Category: Physics Notes PPT

Fet Transistor stands for Field-Effect transistor. The field-effect transistor (FET) is a type of transistor that controls the flow of current in a semiconductor using an electric field. 

FETs are three-terminal devices with a source, gate, and drain. The application of a voltage to the gate, which modifies the conductivity between the drain and source, controls the flow of current in FETs.

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The first patent for FET transistors was filed by Julias Edgar in 1926. Since then much development has taken place. Another patent was filed by Oskar Heil in 1934. The junction gate that is used in field-effect transistors was created at the Bell Labs by William Shockley. Many other advancements in FET Transistors have been made over the years. 

Working of FET Transistor

The Fet transistor is a voltage-operated device in which the voltage applied is used to control the current flowing. It is also known by the name unipolar transistor as they undergo an operation of a single-carrier type. The input impedance is high in all forms and types of FET. The conductivity is always regulated with the help of applied voltage from the field-effect transistor’s terminal. Moreover, the density of the carrier charge affects conductivity. 

A FET transistor is a device with three major components: Source, Drain, and Gate. The source is one of the terminals of the FET transistor through which most of the carriers enter the bar. The Drain is the second terminal through which the majority of carriers lead the bar. The Gate has two terminals that are internally connected with each other. 

Since the gate in a FET transistor is reverse biased, the gate current is practically zero. The drain supply is connected to the source terminal leading to the electrons flow which provides the necessary carriers.  

FET Transistor – Types and Its Working Principles

There is another subdivision of FET Transistors. In one of the types, the current is taken up primarily by the majority carriers and is therefore called majority charge carrier devices. There are minority charge carrier devices, as well,  in which the current flow is primarily due to minority carriers. 

The two terminals, source, and gate have a potential between them which in turn has the conductivity of the channel as a function of it. The three terminals i.e. source, drain, and gate are there for every FET Transistor. The function of the gate terminal is similar to the gate in real life as the gate can open and close and can either choose to permit the passage of electrons or stop them altogether. 

FETs are categorised as:

1. Junction Field Effect Transistor (JFET)

2. Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

1.Junction Field Effect Transistor (JFET)

The Junction FET transistor is a form of field-effect transistor that can be used to control a switch electrically. Between the sources and the drain terminals, electric energy travels through an active channel. 

The channel is strained and the electric current is switched off by supplying a reverse bias voltage to the gate terminal.

Working Principle:

The working of these JFETs is based on the channels that form between the terminals. Either an n-type or a p-type channel can be used. It’s called an n-channel JFET because it has an n-type channel, and it’s called a p-channel JFET because it has a p-type channel.

FET transistors are made in the same way as N-P-N and P-N-P transistors are made in BJT (Bipolar Junction Transistor). These JFETs have a channel that can be either n or p-type.

• It is classified as an n-channel JFET or a p-channel JFET depending on the channel.

• The source terminal connects the positive side of an n-channel JFET.

• In this n-channel JFET, the drain terminal has the largest potential compared to the gate.

• The connection created by the drain and gate terminals is in reverse bias. As a result, the depletion region around the drain is wider than the source.

• The majority of the charge carriers, which are electrons, flow from the terminal drain to the source.

• As the potential at the drain rises, the flow of carriers rises with it, and the flow of current also rises with it. 

• However, when the voltages at the drain and source are increased to a particular level, the current flow is stopped.

• The JFET is well-known for its ability to control current through the application of input voltages. In this transistor, the input impedance is at its highest point.

• There is no current evidence at the gate terminal when the JFET is in its optimum mode.

That is how an n-channel JFET operates. Only a change in the polarities of the supplies causes the FET to operate as a p-channel JFET.

2.Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

MOSFETs work by applying a voltage to channels that already exist or form. MOSFETs are classified into two types based on their operation modes: 

In the enhancement mode, the gate voltage induces the channel, whereas, in the depletion mode, the MOSFET operates owing to the existing channel.

There are two types of MOSFET depletion models: n-type and p-type. The only difference is the substrate deposition. The formation of the depletion zone is caused by a concentration of carriers that are preferred by the majority. Conductivity is affected by the width of the depletion.

A channel is formed in the enhancement mode when a voltage applied to the gate terminal exceeds a threshold voltage. It could be n-type for a P-type substrate and p-type for an N-type substrate. The enhancement mode is classified as N-type Enhancement MOSFET or P-type Enhancement MOSFET based on the channel formation. MOSFETs of the enhancement type are more commonly used than those of the depletion type.

Difference Between FET and MOSFET

The main difference between the two major types of FET transistors – JFET and MOSFET- is that JFET (Junction Field Effect Transistor) is a three-terminal semiconductor device while MOSFET (Metal oxide semiconductor field-effect transistor) is a four-terminal semiconductor device. JFET can only operate in the depletion mode. While MOSFET can operate in the enhancement as well as the depletion mode. The input impedance is higher in MOSFET making them more resistive. In comparison to the price, MOSFET is more expensive than JFET. 

Due to high input impedance, FET transistors are commonly used in and as input amplifiers in electronic voltmeters, oscilloscopes, and other measuring devices. T
hey also occupy little space which makes them more efficient for other devices. 

