Physics MCQ's ( Basic + Advance - Must have) Pocket book pdf

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Why does ice float on water ?

"Ice is less dense than water because of its intermolecular forces."

Water contains hydrogen bonds (a type of intermolecular force of attraction) between the H (hydrogen) of one atom and the O (oxygen) of another atom.  As the water gets colder and the kinetic energy of the molecules decreases, the hydrogen bonds keep the water molecules apart, forming hexagonal structures with water molecules at each vertex.  In between the water molecules is nothing.  In liquid water, the molecules of water can be much closer together; the hydrogen bonds are more flexible.  Therefore, the solid ice, with its molecules kept at a fairly fixed distance and the crystals holding lots of "nothing" among the water molecules, is less dense than the liquid water.

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This picture will make you crazy..!!

Some Best Optical Illusions

Want To know about what causes Optical Illusions ?? 

Here is the answer

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Can anything travel faster than light ??

What Travels Faster Than the Speed of Light ??

 

Many people wants to know that ,can anything travel faster than speed of light..?? or simply What Travels Faster Than the Speed of Light ??If you have the same question than here is the answer :

Most textbooks say that nothing can go faster than light, but that statement actually should be qualified: The answer is yes, you can break the light barrier, but not in the way we see in the movies. There are, in fact, several ways to travel faster than light:

1. The Big Bang itself expanded much faster than the speed of light. But this only means that "nothing can go faster than light." Since nothing is just empty space or vacuum, it can expand faster than light speed since no material object is breaking the light barrier. Therefore, empty space can certainly expand faster than light.

2. If you wave a flashlight across the night sky, then, in principle, its image can travel faster than light speed (since the beam of light is going from one part of the Universe to another part on the opposite side, which is, in principle, many light years away). The problem here is that no material object is actually moving faster than light. (Imagine that you are surrounded by a giant sphere one light year across. The image from the light beam will eventually hit the sphere one year later. This image that hits the sphere then races across the entire sphere within a matter of seconds, although the sphere is one light year across.) Just the image of the beam as it races across the night sky is moving faster than light, but there is no message, no net information, no material object  that actually moves along this image.

3. Quantum entanglement moves faster than light. If I have two electrons close together, they can vibrate in unison, according to the quantum theory. If I then separate them, an invisible umbilical cord emerges which connects the two electrons, even though they may be separated by many light years. If I jiggle one electron, the other electron "senses" this vibration instantly, faster than the speed of light. Einstein thought that this therefore disproved the quantum theory, since nothing can go faster than light.

But actually this experiment (the EPR experiment) has been done many times, and each time Einstein was wrong. Information does go faster than light, but Einstein has the last laugh. This is because the information that breaks the light barrier is random, and hence useless. (For example, let's say a friend always wears one red sock and one green sock. You don't know which leg wears which sock. If you suddenly see that one foot has a red sock, then you know instantly, faster than the speed of light, that the other sock is green. But this information is useless. You cannot send Morse code or usable information via red and green socks.)

4. The most credible way of sending signals faster than light is via negative matter. You can do this either by:

a) compressing the space in front of your and expanding the space behind you, so that you surf on a tidal wave of warped space. You can calculate that this tidal wave travels faster than light if driven by negative matter (an exotic form of matter which has never been seen.) b) using a wormhole, which is a portal or shortcut through space-time, like the Looking Glass of Alice.

In summary, the only viable way of breaking the light barrier may be through General Relativity and the warping of space time. However, it is not known if negative matter exists, and whether the wormhole will be stable. To solve the question of stability, you need a fully quantum theory of gravity, and the only such theory which can unite gravity with the quantum theory is string theory (which is what I do for a living). Sadly, the theory is so complex that no has been able to fully solve it and give a definitive anwer to all these questions. Maybe someone reading this blog will be inspired to sovle string theory and answer the question whether we can truly break the light barrier.

 

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What is the speed of electricity?

What is the speed of electricity ?

The speed of electricity really depends on what you mean by the word "electricity". This word is very general and basically means, "all things relating to electric charge". I will assume we are referring to a current of electrical charge traveling through a metal wire, such as through the power cord of a lamp. In the case of electrical currents traveling through metal wires, there are three different velocities present, all of them physically meaningful:

1. The individual electron velocity
2. The electron drift velocity
3. The signal velocity

In order to understand each of these speeds and why they are all different and yet physically meaningful, we need to understand the basics of electric currents. Electric currents in metal wires are formed by free electrons that are moving. In the context of typical electric currents in metal wires, free electrons can be thought of as little balls bouncing around in the grid of fixed, heavy atoms that make up the metal wire. Electrons are really quantum entities, but the more accurate quantum picture is not necessary in this explanation. (When you add in quantum effects, the individual electron velocity becomes the "Fermi velocity".) The non-free electrons, or valence electrons, are bound too tightly to atoms to contribute to the electric current and so can be ignored in this picture. Each free electron in the metal wire is constantly flying in a straight line under its own momentum, colliding with an atom, changing direction because of the collision, and continuing on in a straight line again until the next collision. If a metal wire is left to itself, the free electrons inside constantly fly about and collide into atoms in a random fashion. Macroscopically, we call the random motion of small particles "heat". The actual speed of an individual electron is the amount of nanometers per second that an electron travels while going in a straight line between collisions. A wire left to itself carries no electric signal, so the individual electron velocity of the randomly moving electrons is just a description of the heat in the wire and not the electric current.

