Category: Physics Notes PPT

What is Convection?

Convection is a process of heat transfer by the movement of fluids (gas or liquid) between regions of various temperatures. If convection occurs naturally it is called natural convection or free convection. If convection is forced to occur, say, if fluid is circulated using a pump or a fan, it is called forced convection.

What are Convection Currents?

Convection currents are generated by the differences in densities of the fluid that occur due to temperature gradients. The activity that results from the continuous replacement of the heated fluid in the area of the heat source by the nearby present cooler fluid is called a natural convection current. The heat and the mass transfer which is enhanced due to this natural convection current are called natural convection heat and mass transfer. 

In natural convection, the heat and matter are said to be moved from one location to the other one. The convection currents are linked with natural convection in which the fluid motion occurs naturally such as buoyancy (Optimism).

The convection current cannot take place in solids as the particles within the solids cannot flow easily and most of the free movement is mainly because of the difference in the density that is caused due to a huge transfer of heat between the plates.

Examples of Convection Current:

1. Refrigerator:

In the case of a refrigerator, the freezer unit is present at the top of it. The main reason is that the warm air present inside the refrigerator will rise upwards and the cold air in the freezer point will move downwards and it will keep the lower portion of the refrigerator in a warm condition.

2. Thunderstorm:

A thunderstorm can be the best example of convection currents. The warm water in the air rises upwards and turns into saturated water drops which form the clouds. In this process, the smaller clouds run into each other and hence the bigger clouds are formed. The thunderstorms or cumulonimbus clouds are formed on reaching the final growth stage.

3. Steaming Beverage:

Steaming beverages are a simple example of convection. It is usual that steam comes out from a cup of hot coffee or tea. The warm air that is in the steam rises upwards which is due to the heat of the fluid.

4. Campfires:

The reason for the hotness above the campfire than the heat next to it is due to the convection currents. If you place your hands in front of the campfire (of course, at a safe distance; in any case, do not place the hand above the fire), you can feel the heat which is due to the presence of a few convection currents that rises up towards you.

Why do Convection Currents Form?

The difference in temperature level causes the particles to move to result in the creation of the current. In plasma and gases, the difference in temperature leads to regions of low and high density, where molecules and atoms move to fill in the low-pressure areas. Unless energy sources like sunlight, heat, etc are present, convection currents will continue until a uniform temperature is reached.

How are Convection Currents Created?

Based on three physical assumptions, convection currents are created. They are as follows:

Heat Source:

The presence of a heat source is important because the convection currents are generated by the differences in density of the fluid that occurs due to temperature gradients. In the case of natural convection, the fluid surrounding the heat source receives heat. Due to thermal expansion, it becomes less dense and rises above. The fluid’s thermal expansion plays an important role in the creation of the convection currents. In simple words, the components that are denser or heavier will move downwards while the less dense or lighter components will move upwards which leads to bulk fluid movement.

Presence of Proper Acceleration:

Natural convection occurs only in a gravitational field or when there is a presence of proper acceleration like Centrifugal force, Coriolis force, etc. It essentially does not operate in the earth’s orbit. For instance, other heat transfer mechanisms are required to prevent electronic components from overheating in the orbiting International Space Station.

Proper Geometry:

The magnitude and the presence of natural convection will also depend on the geometry of the problem. In the gravitational field, the presence of a fluid density gradient does not ensure the existence of natural convection currents. 

This problem can be demonstrated from the following figures, where the fluid is enclosed by two large horizontal plates with different temperatures. 

Case A: 

In this case, the temperature of the lower plate is higher than that of the upper plate. Here, there is a decrease in the density in the direction of the gravitational force. This geometry induces fluid circulation and through natural circulation, heat transfer occurs. Being warm in the process, the heavy fluid will move down while the lighter fluid will move upwards, cooling as it moves. 