Conclusion

The article covers some important and key characteristics of FET Transistors. This foundational knowledge can be further used in understanding more concepts related to electricity and current. The definition of FET, types of FET, and how it regulates the circuits are the key highlights of this article.

Ferromagnetism or the meaning of ferromagnetism is a mechanism through which certain materials form permanent magnets. With the aid of a strong electrostatic field, these materials can be permanently magnetized. Ferromagnetic metal ions are grouped into small regions called solid-state domains. So every domain is acting like a tiny magnet. The domains of a ferromagnetic unmagnetized piece are randomly oriented so that their magnetic moments are canceled out. When this material is put in a magnetic field, all domains are oriented in the direction of the magnetic field, creating a powerful magnetic effect. Also, when the magnetic field is withdrawn and the ferromagnetic material becomes a permanent magnet, this order of domains remains the same. There are many different forms of magnetism, but ferromagnetism is of the strongest form and is responsible for the widespread occurrence of magnetism in magnets experienced in everyday life. 

 

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Examples of Ferromagnetic Materials

1. Co (Cobalt)

2. Fe (Iron)

3. MnBi

4. Ni (Nickel)

5. [Nd_{2}Fe_{14}B]

6. MnSb

7. [CrO_{2}] (Chromium dioxide)

8. MnAs 

 

Properties of Ferromagnetic Materials

• When a rod of this material is placed in a magnetic field, it quickly aligns itself in the field track.

• These substances show the permanent magnetism even in the absence of magnetic field

• When the substances are heated at high temperatures, the ferromagnetic substances transform to paramagnetic

• Permeability of ferromagnetic material is greater than 1.

• The mechanism of ferromagnetism is absent in liquids and gases.

• The intensity of magnetization (M), relative permeability ([mu_{r}]), magnetic susceptibility ([chi_{m}]), and magnetic flux density (B) of this material will always be positive.

 

[X_{m} = frac{M}{H}]

 

[mu_r = 1 + X_m]

 

B = [mu_0](H + M)

 

[mu_0 rightarrow] Magnetic Permittivity of the free space.

H [rightarrow] Applied Magnetic Field Strength.

 

Hysteresis Loop

The hysteresis loop is formed by altering the magnetizing force while at the same time measuring the material’s magnetic flux. When a ferromagnetic material is magnetized in one direction, removal of the imposed magnetizing field will not relax back to zero magnetization. A field in the opposite direction needs to drive it back to zero. When an alternating magnetic field is applied to the object, a loop called a hysteresis loop can be traced for its magnetization.

 

The absence of magnetization curve re-traceability is the property called hysteresis, which is due to the presence of magnetic domains in the material. Upon reorientation of the magnetic domains, it takes some energy to turn them back.

 

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This property is useful as a magnetic “memory” of ferromagnetic materials. The magnetic memory aspects of iron make them useful for recording audiotape and for storing data magnetically on computer disks.

 

Curie Temperature

There is a temperature, over which the ferromagnetic material is paramagnetic. This specific temperature is called temperature Curie. This is, if we rise above Curie temperature, it will cause the ferromagnetic materials to lose their magnetic properties. Curie temperature is represented by [T_{c}]. Thermal energy interrupts the magnetic ordering of the dipoles in the ferromagnetic material. 

[E_{thermal} = k_{B} T]

Curie’s law is given by X = [frac{C}{T}]

[k_{B} rightarrow] Boltzmann Constant

C [rightarrow] Curie Constant

 

Examples, 

• Ni – 627 K

• Gd – 293 K

• Co – 1388 K

 

What is Antiferromagnetism?

Antiferromagnetic materials are weakly magnetized in the direction of the field, in the presence of a strong magnetic field. This property of the materials is called antiferromagnetism and antiferromagnetic materials are called the materials which exhibit this property. The magnetic moments are aligned in opposite directions in antiferromagnetic materials and are equal in magnitude. Thus, when antiferromagnetic material is unmagnetized the net magnetization is zero due to the exact cancellation of magnetic moments of the adjacent atoms when added in a line.

 

Application of Ferromagnetic Materials

Ferromagnetic materials have many applications for electrical, magnetic storage, and electromechanical equipment.

• Permanent Magnets: Ferromagnetic materials are used for making permanent magnets because its magnetization lasts longer. 

• Transformer Core: A material used to make the transformer core and choke is subjected to very rapid cyclical changes and the material must also have strong magnetic induction. Ferromagnetic materials are highly used to serve the purpose. 

• Magnetic Tapes and Memory Store: The magnetization of a magnet is not only dependent on the magnetization field but also on the magnetization cycle it has undergone. Thus, the specimen’s magnetization value is a record of the magnetization cycles that it has undergone. Thus, such a machine will serve as a memory storage unit.