Now, if you connect the wire to a battery, you have applied an external electric field to the wire. The electric field points in one direction down the length of the wire. The free electrons in the wire feel a force from this electric field and speed up in the direction of the field (in the opposite direction, actually, because electrons are negatively charged). The electrons continue to collide with atoms, which still causes them to bounce all around in different directions. But on top of this random thermal motion, they now have a net ordered movement in the direction opposite of the electric field. The electric current in the wire consists of the ordered portion of the electrons' motion, whereas the random portion of the motion still just constitutes the heat in the wire. An applied electric field (such as from connecting a battery) therefore causes an electric current to flow down the wire. The average speed at which the electrons move down a wire is what we call the "drift velocity".

Even though the electrons are, on average, drifting down the wire at the drift velocity, this does not mean that the effects of the electrons' motion travels at this velocity. Electrons are not really solid balls. They do not interact with each other by literally knocking into each other's surfaces. Rather, electrons interact through the electromagnetic field. The closer two electrons get to each other, the stronger they repel each other through their electromagnetic fields. The interesting thing is that when an electron moves, its field moves with it, so that the electron can push another electron farther down the wire through its field long before physically reaching the same location in space as this electron. As a result, the electromagnetic effects can travel down a metal wire much faster than any individual electron can. These "effects" are fluctuations in the electromagnetic field as it couples to the electrons and propagates down the wire. Since energy and information are carried by fluctuations in the electromagnetic field, energy and information also travel much faster down an electrical wire than any individual electron.

The speed at which electromagnetic effects travel down a wire is called the "signal velocity", "the wave velocity", or "the group velocity". Note that some books insinuate that the signal velocity describes a purely electromagnetic wave effect. This insinuation can be misleading. If the signal traveling down an electric cable was an isolated electromagnetic wave, then the signal would travel at the speed of light in vacuum c. But it does not. Rather, the signal traveling down an electric cable involves an interaction of both the electromagnetic field fluctuations (the wave) and the electrons. For this reason, the signal velocity is much faster than the electron drift velocity but is slower than the speed of light in vacuum. Generally, the signal velocity is somewhat close to the speed of light in vacuum. Note that the "signal velocity" discussed here describes the physical speed of electromagnetic effects traveling down a wire. In contrast, engineers often use the phrase "signal speed" in a non-scientific way when they really mean "bit rate". While the bit rate of a digital signal traveling through a network does depend on the physical signal velocity in the wires, it also depends on how well the computers in the network can route the signals through the network.

Consider this analogy. A long line of people is waiting to enter a restaurant. Each person fidgets nervously about in their spot in line. The person at the end of the line grows impatient and shoves the person in front of him. In turn, when each person in the line receives a shove from the person behind him, he shoves the person in front of him. The shove will therefore be passed along from person to person, forwards through the line. The shove will reach the restaurant doors long before the last person in line personally makes it to the doors. In this analogy, the people represent the electrons, their arms represent the electromagnetic field, and the shove represents a fluctuation or wave in the electromagnetic field. The speed at which each person fidgets represents the individual electron velocity, the speed at which each person individually progresses through the line represents the electron drift velocity, and the speed at which the shove travels through the line represents the signal velocity. Based on this simple analogy, we would expect the signal velocity to be very fast, the individual velocity to be somewhat fast, and the drift velocity to be slow. (Note that in physics there is also another relevant speed in this context called the "phase velocity". The phase velocity is more of a mathematical tool than a physical reality, so I do not think it is worth discussing here).

The individual electron velocity in a metal wire is typically millions of kilometers per hour. In contrast, the drift velocity is typically only a few meters per hour while the signal velocity is a hundred million to a trillion kilometers per hour. In general, the signal velocity is somewhat close to the speed of light in vacuum, the individual electron speed is about 100 times slower than the signal velocity, and the electron drift speed is as slow as a snail.

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What is electricity and How Electricity is Produced ?

What is electricity ?

Electricity is a form of energy that starts with atoms. You can't see atoms because they're too small, but they make up everything around us. There are three parts to an atom: protons, neutrons and electrons. Electricity is created when electrons move from atom to atom. There are a number of ways to make electrons move, but most electricity is produced at power plants.

