Molecular Definition of Pressure

Let Us Study About Pressure

Pressure is determined by the flow of mass from a high pressure region to a low pressure region. Pressure measurements are made on the fluid states--liquids and gases. Air exerts a pressure which we are so accustomed to that we ignore it. The pressure of water on a swimmer is more noticable. You may be aware of pressure measurements in relations to the weather or your car or bicycle tires.

PRESSURE is a force exerted by the substance per unit area on another substance. The pressure of a gas is the force that the gas exerts on the walls of its container. When you blow air into a balloon, the balloon expands because the pressure of air molecules is greater on the inside of the balloon than the outside. Pressure is a property which determines the direction in which mass flows. If the balloon is released, the air moves from a region of high pressure to a region of low pressure.

Atmospheric pressure varies with height just as water pressure varies with depth. As a swimmer dives deeper, the water pressure increases. As a mountain climber ascends to higher altitudes, the atmospheric pressure decreases. His body is compressed by a smaller amount of air above it. The atmospheric pressure at 20,000 feet is only one-half of that at sea level because about half of the entire atmosphere is below this elevation.

Atmospheric pressure at sea level can be expressed in terms of 14.7 pounds per square inch. The pressure in car or bicycle tires is also measured in pounds per square inches. A car should have 26-30 lb/sq.in. and bicycle tires 40-60/sq.in.

Molecular Definition of Pressure

From the kinetic theory of gases, a gas is composed of a large number of molecules that are very small relative to the distance between molecules. The molecules of a gas are in constant, random motion and frequently collide with each other and with the walls of any container. The molecules possess the physical properties of mass, momentum, and energy. The momentum of a single molecule is the product of its mass and velocity, while the kinetic energy is one half the mass times the square of the velocity. As the gas molecules collide with the walls of a container, as shown on the left of the figure, the molecules impart momentum to the walls, producing a force perpendicular to the wall. The sum of the forces of all the molecules striking the wall divided by the area of the wall is defined to be the pressure. The pressure of a gas is then a measure of the average linear momentum of the moving molecules of a gas. The pressure acts perpendicular (normal) to the wall; the tangential (shear) component of the force is related to the viscosity of the gas.

The scientific units of pressure can be determined from its definition:

A force applied to surface with an area A will have will result in a pressure P as defined above. Force has the units "mass length/time^2" and area has the units length^2. Inserting this into the equation above results in the units of pressure as

A pascal is the SI unit of pressure.

Consider gas molecules in a rectangular box. Every time a molecule collides with a wall of the box the collision results in a force on the box. These forces combine and result in the pressure of the gas.

Latent Heat

Let Us Learn About Latent Heat

When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. The potential energy stored in the interatomics forces between molecules needs to be overcome by the kinetic energy the motion of the particles before the substance can change phase.

Latent heat is the energy absorbed or released when a substance changes its physical state. Latent heat is absorbed upon evaporation, and released upon condensation to liquid (as in clouds). Latent heat is also absorbed when water melts, and released when it freezes.

Latent heat is the name given to energy which is either lost or gained by a substance when it changes state, for example from gas to liquid. It is measured as an amount of energy, joules, rather than as a temperature.

If we measure the temperature of the substance which is initially solid as we heat it we produce a graph like

Temperature change with time. Phase changes are indicated by flat regions where heat energy used to overcome attractive forces between molecules

Starting a point A, the substance is in its solid phase, heating it brings the temperature up to its melting point but the material is still a solid at point B. As it is heated further, the energy from the heat source goes into breaking the bonds holding the atoms in place. This takes place from B to C. At point C all of the solid phase has been transformed into the liquid phase. Once again, as energy is added the energy goes into the kinetic energy of the particles raising the temperature, (C to D). At point D the temperature has reached its boiling point but it is still in the liquid phase. From points D to E thermal energy is overcoming the bonds and the particles have enough kinetic energy to escape from the liquid. The substance is entering the gas phase. Beyond E, further heating under pressure can raise the temperature still further is how a pressure cooker works.

Latent Heat of Fusion and Vaporization

The energy required to change the phase of a substance is known as a latent heat. The word latent means hidden. When the phase change is from solid to liquid we must use the latent heat of fusion, and when the phase change is from liquid to a gas, we must use the latent heat of vaporization.

The energy require is Q= m L, where m is the mass of the substance and L is the specific latent heat of fusion or vaporisation which measures the heat energy to change 1 kg of a solid into a liquid.