Case B: 

In this case, the temperature of the lower plate is lower than that of the upper plate. Here, the density increases according to the direction of the gravitational force. This geometry leads to a stable temperature gradient, stable conditions and does not induce the circulation of the fluid. Also, the heat transfer occurs only through thermal conduction. 

Convection is different from conduction, which is the transfer of the heat between the substances in direct contact with each other. Convection currents transfer the heat through the mass movement of fluids such as water, molten rock, or air from one place to the other.

Convection in Ocean

In the oceans, the convection drives ocean currents like the Gulf Stream and the other currents which turn over and mix up the waters. From the higher latitudes, the cold polar water is drawn downwards and sinks into the bottom of the ocean. It is pulled downwards to the equator as the light and warm water rises upwards to the surface of the ocean. In order to replace the cold water that is pulled towards the south direction, the warm water is pulled towards northward. The soluble nutrients and heat are distributed around the world due to this process.

Convection in Air

The circulation of air in the earth’s atmosphere is driven by convection. Near the equator of the earth, the sun heats the air which becomes less dense and rises upwards. It cools down as it rises and becomes less dense than the air that is around, spreading out and descending again towards the equator. The constantly moving cells of cold and warm air are known as the Hadley Cells. It drives the circulation of air continuously at the surface of the earth is what we call wind. The atmospheric convection currents are also the reason for the clouds to be upwards.

Convection in Earth

There is a belief among geologists that the molten rock deep within the earth is circulated by the convection currents. Being in a semi-liquid state, the rock should behave like any other fluids, rising up from the bottom of the mantle after getting hotter and less dense from the heat of the core of the earth. The rock becomes relatively denser and cooler, sinking back down to the core as it loses heat into the earth’s crust. The constantly circulating cells of the cool and hot molten rock are considered to help in
heating the surface. Also, some Geologists believe that the convection currents within the earth are the contributing cause for earthquakes, volcano eruptions, and continental drift.

Convection Currents – Atmospheric Circulation

The atmospheric circulation is the most important phenomena in the terrestrial climate. It is the movement of air in the large scale and is a means by which the thermal energy together with ocean circulation is distributed on the surface of the earth. Each year, the atmospheric circulation of the earth varies but the large-scale structure of the circulation remains quite constant.

The atmospheric circulation is a consequence of the illumination of the earth by the sun and the laws of thermodynamics. It can be viewed as a heat engine driven by the energy of the sun and whose energy sinks ultimately in the blackness in the space and also the wind turbines are powered by the sun.

What Will Happen if Convection Currents on Earth are Stopped? 

Suppose, if all the convection currents on earth are stopped, it would affect us the worst. The amount of heat that is radiated from the sun sets the earth’s surface temperature. If there is no convection, then the equator will get hotter and hotter and the north and south poles will become cooler and cooler. 

The oceanic currents from the tropical regions will bring the warm water more towards the north and the currents from the cooler regions will bring the cool water towards the equator. Hence, if the convection is stopped completely, ocean currents will occur and the very low and very high temperatures will force the living beings on the earth to move away from the equator and poles.

Most of the rocks present in the earth convect on a large scale. The rocks can drift very slowly even though they are solids. Convection helped the formation of large islands. There won’t be any new volcanoes on islands if the rocks stop flowing inside the earth.

Impact of Convection on the Earth’s Climate

The convection that happens in the deep surface of the Earth’s mantle also will impact the climate and surface of the earth. Through the movements of the Ocean and Continental plates, convection influences the atmosphere. A massive amount of air is circulated by the atmosphere and the position of the basins and continents in the ocean changes as to how weather and air movement around the globe. The air and ocean current fluctuations allow the precipitation to move towards the various areas of the globe.

Also, it is supposed that the convection that happens in the earth’s mantle is responsible for the creation of the Earth’s magnetic field. Due to the flow of liquid iron through the mantle, the earth’s magnetic field arises and creates electric currents.

