SOUND - Important Notes

1. A vibrating body produces sound.
2. Sound has energy
3. Sounds are produced by vibrating bodies and the air that passes through orifices of the instruments.
4. Sound needs a medium to propagate.
5. The to and fro motion of a body from its mean position is known as one vibration.
6. The maximum displacement of vibrating body from its mean position is called as amplitude.
7. The number of vibrations per second is called as frequency.
8. The shrillness of a sound is known as pitch.
9. The pitch of the sound depends upon its frequency.
10. Normal sound consists of mixed frequencies.
11. The sounds which are pleasant to hear are called as music. Music is a combination of sounds that are produced in an order and pleasant to hear.
12. The sounds which are unpleasant to hear are called as noise.
13. The sounds that a normal human being can hear are called as audible sounds.
14. The sounds that a normal human being cannot hear are called inaudible sounds.
15. Frequency of the audible sounds ranges from 20 Hertz (20 vibrations/sec) to 20000 Hertz(20000 vibrations/sec). 

 

All about Force?

Force definition

The push or pull of a body by an object is called force on the body.

or

Force is an external influence acting on a body to change its state of motion.

Types of Forces

Contact Force 

Force which results when there is a direct physical contact between two interacting objects is known as contact force. 

Types of Contact Forces

Muscular Force

The force which we exert by using our muscles is called as muscular force.

Friction Force

The force which resists the motion of an object over the surface of other.  The direction of friction force is always opposite to the direction of the motion relative to the surface.

Normal Force

The friction force, on an object, which acts perpendicular to the surface of  its plane is called as Normal Force. 

Tension

Tension is a pulling force acting on a object by means of a string against the gravitational force.  

Field Force

The force which occurs without any physical contact is known as force at a distance or field force.

Field is region of imaginary lines around an object. When an other body is placed in this region, it will experience the force. The greater the density of these imaginary lines, the stronger the forces in that region.

Types of Field Forces

Magnetic Force

Magnetic force is a field force responsible for attraction of like poles and repulsion of unlike poles.

Electrostatic Force

Force exerted by a charged body on another charged body is known electrostatic force. This force is due to electric field from the charge.

Gravitational Force

The force of attraction that exists between any two masses n the universe is known as Gravitational force. This force is due to electric field from the charge. Gravitational force is also non contact force and hence it is also a field force.

Net Force

Multiple forces can act on an object simultaneously at a time. When the forces act in same direction then net force will be addition of all forces. When the forces are in opposite direction, then net force will be difference of the forces. When the Net force is zero, there is no change in state of object. 

The direction of motion of the object will be in the direction of Net Force. If the Net Force acts in direction of motion, the speed of the an object moving with constant speed also increase. If the Net Force acts in a direction opposite to the motion, then it either slows down the object or brings it to rest or it may change the direct of motion. 

The unit of force in SI system is Newton(N). 

Pressure

The force acting perpendicularly on unit area of a surface is called as pressure.

Pressure = Force/Area

The unit of pressure in SI system is Newton/Meter² or N/m². 

  

What is accelerated Thermal Ageing Test?

 What is Thermal Ageing?

Ageing of the materials with temperature could happen and differs for various materials. Important safety systems/equipments need to be evaluated for the effect of temperature on the materials used in the system. Even storage for long years shall also be considered for some materials like polymers to estimate the ageing effect. 

Ageing effect couldn’t be estimated practically for operating conditions or storage conditions as the duration is very high (tens of years) and the qualification of product couldn’t happen. Hence, Ageing effect due to temperature could be evaluated by exposing the unit to higher temperature for accelerated time. This is called as Accelerated Thermal ageing test.

 

How to calculate time required for Thermal Ageing of an instrument at accelerated temperature?

There are different methodologies to do Thermal Ageing test. Most commonly used methodology is application of Arhenius equation. Usually, this methodology is applicable for Polymer/Organic materials for which temperature effect is more.    

Thermal Ageing is due to the chemical process induced by the temperature, which could lead to change the bulk properties of the material over a time.  

The Arhenius methodology has been developed to simulate the accelerated ageing mechanism. As per this methodology, deterioration of materials in service is due to chemical reaction. These occur internally, sometimes between components of the material, and sometimes with compounds in the environment such as oxygen or water vapor. Chemical reactions occur more rapidly at higher temperatures.

 

Arrhenius showed that temperature dependence of chemical reactions follows an exponential equation. He postulated a consistent correlation between the amount of physical change and chemical reaction so that the time to reach a selected amount of physical change will vary according to an equation. The rate of thermal aging is the slope of the graph using the Arrhenius equation.

 

Accelerated Time calculation for Thermal Ageing Test

As per Arhenius equation,

where,

‘E’ is activation energy for the material under consideration; in eV

‘K’ is Boltzman Constant i.e. 8.617 x 10^-5 eV.K^-1 

‘t1’ is ageing time required at accelerated temperature ‘T1’.

‘t2’ is service life time operating temperature ‘T2’.