 

Ferromagnetism vs Paramagnetism vs Diamagnetism

Ferromagnetism is a process through which a metal or substance is turned into a highly magnetic substance or a strong permanent magnet when it is subjected to an electromagnetic field. When these substances are brought out of the magnetic field, they remain as strong permanent magnets. This process can only be found in solids and is absent in liquids and gasses. But, when the ferromagnetic substances are exposed to extreme heat, they lose their magnetic property. Paramagnetism is a similar mechanism in which some objects or metals are weakly attracted to magnetic objects. When ferromagnets are subjected to extreme heat, it disrupts the perfectly aligned atoms and changes the object into a paramagnet. On the other hand, diamagnetism is a process in which a substance acts as a weak magnet when the orbital motion of the atoms of the object is changed by an external electromagnetic field. Both the paramagnets and diamagnets are weak magnets and to find the difference between both, the rule of thumb is, a
n object is said to be paramgent if all the electrons within the objects are unpaired and an object is said to be diamagnetic if all the electrons in the object are paired. 

 

Fun Facts

• Magnetar is the most powerful magnet in the universe.

• Hammering a magnet can cause its magnetic properties to lose out. Heating up a magnet is another means of destroying its magnetic properties. This is because the molecules lose their alignment north-south and get arranged in random directions.

Before going through force and pressure in detail. Let us first have an overview of it. Force is applied in order to perform a work. If we talk about pressure then pressure is the force that is applied per unit area. Force is applied perpendicular to the surface per unit area. The unit of measurement of pressure is pascal. A Pascal is basically defined as the pressure exerted by one force of Newton on one square unit area.

Where there are similarities between the force and pressure but in actual force is the total effect of one object on the other one whereas If we talk about pressure it us the certain amount of force that is applied on a unit area. That is you can define pressure in the physical quantity of the force that is spread over a unit area. Since we know that force is directly proportional to pressure, in this context if the force applied on an object it will be increased in the same way pressure will also increase. You can go through this article as this article will provide you a brief knowledge of pressure and force. In this article you are going to study about force and pressure, properties of Force, type if force, thrust and pressure and further you will be provided with the Frequently Asked questions related to the force and pressure topic that will increase your knowledge and will help you to get a clear insight of this topic pressure and force.

Force and Pressure Definition

A kind of push or pull is known as force. It is something whose action generates motion in an object. or, it can also be defined as the interaction between two objects. Force has a magnitude as well as direction. Magnitude is used to express the strength of the force. A change in direction or magnitude or both also changes the force. Exerted force on an object may change the speed with which the object moves, the direction of motion or shape. When we measure the amount of force acting on a unit area, it is called pressure.

We will study more about force, it’s types, physical properties and pressure in this article.

Properties of Force

The force exerted away from the body is a push. And a pull is a force exerted towards the body. Thus, a force is a push or a pull.

When the force is being applied in the direction of motion of the object then it mostly increases its speed and when it’s applied in the opposite direction, the speed decreases.

Thus, the state of motion of an object is explained by its speed and the direction of motion.When the speed is zero, the body is considered to be at a state of rest.

An object cannot move, change its direction or speed by itself. Which implies that an external force acting upon the body can change its speed, motion, direction, and shape.

The resultant of all the forces acting on a body is known as the net force. And, the acceleration of the body is along the direction of the net force.

The force that opposes the relative motion between two surfaces is called frictional force.

Frictional force acts between the surface of the two bodies in contact with each other.

Types of Force

Force is applied to a body in order to perform a work. There are numerous  forces that can be applied that for every force there are a number  of varieties or types. Let’s have a look at these forces.

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1. Frictional Force – The force that opposes the relative motion between two surfaces is called frictional force. The frictional force acts between the surface of the two bodies in contact with each other

2. Muscular Force – When force is caused or carried out by the action of muscles is called muscular force.

3. Magnetic Force – Force acting between two magnetic bodies as a result of their poles is called magnetic force. Magnetic Force is usually an attracting force.

4. Gravitational Force – Earth exerts a pull on all objects or bodies towards itself. This pull is known as gravitational force. As a result of gravitational force, every object in this universe, irrespective of its size and shape, exerts some force on every other object.

5. Electrostatic Force – Electrostatic Force, also known as the Coulomb Interaction is basically a force applied by a charged body on another charged or uncharged body called electrostatic force. In other words, the electrostatic force is an attractive or repulsive force acting between two electrically charged objects.

6. Buoyant Force – It is an upthrust force exerted by fluid opposing the weight of the object that is fully or partially immersed in that fluid. Buoyancy is generally caused by the differences in pressure acting on opposite sides of an object immersed in a static fluid.

7. Tension Force – Tension force acts when a rope or string or any similar object is pulled by forces acting from the opposite direction. It is directed over the length of the wire or the string.

8. Drag Force – Drag force also acts in a fluid like water or air. It is the resistant force caused by the motion of a body through a fluid. Drag force is the one that acts opposite to the direction of the oncoming flow velocity.

9. Spring Force – It is a type of restoring force. This is because the force exerted by the spring is always in the opposite direction to the displacement.

10. Nuclear Force – It is a force that acts between all the particles in the nucleus. That is the force acting between two neutrons, between two protons and between a neutron and a proton. A nuclear force is always an attractive force.

Thrust and Pressure

Force acting on a unit area is known as pressure. The SI unit of pressure is Pascal and it is given by P= Force/Area. Thus, it can be seen that pressure is the ratio of the force to the area over which the force is acting.