Facts About Convection

Since olden times, convection currents have been utilised for various daily life purposes like heating, ventilation, etc.

Convection currents are also helpful in mining sites. This is because, at times, these currents are able to assist ventilation in mines.

To know more about convection current and its occurrence in different mediums, log on to and attend the online sessions conducted by the top experts. Get the best study material for understanding the concepts of convection current in no time and prepare your foundation accordingly.

Students can define couple in Physics within a few lines of explanation. When two equal and parallel forces act opposite to each other, then they both create a couple.

There’s no ambiguity in understanding the couple definition Physics. A couple has the only effect of producing or preventing the turning effect of the body.

A couple can be calculated with the help of the product of the magnitude of both forces & the perpendicular distance between the forces’ line. These force lines are also called action lines.  

Couple Moments Physics

When you try to understand and define couple, the steering wheel of an automobile can be the best option. It would help if you had hand forces to create a couple. Also, the application of a screwdriver twisted by your hand is responsible for the creation of a couple.

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A couple in Physics is a twisting force that does not allow any kind of moment, turning effect throughout the entire process of force engagement. 

This is a different type of force that comes with the application of two other forces. These two forces are responsible for the twisting of an object. A couple is dependent on certain other methods such as several forces, their directions, methods, their nature.

Application of Couple in Physics

You can know different types of applications for couples. Some of them are given hereunder:

• Steering wheel applied by the car driver

• Opening and closing of a water tap

• Winding the spring of an alarm clock

• Unlocking the locker by using a key 

• Opening and closing of a cap of a water bottle, or jug.

• Turning of a screwdriver

Difference Between Couple and Moment

Students can understand the actual difference that lies within a couple and moment with the help of the given table:

Couple Force Formula

As we know, a couple is the combination of two parallel forces that have identical magnitude but opposite directions distinguished by a perpendicular distance.

M[_{c}] = r * F

The above formula is using vector analysis. As both of the forces are acting at a distance, they create a moment. This is called a moment of a couple.

We can name a couple as torque also. It is responsible for the development of the rotational motion of a body. Forces that are involved in a couple have the same magnitude but different directions. 

This is why the resultant force is zero. When you don’t find any resultant force acting on the body, the body won’t possess any type of translational motion.

When there is no translational motion, then the body is at rest. According to research, the moment of the couple in the same plane about any point is not equal to zero. As a result, we obtain that the body is under rotational motion. 

Couple vs Torque

Torque and work have identical dimensions. However, they are not the same if we consider their physical quantities. The moment of force is a vector quantity, whereas work is a scalar quantity. 

It is coincident that both of them possess identical dimensions, but their purposes are completely different. The measurement of the couple is not linked with the axis of rotation. That is why couple and torque both are independent of the axis of rotation.

However, the magnitude of a couple is constant. When a body is under translational equilibrium, the resultant force acting on that body is zero.

Mathematically, we can write it as

ΣF = 0

This expression is ideal for an equilibrium body.

Also, when a body comes under the influence of rotational equilibrium, then the resultant moment acting on a body should be zero. The moment also has the same impact when it has no rotational motion:

ΣM = 0

Force Couple Examples

When the forces caused due to two hands help to turn a steering wheel are considered the best example of a couple. 

Each hand grips of a driver on the wheel at different points have an impact over the shaft. When a driver applies a force, then an equal amount of magnitude and opposite direction helps to rotate the wheel. The movement of screwdrivers is also another example of a couple.

What is the definition of a couple?

A screwdriver is twisted by the equivalent of a couple of hand forces, whereas an automobile’s steering wheel is spun by a pair of hand forces. In contrast, a wrench is operated by a force exerted at only one end, resulting in an imbalanced force on the element being tightened in addition to the turning moment.

A couple is a pair of equal parallel forces pointing in opposite directions in mechanics. The only thing a couple can do is cause or prevent a body from turning. The amount of either force is multiplied by the perpendicular distance between their action lines to compute the turning effect, or moment, of a pair.