Internal Energy

Let Us Learn About Internal Energy

Internal Energy is the energy stored in a system at the molecular Level. The System's Thermal Energy -the Kinetic Energy of the atoms due to their random motion relative to the Center of Mass plus the binding energy (Potential Energy) that holds the atoms together in terms of atomic bonds.

We consider all possible internal changes to the body as making up the total internal energy.

There are two ways to change the internal energy: with work, and everything else. Everything else is defined as heat. Heat is the defined as the transfer of energy to a body that does not involve work or those transfers of energy that occur only because of a difference in temperature. As Bellman would say,

Internal energy is one of the most important concepts in thermodynamics. Energy changes in a body sliding with friction. Warming a body increases its internal energy and cooling the body decreases its internal energy. However, what is internal energy? We can look at it in various ways: let's start with one based on the ideas of mechanics. Matter consists of atoms and molecules, and these are made up of particles having kinetic energy and potential energy. We tentatively define the internal energy of a system as the sum of the kinetic energy of all its constituent particles, plus the sum of all the potential energy of interaction among these particles.

Note that internal energy does not include potential energy arising out of interaction between the system and it surroundings. If the system is a glass of water, placing it on a high shelf, increases the gravitational potential energy arising from the interaction between the glass and the Earth. However, this has no effect on the interaction between the molecules of water, and so the internal energy of water does not change.

We use the symbol 'U' for internal energy. During a change of state of the system, the internal energy may change from an initial value U₁ to a final value U₂. We denote the change in internal energy as .

A Carnot heat engine

Let Us Learn About A Carnot Heat Engine

A Carnot heat engine is a hypothetical heat engine which works on the basis of reversible Carnot cycle. It is assumed as the most efficient heat cycle consisting of two isothermal and two adiabatic processes. This is the engine which could be operated at the maximum efficiency.

In the Carnot heat cycle there are four basic steps in the first step the engine absorbs heat and the gas begins to expand. In this step the temperature of the gas does not change and hence it is an isothermal process all the amount of heat energy is used to expand the gas only. In the second step the gas is still expanding but a cooling is being done so this type of process is called adiabatic expanding. In the third step the gas which is cooled in step two is now recompressed and heat goes to the heat sink. In this third stage there is a decrease in volume but an increase in the pressure of the gas however temperature still stays constant so this is an isothermal compression process. In the fourth step the gas cool gas is compressed again and its temperature goes back to the original temperature again a temperature change is observed so this is an adiabatic compression.

The Carnot engine is a simple heat engine concept, where the gas always stays in the cylinder just being heated and refrigerated.

I roughly reproduced this principle with Realflow, resulting in the gas actually pushing the piston out, and the inertia built by the wheel drives the piston back for a new cycle. The only cheat is the heating/freezing synchronization system which is a simple Python script. Modifying max heat temperature will then alter the speed of the engine, and this, is fun and compelling.

photoelectric effect

Let Us Learn About photoelectric effect

The photoelectric effect is usually observed when light is shined on metallic surfaces. The beam of light that is shone in a metal surface is referred to as the photocathode, and the electrons that it ejects from an atom are called photoelectrons. Shining light on a conductive metal surface can actually cause an electrical current, called a photocurrent , to form. A material that is sensitive to light, such as the metals which can carry an electrical current because of light, are referred to as photosensitive substances.

When light of sufficiently small wavelength is incident on a metal surface, electrons are ejected from the metal. This phenomenon is called as 'photoelectric effec

t' and the ejected electrons are called as 'photoelectrons

There are three ways in which electrons eject out of a material. They are

(i) Thermionic emission

(ii) Field emission

(iii) Photo electric emission

In all the above cases, energy is given to the material but in different forms. If given in the form of heat it is called as Thermionic emission, if in the form of electrical energy, it is field emission and if in the form of light (photons), then it is photoelectric effect.

The light must be energetic enough, which for zinc is in the ultraviolet region of the spectrum.If light were waves, we would expect the free electrons to steadily absorb energy until they escape from the surface. This would be the case in the classical theory, in which light is considered as waves. We could wait all day and still the red light would not liberate electrons from the zinc plate.So what is going on? We picture the light as quanta of radiation (photons). A single electron captures the energy of a single photon. The emission of an electron is instantaneous as long as the energy of each incoming quantum is big enough. If an individual photon has insufficient energy, the electron will not be able to escape from the metal.