Pressure (p) =[ frac{text{Force (Fn)}} {text{Area  (A)} }]

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Thrust, on the other hand, is a reaction force. When a system accelerates a body with a mass in one direction, the acceleration causes a force of equal magnitude in the opposite direction upon the system. The force applied in a perpendicular direction on the surface is called thrust.

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Thrust is measure in Newtons and can be calculated by the formula-

Thrust = Pressure × Area

Some objects that float in water are because of thrust. Also, objects weigh less in water because of the same phenomenon.

If you want to get further knowledge about the topic of pressure and force and know about th
e reaction between both force and pressure you can visit . app provides you with all the information and the study material that you will require to understand this concept. At our main motive isis to make you understand the topic well. The children who ate connected with for their studies already observed a leap and bound changes in their scores and knowledge.  The topics that are discussed upto class 12 are mainly to make your foundation for the coming higher class and gas specially cleared the concepts of the students of these classes so that their foundation remains strong. You just have to download the app if and you can get all sorts of information.

Now before moving to another topic let’s have a look at the Frequently Asked questions related to force and pressure.

It is known that the conduction of heat takes place when the molecules of matter vibrate or agitate and transmit energy to the adjacent molecules. As the neighbouring molecules collide, heat energy is transferred from a higher temperature area to a lower one. This process abides by Fourier’s law. So let us go through the article below to understand what is Fourier’s law. Fourier’s law is also called the law of thermal conduction equations or the law of thermal conductivity. This law is taught in schools in the NCERT book that follow the CBSE curriculum. In order to understand this law index, there are certain concepts that students should learn and not understand like Newton’s law of cooling, ohm’s law, heat transfer, change of state, specific heat capacity, measurement of temperature. 

Fourier’s law is also known as the law of heat conduction, it mainly states that the heat transfer rate through a material is considered to be proportional to the negative gradient present in the temperature, as well as to the area which is at right angles to the gradient, through which the heat flows. This law has two equivalent forms- integral form and differential form. This law is considered to be in an empirical relationship that is based on observation.

If the thermal conductivity of a solid is greater than the ability to conduct heat through it will also be greater. However, if the heat conductivity of the solid is lower than the ability to conduct heat through it will also be lower. The rate at which heat conduction takes place requires certain factors for the medium through which it takes place like the geometry, thickness, material, the temperature difference across the medium also matters.

Key Concepts of the Topic

Key concepts whose knowledge is necessary for the understanding of Fourier’s law-

• Temperature and heat

• Measurement of temperature

• Ideal-gas equation and absolute temperature

• Thermal expansion

• Specific heat capacity

• Calorimetry

• Change of state

• Heat transfer

• Newton’s law of cooling

• Ohm’s law

What is Fourier’s Law?

“Fourier’s law of thermal conduction states that the rate of heat transfer through a material is proportional to the negative gradient in the temperature and the area (perpendicular to the gradient) of the surface through which the heat flows.”

Processes of heat transfer are measurable in the form of rate equations. The rate equation for conduction (a mode of heat transfer) is deduced based on Fourier’s law of thermal conduction. It says that the rate of transfer of heat across a substance is directly proportional to the negative gradient in temperature and area, at 90 degrees to that gradient, in which flow of heat occurs.

The differential form of Fourier’s law can be represented as:

q = – k▽T

where,

• ∇T is temperature gradient (K. m-1)

• k is the conductivity of the materials (W. m-1. K-1)

• q is the heat flux density vector (W. m-2)

Thermal conductivity (k or λ) of a substance is nothing but the proportionality constant acquired in the expression. A body in which energy transfer occurs rapidly by the process of conduction is considered an excellent thermal conductor. Also, it has a significant value of k.

To find the solution of Fourier’s law, the relationship of geometry, temperature difference, and thermal conductivity of the material is derived. Joseph Fourier first introduced this law in the year 1822 and concluded: “the heat flux resulting from thermal conduction is proportional to the magnitude of the temperature gradient and opposite to it in sign”.

Heat flux is the heat transfer rate per unit area normal to the direction of heat transfer. It can also be referred to as heat flux density. Since it is a vector quantity, it has both magnitude and direction.

Features of heat transfer of a solid body can be quantified by the virtue of its property, thermal conductivity. You must remember that Fourier’s law is applicable to all states of matter, be it solid, liquid, or gas. Hence, it can be defined for liquids and gases as well.

The thermal conductivity of a maximum number of solids and liquids varies with temperature, and for gases, it depends on pressure.

[k = k r^{rightarrow},T(r^{rightarrow},t) = frac{q_x^rightarrow}{frac{partial T}{partial x}}]

For a lot of materials that are homogeneous, thermal conductivity can be written as k = k (T). The same type of expression is related to thermal conductivities in y (negative) and z (negative) directions. However, for an isotropic substance, thermal conductivity is not dependent on the path of heat transfer.

kx = ky = kz = k

From the above expression, it can be said that when thermal conductivity and temperature difference increases, conduction heat flux also increases. Generally, a solid substance’s thermal conductivity is more significant than a liquid and gas. This is because of the difference in intermolecular spaces.