How do power plants work ?

Power plants that use water to make electricity are built near rivers.Dams are built across rivers to hold back the water. The water is then directed through big pipes and it falls against the blades of giant turbines. The turbines have blades on them that turn when the water hits them, just like the blades of a pinwheel turn when you blow on them. Once the water hits the blades, it returns to the river.

The turbine blades are attached to a big metal rod, and at the end of that rod are large magnets. When the blades turn, they make the rod and the magnets spin very fast. The magnet end is surrounded by heavy coils of copper wire, and the spinning magnets cause electrons in the wire to begin to move, creating electricity.

What happens to the electricity after that ?

It moves through wires into what's called a power transformer. The electrical voltage (the strength at which electricity flows) is fairly high and the transformer makes it even higher to help it flow through wires called transmission lines. Those wires are attached to wooden or metal poles that you see along roads and throughout communities.

All the wires are made of metal – usually aluminum or copper. That's because metal is a good conductor – electricity travels through it easily. By the way, water is also a good conductor, and because our bodies are mostly made of water, electricity can travel through us easily. That's not something we want to happen though, because if we have electricity going through us we'll likely be seriously hurt or even killed. That's why grown-ups warn you to stay away from high voltage sites and not to stick your fingers in a wall plug.

Electricity travels fast – about 310,000 kilometers per second! If you moved that fast, you could probably make several trips around the world in the time it takes to turn on a light!

Sometimes, when electricity has to travel a long way it gets a little weaker as it moves along the lines. It needs a boost (like you need food to replace the energy you've burned after playing outside all day). That's where substations help. Substations are large box-like power transformers that sit in fenced-in areas. You'll see signs on the fences that say high voltage – stay away and it's really important that you obey those signs (remember what you read about electricity being able to travel easily through your body).

How does electricity get into my house ?

When wires reach your house, another transformer on the power pole makes the electricity just the right voltage so you can use it safely.

The wire is connected to a meter box that keeps track of how much electricity is being used. There are wires in your house connected to plugs, also called outlets. These outlets let you plug in your boom box, television set, or any thing else electrical. What an amazing journey electricity takes to get to your home !!

 

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Nasa has announced that it has found evidence of flowing water on Mars

Nasa Mars water announcement: agency announces it has found proof of flowing water, improving chances of supporting alien life

 

Nasa has announced that it has found evidence of flowing water on Mars — a discovery with potentially huge implications for the possibility of life on the planet.

Scientists have long suspected that the planet might have running water. But the new findings confirm that it is on the planet, combined with “hydrated salts” in a brine.

Normally, water on Mars freezes or evaporates, because of the intense environment on the planet. But the addition of salts means that it is much more stable, allowing it to survive on the Red Planet.

Scientists have long speculated that the Recurring Slope Lineae — or dark patches — on Mars were made up of briny water. But the new findings prove that those patches are caused by liquid water, which it has established by finding the hydrated salts.

The new research is based on an analysis of spectral data from the American space agency Nasa's Mars Reconnaissance Orbiter spacecraft. 

Breaking down reflected light into its different wavelengths provides a chemical "fingerprint" of what a substance is made of. The Mars scientists devised a new method that allowed chemical signatures to be extracted from individual image pixels, providing a much higher level of resolution than had been achieved before.

“Recurring Slope Lineae (RSL) are seasonal flows on warm Martian slopes initially proposed, but not confirmed, to be caused by briny water seeps,” the team behind  the discovery wrote in another paper, due to be delivered this week. “Here we report spectral evidence for hydrated salts on RSL slopes from four different RSL locations from the Compact Reconnaissance Imaging Spectrometer for Mars on board Mars Reconnaissance Orbiter.

“These results confirm the hypothesis that RSL are due to present-day activity of briny water. “

Source:http://www.independent.co.uk/news/science/nasa-mars-water-announcement-agency-announces-that-it-has-found-proof-of-flowing-water-on-mars-a6670446.html

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We know the Earth is rotating, but why?

Why is everything in the Solar System spinning? And why is it mostly all spinning in the same direction?

 

Why is everything in the Solar System spinning? And why is it mostly all spinning in the same direction?

It can’t be a coincidence. Look down on the Earth from above, and you’d see that it’s turning in a counter-clockwise direction. Same with the Sun, Mars and most of the planets.  

4.54 billion years ago, our Solar System formed within a cloud of hydrogen not unlike the Orion Nebula, or the Eagle Nebula, with its awesome pillars of creation. Then, it took some kick, like from the shock wave from a nearby supernova, and this set a region of the cold gas falling inward through its mutual gravity. As it collapsed, the cloud began to spin.

But why?

It’s the conservation of angular momentum.

