physics. These statistical methods are known as Maxwell Boltzmann Statistics.

These statics explained successfully many observed physical phenomenon like temperature, pressure, energy etc; but couldn't explain adequately several other experimentally observed phenomenon like black body radiation, specific heat at low temperature etc.

In order to explain such phenomenon "quantum statistics" was introduced and developed by Fermi, Dirac, Bose, Einstein with new approach by using new quantum idea of discrete exchange of energy between system.

i) Bose-Einstein Statistics
ii) Fermi-Dirac Statistics

Atomic Structure - Important Points for competetive exams

1. Distance of closest approach: It is the distance from which the nucleus of an atom, the alpha particle comes to rest and its kinetic energy is totally converted  into electrostatic potential energy. It is denoted by ro.

ro = (1/4πεₒ)*[(2ze²)/(1/2)*(V²m)]

2. Diameter of atom: 10⁻¹⁰ meter

3. Diameter of Nucleus: 10⁻¹⁴ meter 

4. Impact Parameter(b):  (1/4πεₒ)*[(ze²tan𝜃)/(1/2)*(U²m)]; U is velocity of alpha particle

5. Impact Parameter(b) is inversely proportional to the angle of scattering(𝜃).

6. The equation mvr =n*(h/2π) is called "Bohrs quantisation condition".

7. The equation h𝝂=Ei-Ef is called "Bohr's Frequency condition".

8. Bohr's Radius r = (n²h²εₒ)/πme²

9. Velocity of electron (V) = e²/2nhεₒ 

10. If 'C' is velocity of light; V = [(1/4πεₒ)*(2πe²/Ch)]*(C/n)

11. The factor [(1/4πεₒ)*(2πe²/Ch)] is called fine structure constant. It is denoted by '𝛼'

12. The value of 𝛼=1/137; V = (1/137)*(C/n)

13. Energy of electron En = -(1/4πεₒ)²*(2π²me⁴/n²h²)

14. An electron can have only some definite values of energy while revolving in the orbits n=1,2,3,..... It is called energy quantization.

15. Energy Quantization: 

      E1 = -(1/4πεₒ)²*(2π²me⁴/n²h²) ;
      E2 = (1/4)*E1
      E3 = (1/9)*E1   ............................E∞ = 0

      E = -13.6/n²

 16. Rydberg's constant for Hydrogen (RH) is (1/4πεₒ)²*(2π²me⁴/ch³). Its value is 1.09678 x 10⁷m⁻¹

 17. Value of (1/4πεₒ) is 9x10⁹ C²N⁻¹m⁻².

 18. The charge 'e' of the electron is measured by Millikan's Oil drop method.

 19. The ratio of charge to mass(e/m) for an electron is measured by "Thomson".  

 20. Mass of electron(m) = 9.1 x 10⁻³¹ Kg

 21. Mass of Proton is 1835 times that of mass of electron.

 22. Canal Rays or Positive Rays are discovered by "E. Goldstein". Wien observed that these rays can be deflected in magnetic field and hence they are called Positive Rays.   

23. Rest mass energy of electron is 931 MeV

24. Orbital frequency of electron is (1/T) = (V/2πr)

25. Ionization energy of a Hydrogen atom is 13.6 eV

26. The excitation energy required by the electron to excite from state n1 to state n2 is En₂-En₁

Spectral series of Hydrogen atom

27. Lyman series lie in Ultravoilet region.

28. Balmer series lie in near UV region and visible region.

29. Paschen series lie in infrared region.

30. Brackett series also lie in infrared region. 

31. Pfund series also lie in far infrared region.

                                                          (1/) = R[(1/nf²)-(1/ni²)]

In addition to the above, nf=6, Humprey series results.

32. Velocity of an electron is independent of its mass.

33. Velocity of an electron is inversely proportional to the orbit.

34. The electron in the inner most orbit has highest velocity.

35. Velocity of a electron is independent of its mass.   

36. Orbital frequency is inversely proportional to the cube of 'n' i.e. 𝜈∝(1/n³).

37. If Ep & Ek represents Potential & Kinetic energies of the orbital electron, then Ek = -Ep/2.

38. When a Hydrogen atom is raised from the ground state to an excited state both kinetic energy and potential energy decrease.E∝(1/n²).

