Category: Physics Notes PPT

Wood is the second most used material for construction purposes, making of furniture, flooring, etc. For building constructions, though the stone has taken first place, the boom of wood has increased dramatically in recent times. Let’s see more interesting facts about wood, different physical, chemical, mechanical, properties of wood.

Tissue Composition of Wood

Wood is a natural resource that can be obtained by plants and trees. Basically wood is a combination of different tissues like the xylem, which is a vascular tissue, phloem, and cell walls. The vascular cambium layer which is present inside the bark helps to produce new wood.

Properties of Wood

The properties of wood explain its behaviour, and how it reacts to different substances at different temperatures. These properties explain the characteristics of wood with a generalized meaning. The properties of wood may include physical properties, chemical properties, mechanical properties. The physical properties of wood can be defined as the characteristics of a wood that do not have any change in its size, shape, colour, etc.

Physical Properties of Wood

The characteristics of wood that explain the physical features of various types of wood are nothing but the physical properties of wood. They are explained as follows.

Colour – As the basic use of wood is to make furniture and decor items for the house, colour and appearance play a vital role in choosing a variety of wood. But the colour of the wood varies from type to type. The wood is available in a wide range of colours starting from white to dark brown etc. The colour of the wood also changes with the observation. If a tree can be observed from top to bottom, it appears in some colour and vice versa.

Lustre – Another physical property of wood is its lustre. Luster refers to the tendency of elements with the reflection of light. After colour, priority is given to the lustre of the wood.

Odour and Taste – We commonly observe different odours from different types of wood. For example, the sandalwood and the rosewood give a nice aroma whereas the other timber woods may give some tobacco odour. Also, the new wood sample gives a fresh aroma and it keeps on degrading with time.

The density of Wood – Another characteristic of wood is its density. But what is the density of wood? The density of wood refers to its mass per unit volume. Based on the weight of the wood sample, the density of wood changes. Different types of wood have different densities. Below detailed information about the density of different types of wood is given.

• If the density or specific gravity is 36 then the wood is called very light.

• When density or specific gravity is = 0.36 then the wood is called light.

• If the density or specific gravity is 0.36 – 0.05, then wood is considered moderately heavy.

• When density or specific gravity is > 0.05, then the wood is heavy.

Hardness – Hardness refers to the strength of the wood also the resistance or the capacity of wood which can stand strong for a long time after being affected by several factors. 

Mechanical Properties of Wood

The mechanical properties of wood can be explained as the capability of wood to withstand externally applied forces. These include different types of properties to understand the strength, resistivity, elasticity, durability, etc. many more characteristics of wood. The mechanical properties of wood were further classified into two. One is strength properties and the other is elasticity properties. There are different standards available to check the strength of wood. They are – 

• ISO (International standard institution

• ISI (Indian standard Institution)

• BSI (British Standard Institution)

• PSI (Pakistan Standard Institution)

Compression – The compression of wood depends on the directions of applied forces, like – 

• perpendicular to wood grain

• parallel to wood grain, and 

• an angle to the wood grain.

Tension –  The tension is very effective on wood when it is parallel to the wood’s grain.

Bending – Based on the load put on the wood, bending may occur. It shows the strength and stability of wood even if the load is increased in a parallel direction.

Conclusion

Wood is one of the most important materials used for several household and commercial purposes. Different types of physical properties and mechanical properties of wood explain the characteristics of a particular type of wood. All these properties were generalized to all types of wood. Few differences may occur accordingly.

In subjects like physics the topic radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:

• The electromagnetic radiation, waves of radio, microwaves, infrared rays, visible light, ultraviolet rays, X-rays and gamma radiation i.e., γ.

• The particle radiation such as alpha radiation (α), beta radiation (denoted by β), proton radiation and neutron radiation

• The acoustic radiation  such as ultrasound and seismic waves  that is said to be dependent on a physical transmission medium

• The gravitational radiation that takes the form of gravitational waves or the ripples in the curvature of spacetime.