Think about the individual atoms in the cloud of hydrogen. Each particle has its own momentum as it drifts through the void. As these atoms glom onto one another with gravity, they need to average out their momentum. It might be possible to average out perfectly to zero, but it’s really really unlikely.

Which means, there will be some left over. Like a figure skater pulling in her arms to spin more rapidly, the collapsing proto-Solar System with its averaged out particle momentum began to spin faster and faster.

This is the conservation of angular momentum at work.

As the Solar System spun more rapidly, it flattened out into a disk with a bulge in the middle. We see this same structure throughout the Universe: the shape of galaxies, around rapidly spinning black holes, and we even see it in pizza restaurants.

The Sun formed from the bulge at the center of this disk, and the planets formed further out. They inherited their rotation from the overall movement of the Solar System itself. Over the course of a few hundred million years, all of the material in the Solar System gathered together into planets, asteroids, moons and comets. Then the powerful radiation and solar winds from the young Sun cleared out everything that was left over.

Without any unbalanced forces acting on them, the inertia of the Sun and the planets have kept them spinning for billions of years.

And they’ll continue to do so until they collide with some object, billions or even trillions of years in the future.

So are you still wondering, why does the Earth spin?

The Earth spins because it formed in the accretion disk of a cloud of hydrogen that collapsed down from mutual gravity and needed to conserve its angular momentum. It continues to spin because of inertia.

The reason it’s all the same direction is because they all formed together in the same Solar Nebula, billions of years ago.

 

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What's the difference between fission & fusion ?

What's the difference between fission & fusion ?

 

A nuclear reaction is a process in which atoms collide with other atoms and lose some of their original mass. Because of the principle of energy conservation the lost mass must reappear as generated energy, according to Einstein's equation E = mc². The two types of nuclear reactions used to produce energy are fission and fusion.

In a fission reaction, a heavy atomic nucleus is split into smaller nuclei, other particles and radiation. In a typical reaction, an atom of uranium 235 absorbs a neutron and splits into two lighter atoms, barium and krypton, emitting radiation and neutrons. Under special circumstances (the attainment of a "critical mass") the emitted neutrons can split further atoms, which in turn bring about more splitting, producing a very fast chain reaction. Nuclear power plants exploit the process of fission to create energy.  

In a fusion reaction, two or more light atomic nuclei fuse to form a single heavier nucleus. The mass change in the process is the source of nuclear energy. Fusion within the cores of the sun and other stars generates their radiating energy by fusing two hydrogen atoms to produce a helium atom. Current researchers are using magnetic vacuum chambers and laser beams in an attempt to generate the extreme high-temperatures necessary for the fusion process. If successful, the net energy gain would create a viable alternative energy option.

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How Can We See Black Holes ?

How Can We See Black Holes?  

Can we see a Black holes..? if you have this question in your mind than here is answer for you.

 

Though we cannot "see" a black hole itself (since not even light can escape the hole's gravitational field), we may see the hole's effects on nearby matter. For example, if gas from a nearby star were sucked towards the black hole, the intense gravitation al energy would heat the gas to millions of degrees. The resulting X-ray emissions could point to the presence of the black hole.

Or, if a massive black hole were surrounded by large amounts of orbiting material -- gas, dust, even stars -- their rapid motion close to the hole could be observable via shifts in the energy of the radiation they emit. Evidence along these lines is mount ing, suggesting that black holes may not be that rare in the universe.

However, such evidence remains indirect and therefore inconclusive. To confirm that black holes actually exist, we'll need to be able to observe the gravitational waves they produce as they form or interact.

If scientists could build gravitational wave detectors of sufficient sensitivity, they should be able to measure the vibrations in spacetime generated by black holes as they form from a collapsing star, when they ingest large amoun ts of matter, or if they interact, even collide with a second black hole or another massive object, such as a neutron star. Certain patterns of gravitational waves emitted would reveal the "smoking gun."

So far, the wavelike disturbances in spacetime have eluded detection. In a relativistic universe, there should be no shortage of places in which to hunt for black holes. Much larger and more sensitive detectors are now under construction. With luck, soon gravitation scientists may be shouting "Eureka!"

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10 Mind-Blowing Facts About Black Holes

10 Mind-Blowing Facts About Black Holes

 

Here are 10 interesting facts about black holes

1. It was John Mitchell who actually first proposed the idea of ‘dark stars’ or object. Later, in 20th century the term ‘black hole’ was coined.

2. Black holes are actually leftovers of former stars and are so dense that nothing can flee from their dominant gravitational energy. 

3. If the formation of a black hole has taken place, it can continue to grow by absorbing additional matter.

4. Generally the life cycle of most of the stars end up being a white dwarf or a neutron star, but, black holes are considered to be the last evolutionary stage in the lifetime of a star.