39. The energy difference between two consecutive energy levels decreases as the quantum number 'n' increases.

40. Bohr used conservation of angular momentum to explain his theory.

41. The velocity of an electron in the ground state is e²/2hεₒ = 2 x 10⁶ m/sec 

42. The ground state energy of Hydrogen atom is -13.6 eV. The energy needed to ionise the Hydrogen atom from its second excited state is 1.51 eV.

43. According to Bohr's principle, the relation between principle quantum number(n) and radius of orbit is  r ∝ n²

Heat Transfer due to convection

In this type of heat transfer, molecules of fluids move up bodily due to heating. Such heat transfer occurs when a fluid such as air or water comes in contact with an object whose temperature is higher than that of fluid. As temperature of fluid in contact with hot body increases, the fluid expands and thus becomes less denser and due to buoyant forces it rises & the position is occupied by cooler surrounding fluid and the process continues.

"Convection" is part of many natural process. Atmospheric convection plays an important role in determining global climate patterns & daily weather changes.

The rate of heat transfer by convection depends on the temperature difference between the surfaces and also on their areas. 

Heat Conduction

In this mechanism, heat transfer is due to vibration amplitudes of molecules & atoms present in solids.

Consider a cubicle of solid. Let us maintain one face of cube at high temperature (TH) and other opposite face at low temperature(Tc).

Due to temperature difference an amount of heat energy (Q) passes from  hot face to cold face in time 't'.

Conduction rate  Pcond (amount of energy transferred for uni time) is

Pcond = Q/t = K*A*(TH-Tc)/d;

where 'K' is coefficient of thermal conductivity, a constant for given material.
           'd' is thickness of slab
           'A' Area of slab
            't' is time of conduction

Therefore, Q= K*A(TH-Tc)*t/d

Note: i) 'K' depend on nature of material of which slab is made
         ii) A good thermal conductor has 'K' greater value.

Thermal Resistance to conduction(R-value):

This explains resisting of thermal conductivity. The R-value(thermal resistance) of a slab of thickness 'd' is defined as R=d/k. Thus material having less value of 'K' will have higher R-value and thus acts as a good thermal insulator.

Note:

i) 'R' is properly assigned to specified thickness of slab but not to material of slab.
ii) In steady state, conducting rates thru any no. of materials must be equal.
     Therefore, Pcond = A*(TH-Tc) / Σ(d/K)
iii) Heat is transferred from molecule to molecule by conduction. In this case molecules do not bodily move but simply vibrate.

Black body and its Radiation

A perfectly black body is the one which absorbs all the radiations of all wavelengths incident on it. Since it neither reflects not transmits any radiation it appears black in color what may be the color of incident radiation.

According to Kirchoffs law, a body which is capable of absorbing radiation must also be capable of emitting all possible wavelengths. So a perfectly black body is a good absorber as well as good radiator. When it is heated to a suitable high temperature, it emits radiations of all wavelengths (continuous spectrum). As the radiations emitted by black body is rich in maximum possible wavelengths and hence such Radiations are known as full Radiation or Total Radiation.     

The wavelength of emitted Radiation by a black body depends only on its temperature and is independent of the material of the body.

There is no body acting as perfect black body. The nearest approach is lamp black or platinum black. These are capable of absorbing the visible and a part near infrared but far infrared (heat Radiation) are reflected. So perfectly black body is just an ideal concept. For all practical purposes a lamp blacked surface can be considered as perfectly black body.

Energy Distribution in Black body Radiation

The distribution of energy in black body radiation for different wavelengths and at various temperatures was determined experimentally by Lummer and Pringsheim in 1899. They used the black body as an electrically heated chamber with narrow aperture.

The temperature of heated enclosure is measured by thermocouple.

The parallel beam of Radiation is allowed to incident on a "fluorspar prism" instead of a glass prism. The reason behind not using glass prism is that it absorbs some heat radaition.

The radiation is detected by means of Bolometer. Bolometer is an instrument to detect Thermal Radiation. The Bolometer is a linear type due to Lummer and Kurlabaum and is fitted with galvanometer 'G'. The deflection produced in the Galvanometer gives the intensity of Radiation, Eƛ. This is defined such that quantity Eƛ.dEƛ is the energy, for wavelengths lying between ƛ and ƛ+dƛ emitted per second per unit surface area of black body.   

The wavelengths at different parts of the spectrum was calculated by "Prism Dispersion Formula".

The experiment results are as follows:

1) The emission from a Black body at any temperature is composed of Radiation from all wavelengths.