The Couples’ Characteristics

• The couple does not induce translational motion because the two forces that make up the couple are equal and opposing.

• When it is applied to a body, the net resultant force is zero.

• Because the algebraic sum of the moments of the two forces around any point in their plane is not zero, it causes pure rotational motion in the body.

• The size and direction of a couple’s moment about any point on its plane are both constant.

The Couple’s Moment

The moment of the pair is described mathematically as the product of the force and the perpendicular distance between the two forces’ lines of action. The arm of the Couple refers to the perpendicular distance
between the lines of action of two forces. That is, the product of the applied force and the arm of a pair of forces equals the moment of force.

As a result, the moment of a pair of forces is equal to,

Τ = F × D

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The moment of a pair of forces will be bigger if the formula for the moment of a pair of forces is followed.

• The force’s magnitude is bigger, and

• The arm of the pair of forces is longer, indicating that the perpendicular distance between the two forces’ lines of action is greater.

The Newton-Meter is the SI unit for Couple Moment (N m). The dimensional formula for the Moment of Couple is [left [ ML^{2}T^{-2} right ]].

Couple Moments of Various Types

A couple of moments can be divided into two categories:

Positive moment: When the body rotates in an anticlockwise direction under the impact of a Couple, the moment is said to be positive.

Negative moment: The moment is considered to be negative when the force pair spins in a clockwise direction.

In structures that you assume are friction-less with no heat loss, the overall energy will continue to be the same. (systems like this might be observed in chemistry) which means that for each small motion of the parts of the device, any lack of kinetic energy could be balanced by way of a growth in potential energy and vice versa. The sum of all the little adjustments in kinetic and potential will add to zero at every step, by no means dropping too much or gaining too much at any moment.

 

What is D’Alembert’s Principle?

D’Alembert’s principle is used to convert the dynamics problems into static troubles. The principle of digital work is typical for solving the static issues. The static problem has no accelerations. We will expand the principle of virtual work for dynamic troubles by introducing the idea of inertia work. For each unit of matter within the system with mass m, Newton’s second law states that

F =ma

we will make this dynamics energy look like a statics energy through denying an inertial pressure

F∗ =−ma

and rewriting equation as

F overall = F +F∗ = 0.

D’Alembert’s principle is just the principle of virtual work with the inertial forces introduced to the list of forces that do work.

D’Alembert’s Principle States That,

For a unit of mass of debris, the sum of difference of the pressure acting at the machine and the time derivatives of the momenta is 0 while projected onto any digital displacement.

It is also referred to as the Lagrange-d’Alembert principle, named after the French mathematician and physicist Jean le Rond d’Alembert. It is an alternative shape of Newton’s second law of motion. according to the 2nd law of motion, F = ma whilst it’s far represented as F – ma = zero in D’Alembert’s law. So it can be stated that the object is in equilibrium while an actual force is appearing on it. Here, F is the actual pressure even as -ma is the negative pressure called inertial force.

D’Alembert’s Principle Mathematical Illustration

D’Alembert’s principle can be explained mathematically in the following manner:

in which,

i is the integral used for the identification of variable corresponding to the particular particle within the system

Fi is the entire applied force on the ith location

mi is the mass of the ith debris

ai is the acceleration of ith particles

miai is the time derivative illustration

𝜹ri is the virtual displacement of ith particle

Derivation of D’Alembert Principle

using D’Alembert’s mathematical method, virtual work can be proven the same as D’Alembert’s principle, which is equal to 0.

Examples of D’Alembert Principle

1D motion of inflexible body: T – W = ma or T = W + ma in which T is tension force of wire, W is weight of sample version and ma is acceleration force.

The 2nd motion of inflexible body: For an object moving in an x-y plane the subsequent is the mathematical illustration: Fi= -mrc in which Fi is the full pressure carried out at the ith region, m mass of the frame and rc is the position vector of the center of mass of the body.