(image to be added soon)

Radiation Definition

The term radiation is often said to be categorized as either ionizing or non-ionizing depending on the energy of the radiated particles. Ionizing radiation generally carries more than 10 eV which is enough to ionize the atoms and molecules and break chemical bonds. This is said to be a very important distinction which is due to the large difference in harmfulness to living organisms. Here a common source which is of ionizing radiation that is said to be radioactive materials that emit by symbols that are α, β, or γ radiation which consist of helium nuclei that is we can say that the electrons or the positrons and photons respectively. The other sources which generally include X-rays which is from medical radiography examinations and muons and the mesons, positrons, neutrons and other particles as well constitute the secondary cosmic rays that are produced after primary cosmic rays interact with the planet’s earth’s atmosphere.

So further down the spectrum which is said to be the non-ionizing lower energies of the lower ultraviolet spectrum that generally cannot ionize atoms.

But we can disrupt the interatomic bonds which form molecules thereby thereby breaking down molecules which are rather than atoms a good example which is sunburn which is generally caused by long-wavelength solar ultraviolet. The waves which are said to be  of longer wavelength than the lights which are the UV in visible light so the infrared and microwave frequencies which generally cannot break bonds but can cause vibrations which are in the bonds which are sensed as heat. So the radio wavelengths are generally not regarded as harmful to biological systems. So these are not sharp and delineations of the energies that we can say that there is some overlap in the effects of specific frequencies.

What is Radiation in Science

The word which is known as radiation which generally arises from the phenomenon of waves radiating that is traveling outward in all directions which is  from a source. This aspect usually leads to a system of measurements and physical units that are applicable to all types of radiation.  Like the law of any ideal gas law there is the inverse-square law that approximates a measured radiation intensity to the extent that the source which generally approximates a geometric point.

The term which is radiation with sufficiently high energy that can ionize atoms, that is to say it can knock electrons off atoms which are creating ions. The process of ionization occurs when an electron is stripped or we can say that when it is “knocked out” that too from an electron shell of the atom. 

This is said to be because of living cells and more importantly we can say that the DNA in those cells can be damaged by this ionization so the exposure which is to ionizing radiation is considered which is to increase the risk of cancer. Thus the process of “ionizing radiation” is somewhat artificially separated from particle radiation and electromagnetic radiation that is said to be simply due to its great potential for biological damage. While an individual cell is made of trillions of atoms only a small fraction of those will be ionized at low to moderate radiation powers. The probability of ionizing radiation that is generally causing the disease which is cancer is dependent upon the absorbed dose of the radiation.  It is a function of the damaging tendency which is of the type of radiation that is equivalent to dose and the sensitivity of the irradiated organism or tissue (effective dose).

What is Meant by Radiation

The exposure which is to the radiation that generally causes damage to living tissue that is high doses result in Acute radiation syndrome that is written as ARS with skin burns and then the hair loss that is the internal organ failure and death.

so while any dose may result in an increased chance of cancer and genetic damage and then a particular form of disease cancer thyroid that is cancer that it often occurs when nuclear weapons and reactors are the radiation source which is because of the biological proclivities of the radioactive iodine fission product which is known as iodine-131. However it is calculating the exact risk and chance of the disease which is cancer that is forming in cells caused by ionizing radiation is said to be still not well understood and currently estimates are loosely which is determined by population that is based data from the atomic bombings of Hiroshima and Nagasaki and from follow-up of reactor accidents that is such as the Chernobyl disaster. 

Our universe is one of the most mysterious and interesting thing existing out there in space. We know that our universe is constantly expanding like a hot air balloon. The universe is expanding and inflating regularly that we can barely notice. This expansion and inflation of the universe imply that the distance between the celestial objects is also increasing over the period of time!! I.e., the distant stars and galaxies are moving further away from the earth. This further results in the stretching of their light as they travel through space towards the earth. This stretching makes visible light look redder in shade, which is familiarly known as the cosmological redshift or redshift. 

Now let us have a look at these interesting concepts of redshift, redshift meaning, and redshift light with fascinating facts and concepts.

Redshift Meaning

The famous astronomer Edwin Hubble used the 2.5m which is about a 100-inch long telescope on Mount Wilson, California, to study observe and study the night sky. He found that some of the nebulae which were found to be quite fuzzy, luminous specks in the dark night sky were actually in fact galaxies, just like our milky way, even though every galaxy will be different and could be of widely varying sizes, each containing billions of stars. 