‘T1’ is accelerated temperature

‘T2’ is Operating temperature

For instance, let us consider the cross linked polyolefin cables (XLPO cables) being used for an application where the normal operating temperature is 50 oC and the expected service life is 40 years. Let us calculate the accelerated time required for thermal ageing test at accelerated temperature of 135 oC.

i) Activation energy (E) for XLPO cable is considered as 0.78 eV

[Ref: An Experimental Study on Dielectric Response of the Cross linked Polyolefin (XLPO) Insulation of the Unshielded Cable Under Electric Field and Heat Flux; Publisher: IEEE]

ii) t2 = 40 years

iii) T1 = 135 degC

Time required for accelerated Thermal Ageing test for accelerated temperature of 135 oC is ~43 days.

Picture size and Resolution explained

Pixel dimensions

The number of pixels along the height and width of an image.

The display size of an image on screen is determined by the pixel dimensions of the image plus the size and setting of the monitor.

For example, a 15 inch monitor typically displays 800 pixels horizontally and 600 vertically. An image with dimensions of 800 pixels x 600 pixels would fill this small screen. On a larger monitor with an 800 x 600 setting  the same image would still fill the screen but each pixel would appear larger. Changing the setting of this larger monitor to 1024 x 768, pixels would display the image at a smaller size, occupying only a part of the screen.

Image Resolution

The number of pixels displayed per unit of printed length in an image, usually measured in pixels per inch(ppi).In photoshop, you can change the resolution of an image; in image ready, the resolution of an image is always 72ppi.

When printed, an image of high resolution contains more and therefore smaller pixels than an image with a low resolution. For instance, a 1x1 inch image with a resolution of 72ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1 x 1 inch image with a resolution of 300 ppi contains a total of 90,000 pixels.

Using too high a resolution increases the file size and slows the printing of an image.; further more, the device will be unable to reproduce extra detail provided by high resolution.

Monitor Resolution

The number of pixels or dots displayed per unit length on the monitor, usually measured in dots per inch (dpi). Monitor resolution depends on the size of monitor plus its pixel setting. 

understanding monitor resolution helps explain why display size of an image on screen often differs from its printed size. Image pixels are translated directly into monitor pixels. This means that when image resolution higher than monitor resolution the image appears larger on screen than its specified print dimensions. For example, when you display a 1 x 1 inch 144ppi image on a 72dpi monitor it appears in a 2 x 2 inch area on screen. Because the monitor can display only 72 pixels per inch. It needs 2 inches to display the 144pixels that make up one edge of the image.  

Modern Physics - Graduate level important notes for competitive exams - PART2

•  4n series is called as "Thorium" series.

• 4n+1 series is called as "Neptunium" series.

• 4n+2 series is called as "Uranium" series

• 4n+3 series is called as "Actinium" series

• X-rays & 𝛾-rays of same energies are distinguished by their frequencies

• Spin of a Deuteron  is 1.

• Spin of Muons is 1/2.

• Spin of Pions (π-mesons) is zero.

• Spin of Photon is 1.

• Spin of Graviton is 2.

• Mass of a Proton is 1.0072766 amu

• Mass of a Neutron is 1.0086654 amu

• The total quantum number 'n' specified "Energy" quantization.

• The azimuthal quantum number specified "Angular Momentum".

• The magnetic moment of proton is given by 2.8eħ/2mp

• The magnetic moment of Neutron is given by 1.9eħ/2mp.

• In Davisson-Germer experiment , the angle between incident beam and diffracted beam is called "co-lattitude".

• In heavy atoms the spin-orbit interaction is almost equal to "electrostatic interaction".

• L-S coupling is due to "electrostatic interaction".

• L-S coupling scheme breaks down in strong magnetic field and applies to light atoms.

• In light atoms, "orbit interaction" is less than "electrostatic interaction".

• According to shell model nucleons interact primarily with a general force field rather than directly with one another.

• Magic numbers: 2,8,20,28,50,82,126

• Debroglie wavelength associated with electron accelerated to a potential V is ƛ=12.26/√V A⁰

REFRIGERATION - IMPORTANT NOTES

Refrigerating machines are of two types 1) Vapour compression machine (Frigidaire)" and 2) "Vapour absorption machine". 

Refrigerant: 

The liquid which on evaporation produces cooling is called refrigerant. Some examples are Ammonia(NH₃), Sulphur dioxide, Freon.

Properties of Refrigerant:

1. It should have low boiling point & melting point
2. It should be vapor at normal temperature and pressure.
3. It should have high thermal conductivity
4. "Latent heat of vaporization" of the refrigerant must be large.  
5.  The specific volume should be small in order to reduce the size of compressors. 

The evaporation of liquid under reduced pressure is a cooling process. The reason is that when a substance changes from liquid to vapor phase, it is associated with absorption of heat. If this heat is not taken from any external source, the necessary heat for phase exchange is taken from itself which therefore cools.

Vapor Compression Machine

Production of temperature by making a liquid evaporate rapidly under reduced pressure and circulating evaporating liquid around enclosure.