There is a threshold frequency (i.e. energy), below which no electrons are released.

The electrons are released at a rate proportional to the intensity of the light (i.e. more photons per second means more electrons released per second).

The energy of the emitted electrons is independent of the intensity of the incident radiation. They have a maximum KE.

An analogy

Try this analogy, which involves ping-pong balls, a bullet and a coconut shy. A small boy tries to dislodge a coconut by throwing a ping-pong ball at it – no luck, the ping-pong ball has too little energy! He then tries a whole bowl of ping-pong balls but the coconut still stays put! Along comes a physicist with a pistol (and an understanding of the photoelectric effect), who fires one bullet at the coconut – it is instantaneously knocked off its support.

Ask how this is an analogy for the zinc plate experiment. (The analogy simulates the effect of infrared and ultra violet radiation on a metal surface. The ping-pong balls represent low energy infrared, while the bullet takes the place of high-energy ultra violet.)

Now you can define the work function. Use the potential well model to show an electron at the bottom of the well. It has to absorb the energy in one go to escape from the well and be liberated from the surface of the material.

Units

The electronvolt is introduced because it is a convenient small unit. You might need to point out that it can be used for any (small) amount of energy, and is not confined to situations involving electrically accelerated electrons.

Potential well

It is useful to compare the electron with a person in the bottom of a well with totally smooth sides. The person can only get out of the well by one jump, they can't jump half way up and then jump again. In the same way an electron at the bottom of a potential well must be given enough energy to escape in one 'jump'. It is this energy that is the work function for the material.

Now you can present the equation for photoelectric emission:

Energy of photon E = hf

Picture a photon being absorbed by one of the electrons which is least tightly bound in the metal. The energy of the photon does two things.

Some of it is needed to overcome the work function f.

The rest remains as KE of the electron.

hf = f + (1/2) mv^ ²

A voltage can be applied to bind the electrons more tightly to the metal. The stopping potential V_s is just enough to prevent any from escaping:

hf = f + eV_s

Torque

Let Us Learn About Torque

Torque is a measure of how much a force acting on an object causes that object to rotate. The object rotates about an axis, which we will call the pivot point, and will label 'O'. We will call the force 'F'. The distance from the pivot point to the point where the force acts is called the moment arm, and is denoted by 'r'. Note that this distance, 'r', is also a vector, and points from the axis of rotation to the point where the force acts. (Refer to Figure 1 for a pictoral representation of these definitions.)

Torque is defined as

In other words, torque is the cross product between the distance vector (the distance from the pivot point to the point where force is applied) and the force vector, 'a' being the angle between r and F.

Using the right hand rule, we can find the direction of the torque vector. If we put our fingers in the direction of r, and curl them to the direction of F, then the thumb points in the direction of the torque vector.

Imagine pushing a door to open it. The force of your push (F) causes the door to rotate about its hinges (the pivot point, O). How hard you need to push depends on the distance you are from the hinges (r) (and several other things, but let's ignore them now). The closer you are to the hinges (i.e. the smaller r is), the harder it is to push. This is what happens when you try to push open a door on the wrong side. The torque you created on the door is smaller than it would have been had you pushed the correct side (away from its hinges).

Note that the force applied, F, and the moment arm, r, are independent of the object. Furthermore, a force applied at the pivot point will cause no torque since the moment arm would be zero (r = 0).

Another way of expressing the above equation is that torque is the product of the magnitude of the force and the perpendicular distance from the force to the axis of rotation (i.e. the pivot point).

Let the force acting on an object be broken up into its tangential (Ftan) and radial (Frad) components the radial component of the force has no contribution to the torque because it passes through the pivot point. So, it is only the tangential component of the force which affects torque

There may be more than one force acting on an object, and each of these forces may act on different point on the object. Then, each force will cause a torque. The net torque is the sum of the individual torques.

Rotational Equilibrium is analogous to translational equilibrium, where the sum of the forces are equal to zero. In rotational equilibrium, the sum of the torques is equal to zero. In other words, there is no net torque on the object.

In physics, when an external force is applied on the body, the body gets tendency to rotate over an axis called torque.Torque is considered as a vector quantity over an axis of rotation. The multiplication of the force magnitude and the perpendicular distance of the line. Torque is mainly for change the state of rotation. The other name of torque is a moment of force.