5. There are mainly three types of black holes namely stellar, super massive and miniature black holes, depending on their mass.

6. Since black holes possess strong gravitational force which pulls all of the light into its center, they cannot be seen.

7. Black holes follow the laws of gravity and hence in order to affect the earth, the orbit of a black hole would have to be very close to the solar system, which is not likely.

8. Astronomers are confident that our own Milky Way galaxy has a super massive black hole at its center.

9.Black holes have to hold a massive amount of mass in an incredibly small space to have the required gravity to pull light in. For example, to make a black hole with the mass of Earth, the entire planet would need to be squeezed down to a space 9 millimeters across.

10.When anything (be it planets, suns, galaxies or particles of light) passes close to a black hole, they will be pulled in by its gravity. If something else acting on the object, like say a rocket, is stronger than the black hole's gravity, it can escape the pull.

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Why does water freeze from the top to the bottom..?

Why does water freeze from the top to the bottom..??

The answer lies in a very peculiar property of water.  As you guessed, the density of water of water increases as the temperature is lowered, but below 4^oC this trend is reversed.   Here is a graph from The Physics Factbook. It shows the density of liquid water vs. temperature. The reason it can show liquid water below 0°C is that water can be supercooled, not actually turning to ice for a long time.




So what happens is the water that is just slightly above the freezing point rises to the top so the freezing process starts there.   This is the same reason why ice is at the top of a lake in the winter and not at the bottom.  This is beneficial to ice skaters as well as fish and other aquatic fauna.

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What Causes Optical Illusions?

What Causes Optical Illusions?

These yellow and blue blocks appear to move one after the other but actually not..!!

In simple terms, an optical illusion is caused by the structure of both the eye and brain and how they work together. Because of the anatomical make up of the eye and the complexity of the way images of transmitted from the eye to the brain, optical illusions are not as rare as one might consider.

The Anatomy of the Eye

The eye has two types of receptors on it (cones and rods) that pick up different bits of information on image. Around the retina, these cones and rods rest, waiting to pick up a stimuli and transfer it to the optic nerve. The optic nerve, in turn, transmits the information to the brain for processing.

Cone cells detect color and rod cells detect low-light contrasts. They work together to provide the necessary information to form an imag. However, at the edges of the retina, there are more rods and at the center of the retina, there are more cones. Because of this, based on how someone is looking at an image, they might see things differently. This is an optical illusion. For a better image, simply turning one's head and looking straight at something will provide the cones access and give a more detailed image.

Medical Syndromes Causing Optical Illusions

There is research to suggest that some optical illusions are caused by medical syndromes such as schizophrenia. Because of the nature of the disease, an individual sees something different than what is actually being seen. This is caused by the brain interpreting the information differently than it normally would be.

Effort on the Eye

Because it is more difficult to raise the eye than move the eye horizontally, the eye perceives that vertical distances are greater than horizontal distances. This creates depth in flat surfaces which can – especially when the eyes are fatigued – lead them to perceive something incorrectly and then pass that information onto the brain. The brain takes what information it has and processes it whether it is correct or not. Fatigue in the eyes is an exceptional cause of optical illusions because it takes a little longer for the eyes to focus more effectively.

 

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Voltage Transformer Basics

One of the main reasons that we use alternating AC voltages and currents in our homes and workplace’s is that AC supplies can be easily generated at a convenient voltage, transformed (hence the name transformer) into much higher voltages and then distributed around the country using a national grid of pylons and cables over very long distances.

The reason for transforming the voltage to a much higher level is that higher distribution voltages implies lower currents for the same power and therefore lower I²R losses along the networked grid of cables. These higher AC transmission voltages and currents can then be reduced to a much lower, safer and usable voltage level where it can be used to supply electrical equipment in our homes and workplaces, and all this is possible thanks to the basic Voltage Transformer.

A Typical Voltage Transformer

The Voltage Transformer can be thought of as an electrical component rather than an electronic component. A transformer basically is very simple static (or stationary) electro-magnetic passive electrical device that works on the principle of Faraday’s law of induction by converting electrical energy from one value to another.

The transformer does this by linking together two or more electrical circuits using a common oscillating magnetic circuit which is produced by the transformer itself. A transformer operates on the principals of “electromagnetic induction”, in the form of  Mutual Induction.

Mutual induction is the process by which a coil of wire magnetically induces a voltage into another coil located in close proximity to it. Then we can say that transformers work in the “magnetic domain”, and transformers get their name from the fact that they “transform” one voltage or current level into another.

Transformers are capable of either increasing or decreasing the voltage and current levels of their supply, without modifying its frequency, or the amount of Electrical Power being transferred from one winding to another via the magnetic circuit.