Very hot celestial objects (objects that are present in the outer atmosphere of the earth or space), such as stars are capable of generating visible light, which may travel a very long way before it strikes something. When we gaze at the stars at night, the light from the stars may have been travelling in a composed manner through space for more than hundreds of years. The light from the star strikes your eye and jiggling electrons in your retina, turns into electricity, which is sent along the optic nerve to your brain and hence we can see the star!! 

For thousands of years, human beings have been aspiring to understand the structure and nature of the Universe and seeking to determine its true extent. But, whereas ancient philosophers have believed that the world consisted of a disk, a ziggurat or a cube surrounded by many celestial oceans or some kind of ether (an organic substance), the turtle holding the universe and many more. Later the advancement of modern astronomy opened their eyes to new frontiers. By the 20th century, scientists and cosmologists have begun to understand just how vast (and maybe even unending) the Universe really is.

The universe is constantly expanding, inflating like a hot air balloon. This implies that the distant stars and galaxies are moving away from the earth. As a result of these transitions, it will stretch its light (light from the celestial bodies) as it travels through space towards us, the further it travels, the more it gets stretched. This stretching will make the objects appear red in colour, the more they travelled away they appear redder, and this effect is known as the redshift. 

Redshift is a key concept for astronomers. The term can be understood literally – the wavelength of the light is stretched, so the light is seen as ‘shifted’ towards the red part of the spectrum or redshift between the two spectral lines. 

Laboratory experiments that are performed here on the Earth have determined that each element in the periodic table emits photons only at certain wavelengths which are determined by the excitation state of the atoms. These emitted photons are manifest as either emission or absorption lines in the electromagnetic spectrum of an astronomical object, and by measuring the position of these spectral lines, we can determine which elements are present in the object itself or along the line of sight.

However, when astronomers perform this analysis, they note that for most astronomical objects, the observed spectral lines are all shifted to longer wavelengths, which is usually the red region of the spectrum. This is known as cosmological redshift or just redshift. 

The light or the photons emitted from the distant stars and more distant galaxies is not featureless but has distinct spectral features characteristic of the atoms in the gases around the stars. When these spectra are considered and examined, they are appeared to be shifted toward the red end of the electromagnetic spectrum. This shift is apparently a Doppler shift or Doppler effect and indicates that essentially all of the galaxies are moving further away from the earth over the period of time. 

Using the results from the nearest celestial objects, it becomes evident that the more distant galaxies and the stars are moving away from the earth at the highest accelerated rate. This is the kind of result one would expect for an expanding and inflating universe. The red line of the electromagnetic spectrum below is the transition from n=3 to n=2 of hydrogen and it is familiarly known as the H-alpha line seen throughout all the universe.

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Of course, making these measurements is a bit tedious and trickier than just saying that the star looks redder than it should be. Instead, astronomers and astrophysicists make use of markers in the electromagnetic spectrum of starlight. This is the actual study of spectroscopy. If you shine a flashlight beam through a clear prism, as a result of dispersion, we witness a rainbow emerging out the other side. But if you place a clear container filled with highly concentrated hydrogen gas between the flashlight and the clear prism, gaps appear in the smooth rainbow of colours, places where the light literally goes missing.

Redshift Light

When the universe was just a few minutes old, the surviving protons and neutrons recombined to form an atomic nucleus, mainly of what would become hydrogen and helium. The hydrogen and the helium that formed at a very early time in the universe are still charged, so fog remains impossible to see through. At this point, the foggy material is not unlike what we find inside a star, but of course, it fills the entire universe.

After the heavy uncontrollable action of the few minutes of existence, the universe stays much the same for the few hundred thousand years, continuing to expand and cool down, the hot fog becoming steadily thinner, dimmer, and redded as the wavelengths of light are stretched by the expansion of the universe.

After 380,000 years, when the part of the universe that we will eventually observe from the earth has grown to be millions of light-years across the fog finally clears. Due to the presence of electric charges of the electrons and the nuclei cancel each other out, the complete atoms are not charged so that now the photons can travel uninterrupted, which implies that the universe has slightly got transparent.