Torque in Physics:

Torque in physics:

Consider an external force F is applied on a body. The body can rotate through the point O about an axis and the direction is perpendicular to the paper.

As in figure the distance of the force from the point O is ON=r.

Then the torque of physics can be written as,

r = F. r

Unit of torque:

newton x meter

• The torque value is positive when the rotation of a body is anticlockwise.

• The torque value is negative when the rotation of a body is clockwise.

Two facts about torque in physics:

• If the value of r=0 through O then the torque is also zero. In this case the rotation of body is not possible. That is if the line of force is zero then torque is zero. For example, the sun has a torque is zero and the gravitational force acts on the earth makes an earth to rotate the sun.
• If the line of force distance increases then torque is also increases. That is the increase value of r cause the increase of torque. In this case a small force is required for rotating a body.

Static Electricity

Let Us Learn About Static Electricity

Static electricity was known to people more than 2500 years ago. The Greeks knew about the attractive property of the resin amber. They knew that by rubbing amber with cloth, it could be made to attract small feathers. The Greek name for amber was 'elektron'.

Static electricity is defined as the deposition of electrical charges or electrons on the surface of a material. They are generally caused when a material is rubbed against the surface of another material. Though electrical charges are present they do not constitute to form an alternating current or a direct current. Hence, these are referred to as "static" which explicitly means "electricity at rest".

Causes of Static Electricity

When two materials come into physical contact against each other, the electrons are shifted from the surface of one material to the other’s surface. The material which supplies the electrons attains extra positive charges called as protons and the other material which gains the electrons attain extra negative charges called as neutrons.

The process of static electricity doesn’t purely mean the conduction of electricity. It is found that most of the materials that have the ability to exhibit static electricity are non conductors of electricity.

A common fact to note is the circumstance when static electricity is exhibited. When the humidity of air is higher than usual, there is high probability of the presence of water particles on the elements. This hinders the deposition of electrical charges on its surface since the water molecules are bound to repel the charges. Therefore it is implied that the process of static electricity is better exhibited when the air is less humid.

Practical Uses:
The common uses of static electricity are in Xerography or electro-photography, Air filters, Automotive paints, Spray paintings.

Most of the modern photocopy machines use static electric charges to make photo copies of documents. Recently invented laser and LED printers also use static electricity.

Since static electricity is an inter-combination of electric charges (protons and neutrons) on two materials that are separated from each other, there is a high probability of small electrical and electronic components getting damaged due to its impact. Hence, manufacturers of such devices have started using a number of antistatic devices to avoid the effect.

Proof of Static Electricity

It is well known that opposite charges attract each other whereas like charges repel each other. Therefore the existence of static electricity can be proved if an object with static electrons on its surface attracts another object which has electrons of the opposite charge.

Consider a metal rod which is initially neutral with equal amount of positive and negative charges on it. When the rod is rubbed against a rough surface for a while, it is bound to achieve positive charges on its surface. Now, it is taken near a small metal bead which is neutral. The rod is found to attract the bead thus proving the process of static electricity. The same rod, when taken near another metal bead which has been rubbed against a metal surface and made to conduct negative charges, is found to push away the bead since like charges repel.

Effects:
By experiments, it has been found that an object which has electrons deposited on it attracts an oppositely charged object with a greater attraction than an object with neutral charges. This is because the strong molecular bonding of the electrons with the surface of the material keeps the electrons intact. But when an oppositely charged object is brought near it, the electrons are attracted with a greater force that they break the strong molecular bonding and jump across to the other object. When more and more electrons fly across the objects, the air surrounded by then gets heated up and this induces the other electrons to switch over to the receptor object. This eventually leads to a situation when both the elements acquire neutral charges.

Transistor Action

Let Us Learn About Transistor Action

The term transistor action refers to the control of the large collector-emitter (linking) current by the smaller base (back injection) current in forward active operation, the origin of “current gain” in a BJT

• Two features of the device are essential for transistor action

1) A narrow base, which forces all electrons injected from the emitter

to travel across the base neutral region to the collector

2) A high emitter doping compared to the base doping, making base

(electron) injection the dominant term

Transistor action in a Si/CoSi2/Si structure is reported. The thin silicide layer (less than 100 A), which acts as the base, is a single-crystal metal, essentially continuous and locally exhibiting atomically perfect interfaces with Si. The transistor action is manifested by a common base current gain alpha as high as 0.6 and a voltage gain greater than 10.