A single phase voltage transformer basically consists of two electrical coils of wire, one called the “Primary Winding” and another called the “Secondary Winding”. For this tutorial we will define the “primary” side of the transformer as the side that usually takes power, and the “secondary” as the side that usually delivers power. In a single-phase voltage transformer the primary is usually the side with the higher voltage.

These two coils are not in electrical contact with each other but are instead wrapped together around a common closed magnetic iron circuit called the “core”. This soft iron core is not solid but made up of individual laminations connected together to help reduce the core’s losses.

The two coil windings are electrically isolated from each other but are magnetically linked through the common core allowing electrical power to be transferred from one coil to the other. When an electric current passed through the primary winding, a magnetic field is developed which induces a voltage into the secondary winding as shown.

Single Phase Voltage Transformer

In other words, for a transformer there is no direct electrical connection between the two coil windings, thereby giving it the name also of an Isolation Transformer. Generally, the primary winding of a transformer is connected to the input voltage supply and converts or transforms the electrical power into a magnetic field. While the job of the secondary winding is to convert this alternating magnetic field into electrical power producing the required output voltage as shown.

Transformer Construction (single-phase)

• Where:
•   V_P  -  is the Primary Voltage
•   V_S  -  is the Secondary Voltage
•   N_P  -  is the Number of Primary Windings
•   N_S  -  is the Number of Secondary Windings
•   Φ (phi)  -  is the Flux Linkage

Notice that the two coil windings are not electrically connected but are only linked magnetically. A single-phase transformer can operate to either increase or decrease the voltage applied to the primary winding. When a transformer is used to “increase” the voltage on its secondary winding with respect to the primary, it is called a Step-up transformer. When it is used to “decrease” the voltage on the secondary winding with respect to the primary it is called a Step-down transformer.

However, a third condition exists in which a transformer produces the same voltage on its secondary as is applied to its primary winding. In other words, its output is identical with respect to voltage, current and power transferred. This type of transformer is called an “Impedance Transformer” and is mainly used for impedance matching or the isolation of adjoining electrical circuits.

The difference in voltage between the primary and the secondary windings is achieved by changing the number of coil turns in the primary winding ( N_P ) compared to the number of coil turns on the secondary winding ( N_S ).

As the transformer is basically a linear device, a ratio now exists between the number of turns of the primary coil divided by the number of turns of the secondary coil. This ratio, called the ratio of transformation, more commonly known as a transformers “turns ratio”, ( TR ). This turns ratio value dictates the operation of the transformer and the corresponding voltage available on the secondary winding.

It is necessary to know the ratio of the number of turns of wire on the primary winding compared to the secondary winding. The turns ratio, which has no units, compares the two windings in order and is written with a colon, such as 3:1 (3-to-1). This means in this example, that if there are 3 volts on the primary winding there will be 1 volt on the secondary winding, 3 volts-to-1 volt. Then we can see that if the ratio between the number of turns changes the resulting voltages must also change by the same ratio, and this is true.

Transformers are all about “ratios”. The ratio of the primary to the secondary, the ratio of the input to the output, and the turns ratio of any given transformer will be the same as its voltage ratio. In other words for a transformer: “turns ratio = voltage ratio”. The actual number of turns of wire on any winding is generally not important, just the turns ratio and this relationship is given as:

A Transformers Turns Ratio

Assuming an ideal transformer and the phase angles:  Φ_P ≡ Φ_S

Note that the order of the numbers when expressing a transformers turns ratio value is very important as the turns ratio 3:1 expresses a very different transformer relationship and output voltage than one in which the turns ratio is given as: 1:3.

Transformer Basics Example No1

A voltage transformer has 1500 turns of wire on its primary coil and 500 turns of wire for its secondary coil. What will be the turns ratio (TR) of the transformer.

 

This ratio of 3:1 (3-to-1) simply means that there are three primary windings for every one secondary winding. As the ratio moves from a larger number on the left to a smaller number on the right, the primary voltage is therefore stepped down in value as shown.

Transformer Basics Example No2

If 240 volts rms is applied to the primary winding of the same transformer above, what will be the resulting secondary no load voltage.

 

Again confirming that the transformer is a “step-down transformer as the primary voltage is 240 volts and the corresponding secondary voltage is lower at 80 volts.

Then the main purpose of a transformer is to transform voltages at preset ratios and we can see that the primary winding has a set amount or number of windings (coils of wire) on it to suit the input voltage. If the secondary output voltage is to be the same value as the input voltage on the primary winding, then the same number of coil turns must be wound onto the secondary core as there are on the primary core giving an even turns ratio of 1:1 (1-to-1). In other words, one coil turn on the secondary to one coil turn on the primary.

If the output secondary voltage is to be greater or higher than the input voltage, (step-up transformer) then there must be more turns on the secondary giving a turns ratio of 1:N (1-to-N), where N represents the turns ratio number. Likewise, if it is required that the secondary voltage is to be lower or less than the primary, (step-down transformer) then the number of secondary windings must be less giving a turns ratio of N:1 (N-to-1).