After this long wait for the fog clearance, what do we get to see or witness? Only fading red wavelengths scattered in all directions, which becomes redder and dimmer as the expansion of space continues to stretch the wavelengths of photons. Finally, light radiation ceases to be visible in all directions. The photons from that last glow have been travelling and stretched into space and even steadily appeared to be redder and these photons are detected now as Cosmic Background radiation and they are still arriving on earth from every direction in the sky.

Redshift is often compared to the high-pitched whine of an ambulance siren coming at you, which drops in pitch as the ambulance moves past you and then further away from you. That variation or change in the sound of an ambulance is due to what’s called the Doppler eff
ect. It’s a good comparison because both sound and light travel in the form of waves, which are affected by their movement through air and space.

Did You Know?

• The earth’s atmosphere has not always been as it is today. If we were able to travel 3.5 billion years (to the time when the earth was about one billion years old) we will not be able to breathe, due to the absence of oxygen and the presence of many toxic gases such as hydrogen, helium, etc…

• The atmosphere around 3.5 billion years ago contained no oxygen. It was mostly made of nitrogen, hydrogen, carbon dioxide and methane that are extremely poisonous gases present in the atmosphere, but the exact composition is still unknown. What is known, however, is the huge volcanic eruptions that occurred around the period of time, releasing extremely hot steam, carbon dioxide, ammonia and hydrogen sulphide into the atmosphere. We know that hydrogen sulphide odour is extremely strong and it is poisonous when encountered or subjected to large amounts.

• Now, the earth is the only planet in our solar system that is capable of holding living organisms. The earth is having an ideal atmosphere with appropriate composition of gases. Today, the atmosphere of the earth is made of approximately 78% of nitrogen, 21% of oxygen and around 0.93% of argon. The remaining 0.07% is mostly carbon dioxide which is approximately 0/04% and the mixture of neon, helium, methane, krypton and hydrogen.

Relation between critical angle and refractive index can be formed because both of them are inversely proportional. But, before going into this detail, you must understand these topics separately.

What is Critical Angle?

In optics as a topic of Physics, the critical angle makes reference to a particular angle of incidence, which gives an angle of refraction of 90 degrees. Additionally, a water–air boundary has a critical value of 48.6 degrees. while the critical angle for crown water and glass boundary is 61.8 degrees.

However, the value of critical angle is always dependent on the mediums situated on both sides of a boundary.

Additionally, the equation of critical angle is:

 [theta _c] = sin^-1 n_r / n_i

Here, [theta _c] is the critical angle of refraction, and n_r and n_i are refraction index and incident index, respectively.

What is Refractive Index?

The extent at which rays of light bend when it enters from one medium source to another is known as its refractive index. The refractive index is represented using the alphabet ‘n.’ Moreover, it can be written as n = c/v, where ‘c’ is light speed or velocity of a specific wavelength in the medium air. On the other hand, v is light’s velocity or speed in other media.

Furthermore, three factors determine this refractive index – medium nature, light colour, and physical conditions.

Fact: An optically rarer medium is one where light travels faster through it. Whereas, an optically denser medium is one where light travels slower through it.

Refractive Index and the Critical Angle Relationship

The mathematical representation of their relationship is:

sin C= 1/ µ^a_b

Here, C = critical angle, µ = refractive index, and a and b are two mediums within which light passes.

Furthermore, take a look at this derivation below!

Snell’s Law (also known as the Second Law of refraction) is applied to derive the relation between critical angle and refractive index.

Hence, take a light ray having an incident angle i, refractive angle r = 90 degrees, critical angle = C, and refractive index of rarer and denser medium be µa and µb, respectively.

So, by applying the second Law of refraction or Snell’s Law:

sin i / sin r = µ_a / µ_b

Therefore, µ_b sin C = µ_a sin90^o

Therefore, µ_b / µ_a = 1 / sin C

Thus, with the help of this equation, critical angle and refractive index relation can be stated as:

µ^a_b = 1/ sinC

Solved Numericals

(i) Find out the ratio between sine of incident angle and the sine of reflected angle where their refractive indices are provided. In medium 1 it is 2.33, while in medium 2 it is 1.66.