Did You Know?

Diamond is the hardest material and has the highest thermal conductivity.

Fourier’s Law Derivation

Consider T1 and T2 to be the temperature difference through a short distance of an area. Here the distance is Δx and the area is denoted as A and k is the material’s conductivity. Henceforth, the following equation can be formed (in one dimension):

Qcond = kA (T1 − T2 / Δx) = −kA (ΔT / Δx)

Now when Δx is zero, the previous equation in differential form can be written as:

Qcond = −kA (ΔT / Δx)

Furthermore, the 3D form of Fourier’s law is:

[q^{rightarrow} = -knabla T]

After going through Fourier’s law and related topics, next take a look at a solved example of heat loss.

Numerical Example Showing Loss of Heat through Windows

One of the major reasons for heat loss from a house is through its windows. Find the heat flux rate from a glass window having an area 1.5 m x 1.0 m and width 3.00 mm, provided the temperatures of outer and inner surfaces are 13.0 degrees Celsius and 14.0 degrees Celsius.

Answer:

Over here, temper
atures of the surfaces are given based on the conditions of the inner and outer parts of the house. Hence, the flow of heat takes place from a higher temperature inside the house to lower temperatures outside.

By using Fourier’s law equation, the following can be assumed:

Thermal conductivity of glass k = 0.96 W / m.K

Then, 

Heat flux q = 0.96 W/m.K x 1 K / 3.0 x 10-3m = 320 W / m2

Net loss of heat from the window:

qloss = q . A = 320 x 1.5 x 1.0 = 480 W

Do It Yourself

1. The Fourier Theory Was Published in Which Year?

(a) 1830 (b) 1845 (c) 1824 (d) 1822

2. Determine the Wrong Assumption Made on Fourier’s Law.

(a) No heat is generated internally

(b) Non-linear temperature profile

(c) Heat conduction in steady-state

(d) Homogeneous and isotropic material

Conclusion

By now, you must have understood what is Fourier’s law, its derivation, and other related topics. This makes it easy to learn further concepts.

Frictional Force refers to the force generated by two surfaces that contact and slide against each other. These forces are mainly affected by the surface texture and quantity of force requiring them together. The angle and position of the object affect the volume of frictional force.

 

The main reason behind friction between objects appears is due to the forces of attraction, known as adhesion, between the points of contact regions of the surfaces, which are always minutely irregular. Friction arises from shearing these “fused” junctions and from the action of the irregularities of the harder surface tilling across the softer surface.

If an object is placed against an object, then the frictional force will be the same as the weight of the object. If an object is pushed against the surface, then the frictional force will be increased and become extra than the weight of the object. The full amount of friction force that a surface can apply upon an object can be easily measured with the use of the given formula:

 

F_frict = μ F_norm

 

Two basic experimental facts describe the friction of sliding solids. First, the volume of friction is nearly independent of the area of contact. If a brick is pulled along a table, the frictional force is similar whether the brick is lying flat or standing on end. Second, friction is directly proportional to the weight that presses the surfaces together. If a load of three bricks is pulled along a table, the friction is three times more than if one brick is pulled. Thus, the ratio of friction F to load L is the same. This constant ratio is called the coefficient of friction and is typically symbolized by the Greek letter mu (μ).

 

Mathematically, μ = F/L. Because both friction and load are calculated in units of force (such as pounds or newtons), the coefficient of friction is dimensionless. The value of the coefficient of friction for a case of one or more bricks sliding on a clean wooden table is about 0.5, which indicates that a force is equal to half the weight of the bricks is required just to overcome friction in keeping the bricks moving forward with a constant speed. The frictional force is directed oppositely to the motion of the object. Because the friction thus far described rises between surfaces in relative motion, it is called kinetic friction.

 

If you have a block of brick in your hand and try to slide it on the ground by force then it does not slide that easily, it happens because of frictional force. In the subject of physics, Frictional force is one of the most important topics and hence students must have a good and clear understanding of the same. Therefore, to help the students of physics in understanding the topic of Frictional force, provides a complete explanation of the same in an easy language.

Types of Frictional Forces

Friction is the force that competes with motion between any surfaces that are in touching base. Static, kinetic, sliding, and rolling friction takes place between solid surfaces. Fluid friction takes place in liquids and gases. All four types of friction are described below:

 

1. Dry Friction

a. Static Friction

b. Kinetic Friction

c. Rolling Friction

d. Sliding Friction

2. Fluid Friction 

Dry Friction: Dry friction is the force that competes with one solid surface gliding across another solid surface. Dry friction always opposes the surfaces sliding kin to one another and can have the effect of any opposing motion or causing motion in bodies. The most commonly used for dry friction is coulomb friction. This kind of friction can further be divided into static friction and kinetic friction. These two types of friction are explained in the diagram below. First, imagine a box on a surface. A pushing force is applied parallel to the surface and is slowly being increased. A gravitational force, a normal force, and a frictional force are also acting upon the box.