This is D’Alembert’s principle.

programs of D’Alembert’s principle

D’Alembert’s principle is based totally on the principle of digital work at the side of inertial forces. the subsequent are the packages of D’Alembert’s principle:

Mass falling under gravity

Parallel axis theorem

Frictionless vertical hoop with a bead

One dimensional wave as the name suggests prescribes to own space dimension, i.e., the only independent variable present is time. There are various examples of waves, such as sound waves, ocean waves, or vibrations that are produced by musical instruments as well as electromagnetic radiations producing waves. A wave is studied in classical physics in mechanics, sound, and light. A wave can be described as a disturbance that travels through a medium transferring energy. A single disturbance is called a pulse, and a repetitive disturbance is called a periodic wave. The medium is a series of interconnected particles exhibiting wave-like nature. The particles interact with one another, allowing the disturbance or wave to travel through such mediums.

Types of Waves

Waves can generally be categorized into two different types, namely, travelling and stationary waves.

• Travelling waves, for example, sea waves or electromagnetic radiation, are waves that “move”, implying that they have a recurrence and are spread through space and time where time is the only independent variable. Another method of depicting this property of “wave development” is related to energy transmission– a wave moves over a set distance. The most significant sorts of travelling waves in regular existence are electromagnetic waves, sound waves, and maybe water waves. It is hard to break down waves spreading out in three measurements, reflecting off items, so we start with the least fascinating instances of waves, those limited to move along a line. We should begin with a rope, similar to a clothesline. You take one end free, holding the rope, and, keeping it extended, wave your hand up and back once. On the off chance that you do it sufficiently quickly, you’ll see a solitary knock travel along the rope.

• As opposed to travelling waves, standing waves, or stationary waves, stay in a consistent situation with peaks and boxes in fixed stretches. One method of creating an assortment of standing waves is by pulling a guitar or violin string. While putting one’s finger on a part of the string and then pulling the string with another finger, one has made a standing wave. The examples for this wave include the string wavering in a sine-wave design with no vibration at the closures. There is additionally no vibration at a progression of similarly divided focuses between the closures. These “calm” places are hubs. The spots of greatest wavering are antinodes.
  

The Wave Equation

The One-dimensional wave equation was first discovered by Jean le Rond d’Alembert in 1746. The mathematical representation of the one-dimensional waves (both standing and travelling) can be expressed by the following equation:

 

[frac{partial^{2} u(x, t)}{partial x^{2}} frac{1  partial^{2} u(x, t)}{v^{2} partial t^{2}}]

 

Where u is the amplitude, of the wave position x and time t, with v as the velocity of the said wave, this equation is known as the linear partial differential equation in one dimension. This equation tells us how ‘u’ can change as a function of time and space. 

One-Dimensional Wave Equation Derivation

Let us consider the relationship between the volume ∆v in the direction x and Newton’s law which is being applied to it:

 

[triangle F = frac{triangle mdv x}{dt}] (Newton’s law)

 

Where F is the force acting on the element with volume ∆v,

 

[= triangle Fx = – triangle px triangle Sx = (frac{partial p triangle x}{partial x} + frac{partial p dt}{partial x}) triangle Sx simeq – triangle V frac{partial p}{partial p}{partial x} – triangle V frac{partial p}{partial p}{partial x} = M frac{dvx}{dt}]

 

dt is minuscule; therefore it is not considered, and ΔSx is in the x-direction, so, ΔyΔz and from Newton’s law).