Transformer Action

We have seen that the number of coil turns on the secondary winding compared to the primary winding, the turns ratio, affects the amount of voltage available from the secondary coil. But if the two windings are electrically isolated from each other, how is this secondary voltage produced?

We have said previously that a transformer basically consists of two coils wound around a common soft iron core. When an alternating voltage ( V_P ) is applied to the primary coil, current flows through the coil which in turn sets up a magnetic field around itself, called mutual inductance, by this current flow according to Faraday’s Law of electromagnetic induction. The strength of the magnetic field builds up as the current flow rises from zero to its maximum value which is given as dΦ/dt.

As the magnetic lines of force setup by this electromagnet expand outward from the coil the soft iron core forms a path for and concentrates the magnetic flux. This magnetic flux links the turns of both windings as it increases and decreases in opposite directions under the influence of the AC supply.

However, the strength of the magnetic field induced into the soft iron core depends upon the amount of current and the number of turns in the winding. When current is reduced, the magnetic field strength reduces.

When the magnetic lines of flux flow around the core, they pass through the turns of the secondary winding, causing a voltage to be induced into the secondary coil. The amount of voltage induced will be determined by: N.dΦ/dt (Faraday’s Law), where N is the number of coil turns. Also this induced voltage has the same frequency as the primary winding voltage.

Then we can see that the same voltage is induced in each coil turn of both windings because the same magnetic flux links the turns of both the windings together. As a result, the total induced voltage in each winding is directly proportional to the number of turns in that winding. However, the peak amplitude of the output voltage available on the secondary winding will be reduced if the magnetic losses of the core are high.

If we want the primary coil to produce a stronger magnetic field to overcome the cores magnetic losses, we can either send a larger current through the coil, or keep the same current flowing, and instead increase the number of coil turns ( N_P ) of the winding. The product of amperes times turns is called the “ampere-turns”, which determines the magnetising force of the coil.

So assuming we have a transformer with a single turn in the primary, and only one turn in the secondary. If one volt is applied to the one turn of the primary coil, assuming no losses, enough current must flow and enough magnetic flux generated to induce one volt in the single turn of the secondary. That is, each winding supports the same number of volts per turn.

As the magnetic flux varies sinusoidally, Φ = Φ_max sinωt, then the basic relationship between induced emf, ( E ) in a coil winding of N turns is given by:

emf = turns x rate of change

• Where:
•   ƒ  -  is the flux frequency in Hertz,  = ω/2π
•   Ν  -  is the number of coil windings.
•   Φ  -  is the flux density in webers

This is known as the Transformer EMF Equation. For the primary winding emf, N will be the number of primary turns, ( N_P ) and for the secondary winding emf, N will be the number of secondary turns, ( N_S ).

Also please note that as transformers require an alternating magnetic flux to operate correctly, transformers cannot therefore be used to transform or supply DC voltages or currents, since the magnetic field must be changing to induce a voltage in the secondary winding. In other words, Transformers DO NOT Operate on DC Voltages, ONLY AC.

If a transformers primary winding was connected to a DC supply, the inductive reactance of the winding would be zero as DC has no frequency, so the effective impedance of the winding will therefore be very low and equal only to the resistance of the copper used. Thus the winding will draw a very high current from the DC supply causing it to overheat and eventually burn out, because as we know I = V/R.

Transformer Basics Example No3

A single phase transformer has 480 turns on the primary winding and 90 turns on the secondary winding. The maximum value of the magnetic flux density is 1.1T when 2200 volts, 50Hz is applied to the transformer primary winding. Calculate:

a). The maximum flux in the core.

 

b). The cross-sectional area of the core.

 

c). The secondary induced emf.

Electrical Power in a Transformer

Another one of the transformer basics parameters is its power rating. Transformers are rated in Volt-amperes, ( VA ), or in larger units of Kilo Volt-amperes, ( kVA ). In an ideal transformer (ignoring any losses), the power available in the secondary winding will be the same as the power in the primary winding, they are constant wattage devices and do not change the power only the voltage to current ratio. Thus, in an ideal transformer the Power Ratio is equal to one (unity) as the voltage, V multiplied by the current, I will remain constant.

That is the electric power at one voltage/current level on the primary is “transformed” into electric power, at the same frequency, to the same voltage/current level on the secondary side. Although the transformer can step-up (or step-down) voltage, it cannot step-up power. Thus, when a transformer steps-up a voltage, it steps-down the current and vice-versa, so that the output power is always at the same value as the input power. Then we can say that primary power equals secondary power, ( P_P = P_S ).