Solution: Snell’s Law gives [n_1sintheta _i=n_2sintheta _r]

In order to get [frac{sintheta _i}{sintheta _r}] , you must note that this ratio is [frac{n_2}{n_1}]

After substituting for n₁= 2.33 along with n₂ = 1.66

=> 1.66/2.33 = 0.71

(ii) Find the ratio between refractive index of two mediums, 1 and 2. Here, the reflected angle of medium 1 is 300, while that of medium 2 is 450. 

Solution: Snell’s Law gives [n_1sintheta _i=n_2sintheta _r]

So, for getting [frac{n_2}{n_1}],  this ratio is [sintheta _i=sintheta _r]

On putting values for  θ_i= 30 and θ_r= 45

Therefore, [frac{sin45}{sin30}] = 2

Applications of Critical Angle

The principle of critical angle is used in several practical ways in our day-to-day life. The most widely used is fibre optic cables. The concept is the base for the construction of fibre optic cables and how they work.

Applications of total internal reflection are multi-touch screens, spatial filtering of lights, prismatic binoculars, automotive rain sensors, fluorescence microscopes, and ubiquitous fibre optics communications.

Do It Yourself

1. Angle of incidence is equal to the angle of reflection for perfect reflection. Answer true or false.

(a) False 

(b) True

Ans: (b) True

2. The higher the value of the refractive index of a given medium, the bending of light will be

(a) zero 

(b) smaller 

(c) higher 

(d) negative

Ans: (c) Higher 

3. The refractive index of a medium is the relation between light’s speed in vacuum or air, and

(a) Light’s speed in a medium 

(b) Can be a or c 

(c) Speed of sound in a medium

(d) none

Ans: (a) Light’s speed in a medium

Let’s suppose that there are two persons viz: P and Q, each of these is allocated a task of data analysis for a limited duration in which they have to fulfill the deadline. A person’s ability will be ascertained by how much time he takes. If person P meets the expectations of their hiring manager, he gets recruited into the company and if he does not, he fails.

So, here person P takes 2 hrs, while person Q takes 3 hrs, so here person P has more power than Q. 

Now, this person has ability but he needs a push, so that push or the driving force is the potential difference. 

 

Relationship between Watt and Volt

Watt is the SI unit of power while Volt is the SI unit of potential difference. The relationship and differences between Watt and Volt can be understood by carefully examining the differences between Power and Potential Difference. 

In terms of mathematical formulation, 

P = V*I 

Where, P = Power, V = Potential difference, and I = Current flowing between them

The relation between Watt and Volt can be thought of as direct, which means that Watt varies directly with the Volt. This implies the following:

• When the electric power in terms of Watt increases, there is a corresponding increase in the electric potential in terms of Volt while keeping the electric current constant.

• However, if the electric power Watts decreases, the electric potential in Volts decreases by the same amount, when the electric current is kept constant.

Difference Between Watt and Volt

How Does Wattage Work?

Watt: An electrical power is measured in Watts. Let’s take our hose analogy.

Imagine you are spraying water from the hose into a bucket. So, here power is the measure of how quickly the bucket is filling up. 

From the Watt and Volt equation, we understand that it is a straightforward concept that power is the product of electric potential measured in volts and the current flowing through the conductor in Amperes. So, the more the wattage, the brighter is the light. 

We might have seen that a 9 W LED bulb is brighter than a 6 W LED bulb. 

Now, talking about voltage:

How Does Voltage Work?

Voltage: A force that makes electricity move is the voltage and it is measured in volts.

Let’s consider a hose analogy:

Imagine that voltage is the pressure in the garden hose, even if the nozzle of the hose is turned off, the pressure still persists inside it. 

So, in relation to the electricity, even if we switch off the bulb (as the bulb is connected across the circuit), the voltage remains in the energized part of the circuit/ckt.

Volt and Watt Difference

Volt is the measure of the potential difference across the ends of the conductor or a conducting terminal of the wire.