 

[ = frac{rho triangle V dvx}{dt}]

 

From,

 

[frac{dvx}{dt}] as [frac{partial vx}{dt} frac{dvx}{dt} = frac{partial vx}{partial dt} + vx frac{partial vx}{partial x} approx frac{partial vx}{partial x} – frac{partial p}{partial x} = rho frac{partial vx}{partial t}] (This is the equation of motion)

 

[= – frac{partial}{partial x} ( frac{partial p}{partial x}) = frac{partial}{partial x} (frac{rho partial vx}{partial t}) = rho frac{partial}{partial t} (frac{partial vx}{partial x})]

 

[= frac{-partial^{2} p}{partial x^{2}} = rho frac{partial}{partial t} (frac{-1}{frac{K partial p}{partial t}})]

 

[= frac{partial p^{2}}{partial x^{2}} – frac{rho}{K} frac{partial^{2} p}{partial t^{2}} = 0]

 

Rewriting the above equation gives us:

 

[frac{partial^{2} u(x, t)}{partial x^{2}} frac{1  partial^{2} u(x, t)}{v^{2} partial t^{2}}]

Hooke’s Law

When English scientist Robert Hooke was investigating springs and elasticity in the 19th century, he observed that numerous materials had a similar feature when the stress-strain connection was analyzed. The force required to stretch the material was proportional to the extension of the material in a linear area. This is known as Hooke’s Law. Within the elastic limit of a material, Hooke’s law indicates that the strain is proportional to the applied stress. When elastic materials are stretched, the atoms and molecules deform until stress is applied, and then they return to their original state when the stress is removed. Hooke’s law is expressed as – 

 

F = –kx

F is the force, x is the extension length, and k is the proportionality constant, also known as the spring constant in N/m, in the equation.

A curved mirror in which a reflective surface bulges out towards the light source is known as a convex mirror. The convex mirror reflects the light outwards and so it is not used to focus light. As the object comes nearer to the mirror, the size of the object gets larger until it reaches its original size. These mirrors are also known as diverging mirrors.

A concave mirror has a reflecting surface that caves inwards. The mirror also converges the light at one prime focus point; hence they are also called converging mirrors. They are applied to focus light. Depending upon the location of the object with respect to the mirror, the size of the image formed by the concave mirror varies. It can be real or virtual, inverted or erect and magnified, reduced, or be similar in size of the object depending upon the position.

Focal Length of Concave Mirror

This article will help you find the focal length of a concave mirror. Let’s look at the theory to obtain the image of a farther object.

1. Like a plane mirror, the concave mirror obeys the law of reflection of light.

2. Ray of light from an object – The rays of light emitted from a distant object, e.g., distant buildings or sun, are parallel to each other. When the parallel rays from the source fall on the concave mirror along the axis, reflect and meet at the point in front of the mirror, which is known as the mirror’s principal focus.

3. At the focus of the mirror, a real, inverted, and very small image size is formed. 

4. Focal length – Focal length of the concave mirror is the distance between the pole P of the concave mirror and the focus F. By obtaining the Real image of the distant object, the focal length of a concave mirror can be determined, as shown in the di

Focal Length of Concave Mirror Formula

Let’s see the above-shown diagram, 

Focal Length – The space between the pole P of a concave mirror and the focus F is the focal length of a concave mirror. By obtaining the Real image of a distant object at its focus, the focal length of the concave mirror can be estimated as shown in the diagram.

The focal length of the convex mirror is positive, whereas that of the concave mirror is negative. The same can also be proved by using the mirror formula:

[frac{1}{f}]=[frac{1}{v}]-[frac{1}{u}]

Let’s see how

Since we know that an object is always placed at the left side or direction opposite the incidence ray of the mirror, the object distance will always be negative.

u = -u

v = -v (Image distance is negative since images produced by concave mirrors are usually on the left side or direction opposite to the incidence ray)

Using mirror formula,

 [frac{1}{f}]=[frac{1}{v}]-[frac{1}{u}]

Or [frac{1}{f}]=[frac{u-v}{uv}]

Or [f=frac{uv}{u-v}]

Focal Length of Convex Mirror Using Convex Lens

A curved mirror in which the mirroring surface bulges towards the light source is known as the convex mirror. The light is reflected outwards in a convex mirror; therefore, they are not used to focus light. The convex mirror is also called a diverging mirror or fish-eye mirror.