(a) Static Friction

Static friction occurs earlier to the box slipping and moving. In this area, the friction force will be equal in scale and opposite in direction to the pushing force itself. As the degree of the pushing force increases so does the friction force. If the amount of the pushing force continues to rise, eventually the box will start to slip. As the box begins to slip the type of friction resisting the motion of the box changes from static friction to what is called kinetic friction. The point just before the box slips is called an impending motion. This can also be assumed as the maximum static friction force before slipping. The total of the maximum static friction force is the same as the static coefficient of friction times the normal force existing between the box and the surface. This coefficient of friction is a property that depends on both materials and can typically be looked up in tables.

Static Friction Examples

Static friction acts on two bodies in contact, i.e.,  one body over the other. Let us consider a few real-life examples:

• While walking, our feet’s backward motion exerts pressure on the road as the other foot goes forward.

• Static friction generates when two fabrics slide over each other.

(b) Kinetic Friction

Kinetic friction occurs beyond the point of coming motion when the box is sliding. With kinetic friction, the amount of the friction force opposing motion will be the same as the kinetic coefficient of friction times the normal force between the box and the surface. The kinetic coefficient of friction also rests upon the two things in contact but will almost always be less than the static coefficient of friction.

(c) Rolling Friction

Rolling friction happens when a wheel, ball, or cylinder roll freely above a surface, as in ball and roller bearings. The main cause of friction in rolling appears to be the distribution of energy involved in the twisting of the objects. If a hardball is rolling on a level surface, the ball is somewhat packed down and the level surface is somewhat indented in the regions of contact. The elastic bend or compression produced at the leading section of the part in contact is interference to motion that is not fully compensated as the substances spring back to typical shape at the trailing section. The internal losses in the two substances are parallel to those that keep a ball from bouncing back to the level from which it is released. Coefficients of sliding friction are usually 100 to 1,000 times greater than coefficients of rolling friction for corresponding materials.

 

Rolling Friction Examples

Rolling friction is one of the types of kinetic or dynamic friction, which comes into force when one body rolls over the other. For example,  when a circular disc, a wheel, a ring, or a sphere rolls over the surface, a force that opposes its motion is the rolling friction.

Other examples are:

• A basketball rolling on the court comes to rest after some time.

• A tire rolling on the road or skateboard wheels rolling on the road are all examples of rolling friction.

• The shape, size, and weight are the factors that affect the motion of vehicles; for example, a twenty-wheeler heavy loaded truck will have higher rolling friction than that of a TATA Nano Car.

• Painting the walls using paint rollers.

 

(d) Sliding Friction

Sliding friction is friction that acts on objects when they are slipping over a surface. Sliding friction is weaker than static friction. That’s why it’s easier to slide a piece of equipment over the floor after you start moving than it is to get it moving in the first place. Sliding friction can be valuable. For instance, you use sliding friction when you write with a pen. The pen “point” slides easily over the paper, but there’s just sufficient friction between the pen and paper to leave a mark.

 

Sliding Friction Examples

When an object slides over the other, an opposing force that comes into play is the sliding friction. Illustrative sliding friction examples are as follows:

• Pushing a lawn roller on the grass.

• Rubbing one’s hands together causes heat because of the sliding friction.

• Opening a sliding door/window.

• Dragging mouse on the surface.

• Sliding chalk to make art on the floor.

• Speed car racing.

We must know that static friction is the strongest frictional force, followed by sliding friction and rolling friction is the weakest.

Fluid Friction

Fluid friction takes place between fluid layers that are moving opposite to each other. This internal conflict to flow is named viscosity. In everyday terms, the viscosity of a fluid is branded as its “thickness”.

 

All actual fluids give some resistance to shearing and therefore are viscous. It is very helpful to use the concept of an ideal fluid that offers no resistance to shearing and so is not viscous.

 

Fluid Friction Examples

• While swimming, a swimmer experiences an opposing force; this force is the fluid friction force.

• While sucking a drink through a straw, you experience some friction; that friction is fluid friction.

• During skydiving, the parachute slows down because of the air resistance.

• Flying a paper made an aeroplane.

 

Examples of Fluid Friction

If there is a wet surface between two thin glass plates, you will see that plates get stuck and the bottom plate doesn’t fall when you hold only the top one.

 

How to calculate the Force of Friction

Calculate the force of friction using the formula:

 

F = μn

 

Where N is the normal force and μ is the friction coefficient for your tools and whether they are stationary or moving. The normal force is equivalent to the weight of the object, so this can also be written as:

 

F = μmg

 

Given m is the mass of the object and g is the acceleration due to gravity. The friction behaves to oppose the motion of the object.

 

Calculating the Force of Friction

Find the Normal Force.

 

The “normal” force defines the force that the surface an object is resting on exerts on the object. For a motionless object on a flat surface, the force must exactly resist the force due to gravity, otherwise, the object would move, according to Newton’s laws of motion. The “normal” force (N) is the term for the force that does this.

 

It constantly acts perpendicular to the surface. This means that on a sloppy surface, the normal force would still point straight away from the surface, while the force of gravity would point directly downwards. 