2) At a given temperature, the energy is not uniformly distributed. As the temperature of the black body increases, the intensity of radiation for each wavelength increases. This shows that the total amount of energy is radiated per unit area per unit time increases with rise of temperature.

i.e., T 𝛼 Eƛ

3) The total energy of radiation at any temperature is given by the area between the curve corresponding to that temperature and horizontal axis. The increase in area found in accordance with Stefans law.

4) The amount of radiant energy emitted is small at very short and very long wavelengths. At a particular temperature, the spectral radiance Eƛ is maximum at particular wavelength ƛm. Most of the energy is emitted at wavelengths not very  different from ƛm.

5) The wavelength corresponding to the maximum energy represented by the peak of the curve shifts towards shorter wavelengths as the temperature increases. This is called Wiens Displacement. According to this law ƛm x T = constant.

This shows that as the temperature is increased, the black body emits the radiation of shorter wavelengths such that the product of temperature 'T' and maximum wavelength ƛm is a constant. 

The constant is called Wiens Displacement constant and has value 0.2896 x 10⁻² mK. 

There is change in wavelength due to Doppler effect.

Laws Related to Black Body:

a) Kirchoffs Law
b) Stefan-Boltzmann law
c) Wiens Law
d) Rayleigh-Jeans Law
e) Plancks Law

Important points to be noted:

i)  Wiens formula agrees in short wavelength region.
ii)  Rayleigh-Jeans formula agrees for long wavelength region.
iii) Plancks formula covers the entire region.
iv) When radiation is passed through a black body is passed through a prism, acontinuous spectrum is obtained. The energy is distributed in various wavelengths varying from 0 to infinity.
v) The law that connects the intensity with the wavelength is known as law of distribution of intensity of black body radiation.
vi) According to Stefans law, u=𝝈T⁴ where 𝝈 is Stefans constant and 'u' is energy density.

Spectral Energy Density, Total Energy Density, Emmisive Power & Absorptive Power

Spectral Energy Density: 

Spectral Energy Density for a particular wave length is the energy per unit volume per unit range of wavelength.

Total Energy Density:

Total Energy Density of Thermal Radiation at any point is the total radiant energy per unit volume around that point due to all wavelengths.

Emmisive Power:

The emmisive power of a body at a given temperature and for a given wavelength, is defined as the ratio of the radiant energy absorbed for second by unit surface area of the body per unit wavelength range.

Absorptive Power

The absorptive power of a body at a given temperature and wavelength is defined as the ratio of radiant energy absorbed per second by unit surface area of the body to the total energy falling per second on the same area.   

Concept of Thermodynamics - Zeroeth Law of Thermodynamics

For a system to be in thermodynamic equilibrium the following conditions must be full filled:-

i) Mechanical equilibrium

ii) Thermal equilibrium

iii) Chemical equilibrium

Mechanical equilibrium:

When there is no unbalanced force between system and its surroundings, the system is said to be in mechanical equilibrium.

Thermal equilibrium:

When the temperature in all parts of system is same as that of surroundings, the system is said to be in thermal equilibrium.

Chemical equilibrium:

If the chemical composition is same throughout the system and surroundings it is said to be in chemical equilibrium.

Zeroeth Law of Thermodynamics:

This law was first enunciated by RH Flower in 1831. According to this law when two systems ‘A’ and ‘B’ are in thermal equilibrium with another system ‘C’ then ‘A’ and ‘B’ will also be in thermal equilibrium.

What does First Law of Thermodynamics infer us?

• It is impossible to derive any work without expenditure of an equivalent amount of energy in some other forms. 

• Heat absorbed by the system should be taken positive. Heat rejected by the system should be taken negative. 

• For an ideal gas the total kinetic energy (KE) of all its molecules is called internal energy(U). For such a gas the internal energy depends only on Temperature.

What is ENTROPY - Very important Points to be noted

1) The thermal property which remains constant during an adiabatic process is called as entropy.
     i.e. dQ/T= constant

2) It is a measure of randomness or disorderliness of molecules.

3) It is independent of the path of thermal cycle.

4) The increase in entropy implies transition from ordered state to disorder state.

5) It is an index of unavailable energy of a system.

6) Entropy could also be termed as thermal inertia since more entropy results in less amount of heat energy being converted into work.

7) The increase in Entropy of a system implies transition of thermal energy from more available energy to less available form for conversion into work.

8) The net change in entropy is zero for any reversible cycle. This statement is called as clausius theorem.