The image created by a convex lens is erect and virtual since the focal point (F), and center of curvature (2F) are both imaginary points within the mirror that cannot be reached. As a result, the image formed by these mirrors cannot be projected on the screen as the image is inside the mirror. Hence the focal length cannot be determined directly. Initially, the size of the image is smaller than the object, but it gets larger as the object approaches the mirror. The diagram below shows the convex mirror.

(the )

The focal length of a convex mirror can be determined by introducing the convex lens between the object and the convex mirror. With the help of a convex lens side by side with an object, an image can be obtained when the convex mirror reflects the rays along the same path, i.e. when rays fall naturally on the mirror. The space between the screen and the mirror is the radius of curvature, which is denoted by R.

By using the formula below, the focal length f of the convex mirror can be calculated.

[F=frac{R}{2}]

Where,

R-Radius of curvature

A mirror with a reflecting surface facing outwards is a Convex mirror, whereas a mirror with a reflecting surface facing inwards is a Concave mirror. The coating of the Convex mirror is on the outside of the spherical surface while the coating of the Concave mirror is on the inside. 

For a Convex Mirror, the principal focus is behind, whereas, for a Concave Mirror, the principal focus is at the front. A point at which the reflected rays meet or appear to meet is the Principal focus.

 

To find the focal length of a Concave mirror:

The various ways to obtain the focal length of the concave mirror:

i)A spherical mirror whose reflecting surface is curved inwards and follows laws of reflection of light is a Concave mirror.

ii) The light rays that come in from a distant object are considered to be parallel to each other.

iii) The parallel rays of light will meet the point in the front of the mirror if the image formed is real, inverted, and small in size.

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iv) The image formed by the convex lens is real and can be obtained on the screen.

v)  the symbol ‘f’ is used to denote the difference between the principal axis P and the focus F of the concave mirror.  

To find the focal length of a Convex mirror:

The various ways to obtain the focal length of the convex lens:

• The middle part of the convex lens is thicker and the edges are thinner.  This is known as a converging lens.

• The refracted rays from the parallel beam of light converge on the other side of the convex lens.

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• The image would be real if the image is obtained at the focus of the lens,  inverted and very small.

• ‘f’ is the focal length which is the difference between the optical center of the lens and the principal focus.

• As the image formed by the lens is real, the image can be obtained on the screen.

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The procedure of determining the focal point of a Concave Mirror can be explained as follows: 

• The distance between the selected object should be more than 50 ft.

• The concave mirror placed on the mirror stand and the distant object should be facing each other.

• The screen should be in front of the reflecting surface of the mirror a
  nd to be able to get a sharp image, adjustments should be made to the screen.

• The distance between the concave mirror and screen can be determined by using a meter scale. The distance and focal length of the mirror will be the same as the given Concave Mirror.

• To calculate the average focal length, we will have to repeat the above procedure three times.

The procedure of determining the focal point of a Convex Lens can be explained as follows: 

• Arrange both the lens and the screen of them on the wooden bench.

• The lens should be placed on the holder in such a way that it is facing a distant object.

• Holder should be placed with the screen on the bench.

• The position of the screen should be such that the sharp image of the distant object is obtained on it.

• The difference between the two positions i.e. of the lens and of the screen has to be equal to the focal length of the given convex lens.

• Shift the focus towards various other distant objects in order to calculate the focal length of the convex lens.

A dielectric material is a poor conductor of electricity i.e, an insulator, meaning that when a voltage is applied, no current can pass through the material. At the atomic scale, however, certain adjustments do happen. It is polarized when a voltage is applied across a dielectric surface. Because atoms consist of a positively charged nucleus and negatively charged electrons, polarization is an effect that slightly shifts electrons towards the positive voltage. They do not move far enough to generate a current flow through the material – the shift is microscopic but has a very important impact, especially when dealing with condensers. 