 

The normal force can be simply defined in most cases by:

 

N = mg

 

Here, m denotes the mass of the object, and g represents the acceleration due to gravity, which is 9.8 m/s². This just matches the “weight” of the object.

 

For sloppy surfaces, the strength of the normal force is minimizing the more the surface is inclined, so the formula becomes:

 

N = mg cos (θ)

 

With θ represent the angle the surface is inclined to.

 

For a simple case calculation, consider a flat surface with a 2-kg block of wood resting on it. The normal force would point directly upwards (to support the weight of the block), and you would measure:

 

N = 2 kg × 9.8 N/kg = 19.6 N

 

Find the Right Coefficient.

 

The coefficient relies on the object and the specific situation you’re working with. If the object is not already moving across the surface, you use the coefficient of static friction μstatic, but if it is moving you will use the coefficient of sliding friction μslide.

 

Usually, the coefficient of sliding friction is less than the coefficient of static friction. In other words, it is easier to slide something that is already sliding than to slide something unmoving.

 

The nature of the material also disturbs the coefficient. For instance, if the block of wood from earlier was on a brick surface, the coefficient would be 0.6, but for clean wood, it can be somewhere from 0.25 to 0.5. The static coefficient of friction for ice on ice is 0.1. Again, the sliding coefficient decreases even more, to 0.03 for ice on ice and 0.2 for wood on wood.

 

The formula for the force of friction states:

 

F = μN

 

For instance, consider a woodblock of 2-kg mass on a wooden table, being pushed from still. In this case, you can use the static coefficient, with μstatic = 0.25 to 0.5 for wood. Taking μstatic = 0.5 to take full advantage of the potential effect of friction, and remembering the N = 19.6 N from earlier, the force is:

 

F = 0.5 × 19.6 N = 9.8 N

 

Remember that friction only offers force to resist motion, so if you start pushing it slightly and get firmer, the force of friction will escalate to a maximum value, which is what you have just calculated. Physicists sometimes mark Fmax to make this point clear.

Once the block is in motion, you use μslide = 0.2, in this case:

 

F_slide = μ_slide N

 

= 0.2 × 19.6 N = 3.92 N

 

Problems on Frictional Force

Problem 1 – A 50 N of force is applied to the 6 kg of the box. If the coefficient of friction is 0, 3, calculate the acceleration of the box?

Solution 1– F_normal = 60 N – 40 N = 20 N

Friction force is –  F_friction = µ.F_normal = 0, 3.20N = 6N

Net force in –Y to Y = zero,

But, in –X + X direction net force is not zero

F_net = m.a

F_net = m.a

F_x – F_friction = m.a

F_x – F_friction = m.a

30 N – 6 N = 6 a

a = 4m/s²

 

Problem 2 – A block of mass M = 10 kg is placed on a surface that is inclined at angle θ = 45°. 

Mentioning that μs = 0.5 is the coefficient of static friction is between the block and the surface.

Solution 2 – The minimum force essential to stop slipping is the minimum force that will inhibit the block from sliding down the incline.

F_min = 10 g sin(45°) – 10 g cos(45°) x 0.5.

The maximum force that can be applied without causing the block to slip is the maximum force that can be applied without causing the block to slide up the incline.

F_max = 10 g sin(45°) + 10 g cos(45°) x 0.5.

F_min = 34.65 N, F_max = 103.94 N

These two devices work based on Faraday’s law of electromagnetic induction principle. The “Generators” generate current, and transformers convert between current and voltage. Here, we discuss in detail the important concepts of generators and transformers. 

Table of Content

1. AC generator- types, applications

2. DC generator- types, applications

What is a Generator?

A generator is defined as a machine that, with the help of magnetic induction, changes the mechanical energy into electrical energy.This is possible due to the revolution of coils in a magnetic field, i.e., a generator consisting of exterior fields also maybe because of the revolution of two electromagnets around a fixed coil, i.e. a generator consisting of internal fields.

 

An Electric Generator: Working Principle

The generator is made of a rectangle-shaped coil having several copper wires which wound over an iron core. This coil is called the armature. The function of this armature is used to increase the magnetic flux. A strong permanent magnet is being placed, and the armature rotates in between these magnets. Here the magnetic lines produced are perpendicular to the armature’s axis. There are two slip rings also connected to the armature’s arms. These rings are used for providing movable contact, and two metallic brushes are also connected to the slip rings, which help in passing current from the armature to the slip rings. Finally, the current is passed through a load resistance that is connected across the two slip-rings.

 

The position of the armature keeps changing at different time gaps. At the stage when the magnetic field lines are positioned perpendicular to the coil, the coil is then rotated in the magnetic field to increase the induced e.m.f produced. It occurs in this position as the number of intercepting-magnetic field lines are maximum here.

 

Types of Generators:

The generators are classified further into two types as AC generators and DC generators:

1. AC Generators:

AC-generators are also known as alternators. The principle of its working is based on electromagnetic induction.

AC generators are classified into two types:

1. Induction Generator: It does not require any DC excitation, frequency control or regular control. The induction concepts happen when inductor coils turn in the magnetic field, producing a current and a voltage.