Let’s have a look at equation (1):

 P = V * I

1 Watt  = 1 Volt * 1 Ampere

Or,

1 volt = 1 Watt/1 Ampere ….(2)

From eq (2), Watt is the rate at which the electrical work is performed when one ampere of current flows through the conductor on applying the potential difference of one volt across the ends of the conductor.

Now, let’s give a new meaning to the Watt and Volt Relation in terms of AC and DC:

Watt Volt Relation

Watt Related to Volt

The relation between Watt and Volt is direct. It means that Watt varies directly with the Volt. This implies the following things:

• When the electric power in terms of Watt increases, the electric potential in terms of Voltage increases by the same amount while keeping the electric current constant.

• However, when the electric power Watts decreases, the electric potential in Volts decreases by the same amount, while keeping the electric current constant.

Let us consider that there are two persons P and Q respectively, each of these is allocated a task of physical labour for a limited duration in which they have to fulfill the deadline. A person’s ability will be judged by how much time he takes. So, if the person P or Q meets the expectations of their hiring manager, he gets recruited into the company and if he does not, he fails.

Now, if person P does the work in 2 hours and Q does the work in 3 hours then we can ascertain that P has more power than Q. So the potential difference is the capacity created between two ends which has an empty space between them and Power is the actual energy that is transmitted between this space per unit of time. 

This is all about watt and volt and their relationship. Understand the definitions and concepts to realize how they are related and grab hold of this topic well.

The conservation of energy is one of the most predominant laws in physics. We are aware of the fact that energy has many important forms, also each form can be converted to any other. We know that parameters like distance, time, motion, velocity and acceleration are all relativistic in nature, then, we can say that energy must be a relativistic quantity too. In other words, the energy of the object under consideration is depending on the inertial frame of reference that we are in. 

According to classical physics, the total amount of energy in a system remains constant. Relativistically, energy is still conserved, provided its definition is moderated to include the possibility of mass changing into energy, as in the reactions that occur within a nuclear reactor. Relativistic energy is defined in such a way that it will be conserved in all inertial frames, just like in the case of relativistic momentum. 

Relativistic Energy Formula:

Let us start with the derivation of the relativistic energy formula. When we are calculating the non-relativistic energy we assume that the change in kinetic energy is equal to the work done on the system or the object under consideration. In the relativistic case also assume the same theorem and estimate the relativistic energy. 

To derive the relativistic energy formula we assume that the mass-energy principle holds good under relativity. According to the work-energy theorem, it states that the net work done on a system goes into kinetic energy. In other words, we say the change in kinetic energy can be evaluated by calculating the work done on the system or the object that we were considering. The relativistic energy formula is also known as the energy-momentum relation. 

In classical physics, the kinetic energy is given by the product of mass and square of the velocity, this kinetic energy is valid for the object which has a velocity less than the speed of light. But we know that in relativistic mechanics we assume that the particles are moving with the speed of light. The potential energy of the particle is considered to be almost zero or negligible.

The relativistic energy is also known as the relativistic kinetic energy and it can be derived by a small derivation as given below.  We know that according to the work-energy theorem, it states that the net work done on a system goes into kinetic energy. In other words, we say the change in kinetic energy can be evaluated by calculating the work done on the system or the object that we were considering. Therefore mathematically we write,

[Rightarrow E=int_{0}^{r}dW=int_{0}^{r}F.dr=int_{0}^{r}Fdr]…….(1)

Since both applied force and the displacement are in the same direction we are not considering the scalar product. 

We know that according to Newton’s law of motion force F is equal to,

[Rightarrow F=frac{dp}{dt}=frac{d(mv)}{dt}]…….(2)

And the velocity of the particle is given by,

[Rightarrow v=frac{dr}{dt}]…………(3)

Substituting the values in the equation (1) we get,

[Rightarrow E=int_{0}^{t}frac{d}{dt}(mv)vdt=mint_{0}^{v}vdv=frac{mv^{2}}{2}]..(4)

Equation (4) resembles the classical kinetic energy formula of the particle moving with velocity v. But, here we are considering the relativistic case and hence substituting the value of mass by rest mass and velocity by Lorentz transformation we get,

[Rightarrow m=gamma m_{0}c^{2}]

Substituting the value of relativistic mass and velocity we finally calculate the relativistic energy.