 

Upon removal of the voltage source from the material, it either returns to its original non-polarized state or remains polarized if the material’s molecular bonds are weak. The distinction between dielectric terms and isolator terms is not very well known. All-dielectric materials are insulators but one that is easily polarized is a good dielectric.

 

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Dielectric Constant

The dielectric constant is an object’s ability to retain as much energy in the form of an electrical field as up to the degree that a substance concentrates electrical flux. It can also be regarded as the object’s permittivity ratio to the permittivity of free space.

Types of Dielectric Materials

Dielectrics are grouped according to the type of molecule present in the material. There are two types of dielectrics – Non-polar dielectric and polar dielectric.

Polar Dielectric

The center of mass of positive particles in polar dielectrics doesn’t coincide with the center of mass of negative particles. There’s a dipole moment here. The shape of the molecules is asymmetrical. When applying the electric field, the molecules align with the electric field. The random dipole moment is observed when the electric field is removed, and the net dipole moment in the molecules becomes zero.

 

Example: H₂O, HCl.

 

Non-Polar Dielectric

The center of mass of positive particles and negative particles coincides within the non-polar dielectrics. These molecules do not have a dipole moment. These molecules are in the form of symmetry.

 

Example: H₂, O₂, N₂.

 

Example of Dielectric Material

A dielectric material may be vacuum, solids, liquids, and gases. 

• Ceramics, paper, mica, glass, etc. are some examples of solid dielectric materials. 

• Distilled water, transformer oil, etc. are liquid dielectric materials. 

• Dielectric gases are nitrogen, dry air, helium, various metal oxides, etc. A perfect vacuum is also a dielectric.
  

Applications of Dielectric Material

• Dielectrics are used in capacitors for storing energy. 

• Ceramic dielectric is used in the Oscillator Dielectric Resonator. 

• The high permittivity dielectric materials are used to improve the performance of a semiconductor device. 

• Mineral oils are used as a dielectric liquid in electrical transformers, and they assist in the cooling process.

• Electrets, a specially treated dielectric material, serves as an electrostatic equivalent to the magnets.

• Plastic films were used as films in a variety of applications such as condenser insulation between foils and slot insulation in rotating electric machines.

• Today, the main uses of liquid dielectrics, mainly hydrocarbon mineral oils, are as an insulating and cooling medium for transformers, earth reactors, shunt reactors, rheostats, etc. 
  

What are Dielectric Properties?

Similar to an ideal capacitor, dielectric stores and dissipates electric energy. The main properties of dielectric material include Electric Susceptibility, Dielectric polarization, Dielectric dispersion, Dielectric relaxation, Tunability, etc

 

Electric Susceptibility: Electric susceptibility measures how easily a dielectric material will be polarized when subjected to an electric field. That quantity also determines the material’s electrical permeability.

 

Dielectric Polarization: An electric dipole moment is a measure of the negative and positive charge separation within the system. The relationship between the moment of a dipole (M) and the electric field (E) gives rise to dielectric properties. When the electric field applied is removed, the atom returns to its original state. It happens in an exponential manner of decay. The time that the atom takes to reach its original state is called the relaxation time.

 

Dielectric Breakdown: When higher electrical fields are applied the insulator begins to conduct and act as a conductor. Dielectric materials lose their dielectric properties under these conditions. The phenomenon is called Dielectric Breakdown. That is a process that is irreversible. That leads to dielectric material failure.

 

Dielectric Dispersion:  P(t) is the maximum polarization attained by the dielectric.

 

P(t) = P[[ 1- e (frac {-t}{tr})]]

 

tr is the relaxation time for a particular polarization process, The period to relax varies with various mechanisms of polarization. Electronic polarization followed by ionic polarisation is very rapid. The polarization of orientation is slower than ionic polarisation. The polarization of space charges is very slow.

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