Power in a Transformer

 

Where: Φ_P is the primary phase angle and Φ_S is the secondary phase angle.

Note that since power loss is proportional to the square of the current being transmitted, that is: I²R, increasing the voltage, let’s say doubling ( ×2 ) the voltage would decrease the current by the same amount, ( ÷2 ) while delivering the same amount of power to the load and therefore reducing losses by factor of 4. If the voltage was increased by a factor of 10, the current would decrease by the same factor reducing overall losses by factor of 100.

Transformer Basics – Efficiency

A transformer does not require any moving parts to transfer energy. This means that there are no friction or windage losses associated with other electrical machines. However, transformers do suffer from other types of losses called “copper losses” and “iron losses” but generally these are quite small.

Copper losses, also known as I²R loss is the electrical power which is lost in heat as a result of circulating the currents around the transformers copper windings, hence the name. Copper losses represents the greatest loss in the operation of a transformer. The actual watts of power lost can be determined (in each winding) by squaring the amperes and multiplying by the resistance in ohms of the winding (I²R).

Iron losses, also known as hysteresis is the lagging of the magnetic molecules within the core, in response to the alternating magnetic flux. This lagging (or out-of-phase) condition is due to the fact that it requires power to reverse magnetic molecules; they do not reverse until the flux has attained sufficient force to reverse them.

Their reversal results in friction, and friction produces heat in the core which is a form of power loss. Hysteresis within the transformer can be reduced by making the core from special steel alloys.

The intensity of power loss in a transformer determines its efficiency. The efficiency of a transformer is reflected in power (wattage) loss between the primary (input) and secondary (output) windings. Then the resulting efficiency of a transformer is equal to the ratio of the power output of the secondary winding, P_S to the power input of the primary winding, P_P and is therefore high.

An ideal transformer is 100% efficient because it delivers all the energy it receives. Real transformers on the other hand are not 100% efficient and at full load, the efficiency of a transformer is between 94% to 96% which is quiet good. For a transformer operating with a constant voltage and frequency with a very high capacity, the efficiency may be as high as 98%. The efficiency, η of a transformer is given as:

Transformer Efficiency

where: Input, Output and Losses are all expressed in units of power.

Generally when dealing with transformers, the primary watts are called “volt-amps”, VA to differentiate them from the secondary watts. Then the efficiency equation above can be modified to:

 

It is sometimes easier to remember the relationship between the transformers input, output and efficiency by using pictures. Here the three quantities of VA, W and η have been superimposed into a triangle giving power in watts at the top with volt-amps and efficiency at the bottom. This arrangement represents the actual position of each quantity in the efficiency formulas.

Transformer Efficiency Triangle

 

and transposing the above triangle quantities gives us the following combinations of the same equation:

 

Then, to find Watts (output) = VA x eff., or to find VA (input) = W/eff., or to find Efficiency, eff. = W/VA, etc.

Transformer Basics Summary

Then to summarise this transformer basics tutorial. A Transformer changes the voltage level (or current level) on its input winding to another value on its output winding using a magnetic field. A transformer consists of two electrically isolated coils and operates on Faraday’s principal of “mutual induction”, in which an EMF is induced in the transformers secondary coil by the magnetic flux generated by the voltages and currents flowing in the primary coil winding.

Both the primary and secondary coil windings are wrapped around a common soft iron core made of individual laminations to reduce eddy current and power losses. The primary winding of the transformer is connected to the AC power source which must be sinusoidal in nature, while the secondary winding supplies power to the load.

We can represent the transformer in block diagram form as follows:

Basic Representation of the Transformer

 

The ratio of the transformers primary and secondary windings with respect to each other produces either a step-up voltage transformer or a step-down voltage transformer with the ratio between the number of primary turns to the number of secondary turns being called the “turns ratio” or “transformer ratio”.

If this ratio is less than unity, n < 1 then N_S is greater than N_P and the transformer is classed as a step-up transformer. If this ratio is greater than unity, n > 1, that is N_P is greater than N_S, the transformer is classed as a step-down transformer. Note that single phase step-down transformer can also be used as a step-up transformer simply by reversing its connections and making the low voltage winding its primary, and vice versa as long as the transformer is operated within its original VA design rating.

If the turns ratio is equal to unity, n = 1 then both the primary and secondary have the same number of windings, therefore the voltages and currents are the same for both windings.

This type of transformer is classed as an isolation transformer as both the primary and secondary windings of the transformer have the same number of volts per turn. The efficiency of a transformer is the ratio of the power it delivers to the load to the power it absorbs from the supply. In an ideal transformer there are no losses so no loss of power then Pin = Pout.

In the next tutorial to do with Transformer Basics, we will look at the physical Construction of a Transformer and see the different magnetic core types and laminations used to support the primary and secondary windings.

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What is geostationary satellite?

 

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