9) Clausius Theorem: - The sum of quantities of heat transfer during the isothermal change divided     by absolute temperature of the isothermal in a reversible cycle is zero. Entropy changes linearly in isothermal expansion and remains constant in adiabatic expansion or compression but decreases in isothermal compression.

The shape of Temperature (T) - Entropy (S) diagram (Tephigram) is rectangle.

10) Entropy increases in irreversible process.

11) Definition of second law of thermodynamics in terms of entropy

”Every chemical or physical or natural process in nature takes place in such a manner that total entropy increases or remains constant".

12) The principle of "degradation of energy" states that the available energy tending towards zero.

Specific Heat of Gases

Units and Designation

PRINCIPLE OF ELECTRON SPIN RESONANCE (ESR)

Electron Spin Resonance (ESR) is a branch of absorption spectra in which radiation having frequency in microwave region is absorbed by paramagnetic substances to induce transitions between magnetic energy levels of electrons with unpaired spins.

ESR also called Electronic Paramagnetic Resonance is a spectroscopic technique confined to study of those species having one or more unpaired electrons.

Phenomenon of ESR was invented by Zaveiskii in 1904.


PRINCIPLE OF ESR

When we consider an unpaired electron it is associated with spin. When a magnetic field is applied, the magnetic moment of electron interacts with field and results in splitting of otherwise degenerate field. Now the energy difference between the levels falls in microwave region.So when radiation in microwave range equal to this energy difference is made to incident on substance, transitions occur between these levels absorbing quanta of energy leading to a absorption peak.

Only spin moment contributes towards the magnetic behaviour of electrons.

Consider that system has only spin magnetic moment

μ= -gμBS ....................................(1)

'μ' & 's' are in opposite directions.

For an electron of spin S=1/2, the spin angular momentum quantum number will have values
ms = ±1/2 ..................................(2)

In absence of magnetic field, the two values of 'ms' i.e. +1/2 and -1/2 will give rise to a doubly degenerate spin energy state.

When magnetic field is applied this degeneracy is removed and thus leads to two non degenerate energy levels.

Now the interaction energy is given by

Polar Dielectric in uniform electric field

There are permanent dipoles present in polar dielectric which are randomly aligned in such a way that there is permanent dipole moment Pp. [see fig a]

 
When a dipole is present in an uniform electric field the dipole tries to align itself in the direction of electric field.

Because of this all dipoles in a polar dielectric are partially aligned in the direction of field. This partial alignment is responsible for the induced dipole moment Pi.[see fig b].                 
Therefore, the electric dipole moment is increasing.   

P =  Pp + Pi

The electric dipole moment of a polar dielectric increases
a) by increasing the applied E.
b) by decreasing the temperature
 

Dielectrics - Polar, Non Polar, uses

A dielectric is a non conducting substance introduced between the plates of a capacitor.

What is Non Polar Dielectric?

This is a substance in which the net electric dipole moment is zero because of its symmetrical structure. In this the center of gravity of positive charges and  center of gravity of negative charges
will coincide.

What is Polar Dielectric?

Because of their non symmetrical structure these dielectrics have permanent dipole moment. In this there are permanent electric dipoles present. On this if an external electric field is applied, torque acts on these dipoles rotating them in direction of applied electric field. When an external electric field is applied on non polar dielectric this dielectric gets polarized forming induced charges on the surfaces.      

Uses of Dielectric

1. It maintains mechanical separation between the plates.

2. It decreases the field as well as potential but increases the capacity

3.  Increases capacitance between metal plates
When a non polar dielectric is introduced between the plates it is leading to the displacement of negative charges in the dielectric. Because of the displacement of negative charges, the center of gravity of negative charges is not coinciding the center of gravity of positive charges, thus forming dipoles. This phenomenon of formation of electric dipoles when an external electric field is applied on a non-polar dielectric is known as Polarisation. 

Therefore, induced charges are appearing on the surfaces of dielectric forming their own electric field  Eᵢ. This Eᵢ opposes original electric field Eₒ, thus net electric field E is decreasing.

E=Eₒ-Eᵢ

Therefore, potential between the plates is also decreasing resulting in increase in capacitance.

4. Used for withstanding high potentials
 Any dielectric can withstand a maximum electric field before becoming a partial conductor. This maximum electric field, a dielectric can withstand before reaching breakdown condition is known as dielectric strength of dielectric.
Therefore, heavy capacitors use dielectrics using highest dielectric strengths to withstand large potentials.