2. Synchronous Generators: These are large size generators that are generally used in power plants. These are considered as rotating field or armature types. In the rotating armature type, the armature is positioned at the rotor and the field is at the stator end. The current in the rotor armature is taken through brushes and slip rings. These generators are used for low power requirement applications.

However, the Rotating field type of alternator is widely used due to its high power generation capability, and it does not require slip rings and brushes.

2. Two-phase or Three Phase-Generators:

The two-phase generator generates two different voltages, and each voltage is considered a single-phase voltage. However, both the generated voltages are not entirely dependent on each other.

The three-phase alternator has 3 single-phase windings present apart in such a way that 120º displaces the voltage generated in any of the phases from the other two.

These generators are used in applications like naval, oil and gas extraction, wind power plants and mining machinery etc.

 

Application Advantages of AC Generator:

1. As they do not require brushes, these Generators are generally maintenance-free.

2. These generators are small in size in comparison to DC  generators.

3. Losses are relatively less than DC machines.

4. AC Generator breakers are relatively small in size than DC breakers.

 

DC Generators:

The DC generator is used for converting mechanical energy into direct current electricity.

It is typically found in off-grid type applications. These generators give a continuous power supply directly into electric storage machines and DC power grids without the use of novel equipment. In the case of the DC generator also, the working principle is based on Faraday’s law of electromagnetic induction.

 

When the conductor is placed in the varying field, an electromagnetic force is induced in the conductor. The magnitude of this emf, i.e. induced, can be determined with the help of the emf – equation used for DC generators. Induced current circulation takes place within its closed path. According to Fleming’s right-hand rule, the direction of induced current can be determined.

 

Emf- equation of the DC generator is given as:

 

Eg = P Ф NZ / 60 A

 

Where

1. P is the number of field poles.

2. Φ is the flux produced / pole in Weber.

3. Z is the total no.’s of armature conductors.

4. A is the no.’s of parallel paths in the armature.

5. N is the rotational speed of armature in round per minute (rpm)

 

Types of DC Generators:

There are three main types of DC Generators:

1. Permanent Magnet DC Generator:

There is no need for external field excitation in Permanent magnet type DC generators as it has permanent magnets for producing the flux.

Application: These may be used for low power applications like dynamos etc.

2. Separately Excited DC Generator:

This separately-excited DC generator requires external field excitation for producing the magnetic flux. Here we can also vary the excitation for getting variable output-power.

Application: These are used in the electroplating process and electrorefining applications etc.

3. Self-Excited DC Generator:

Self-excited DC generators can produce their magnetic field when it is as they have residual magnetism in the poles of the stator. These are very simple in design, and there is no requirement of the external circuit to vary the field e
xcitation.

These self-excited DC generators are further classified into three, i.e. shunt, series, and compound-generators.

Application: These generators are used in applications like charging of batteries, welding, ordinary lightening-applications etc.

 

Advantages of DC Generators:

Following are the main advantages of the DC generator:

1. In this case, the cost of cables comes to be less as there is no shielding from radiation required.

2. Here, the fluctuations in the generator can be reduced by a constant arrangement of the coils.

3. In the case of the DC generator, the operating features depend on the field winding etc.

 

Transformer: An Introduction

The device converts the voltage as the higher or lower voltages. There are different voltage levels, used when electrical power is generated, during the transfer.

 

A transformer is usually made of two coils, i.e. primary/field and secondary/inductance, between which are kept apart so that there is no electrical contact in between. When we allow passing a current through the primary coil, there is a generation of the magnetic field which changes. However, it maintains the same frequency. It results in generating an alternating voltage in the secondary coil at the same time. An alternating current passes through a secondary coil during the closed electrical circuit.

 

The greater the difference in between the number of windings in the primary and secondary coils, the greater will be the difference in between their voltages also, so they are directly proportional.

 

Working Principle of Transformer:

The transformer’s working principle is based on mutual inductance between the two circuits, which are linked by a common magnetic flux.

 

Types of Transformers:

Two types of transformers are there, as given below:

1. Step-Up Transformer:

These transformers convert a low voltage into a high voltage. In this case, the number of turns in the primary coil is less than in the secondary coil, i.e. Np

2. Step-Down Transformer:

These transformers convert a high voltage when the current decreases into a low-voltage when the current increases, the no. of turns in the primary coil is greater than the number to the secondary coil, i.e. Np  ˃ Ns.

As per Faraday’s law of electromagnetic induction, the induced e.m.f is given by:

e  = – d Ф / dt

ep  = – d Фp / dt

es  = – d Фs / dt

By using the above equations, we get,

es = Ns x Np x ep

The ratio Ns / Np = K

Apart from this, there may be different types of transformers based on various parameters as follows :

 

Based on Design

1. Core-type transformer

2. Shell-type transformer

Based on the Cooling Method

1. Oil filled self-cooled type.

2. Oil-filled water-cooled type.

3. Airblast type etc.

 

Applications of Transformer:

Following are three basic applications of Transformer:

1. To step up the current and voltage.

2. To step down the current and voltage.

3. Prevention of DC to the next circuit in the DC transformers etc.