Magnetism and electromagnetism - class-X

In an ideal transformer, the primary and secondary voltages are in phase if the secondary is open-circuited, but they are in anti-phase (180 degrees or pi radians) due to Lenz's law when considering the induced EMF relative to the primary input.

Transformer is an electrical device that requires time varying current to link two or more electrical coils through electromagnetic induction and thus transfer energy between the magnetically coupled circuits. Therefore transformer are used only in AC circuits

Since, the starter motor draws a large current resulting in a large potential drop in the battery, which reduced the thermal voltage of the battery hence, the light of a car are dim when the starter is operated.

The iron core of a transformer is made of Laminated silicon steel with special insulated coating to limit Eddy current loss. Electrical steel, also called lamination steel is specialty steel tailored to produce certain magnetic properties, such as a small hysteresis area (small energy dissipation per cycle, or low core loss) and high permeability.
An eddy current is a swirling current set up in a conductor in response to a changing magnetic field. By Lenz's law, the current swirls in such a way as to create a magnetic field opposing the change. To do this in a conductor, electrons swirl in a plane perpendicular to the magnetic field. Because of the tendency of eddy currents to oppose, eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. 

In electric motor electrical energy is converted into mechanical energy hence, option D is correct.

Transformers are found everywhere alternating current is used. This includes both large power stations to the power cord for our laptop computer. A transformer is an electrical device that trades voltage for current in a circuit, while not affecting the total electrical power. This means it takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit.
A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
Therefore, to convert 12V a.c. to 200V a.c., a step up transformer is used.
This transformer works in the principle of Electromagnetic induction which states that rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil.
Say we have one winding which is supplied by an alternating electrical source. The alternating current through the winding produces a continually changing flux or alternating flux that surrounds the winding. If any other winding is brought nearer to the previous one, obviously some portion of this flux will link with the second. As this flux is continually changing in its amplitude and direction, there must be a change in flux linkage in the second winding or coil. According to Faraday's law of electromagnetic induction, there must be an EMF induced in the second. If the circuit of the later winding is closed, there must be an current flowing through it. This is the simplest form of electrical power transformer and this is the most basic of working principle of transformer.

A step up transformer increases the voltage This transformer has more windings on the "secondary" side of the transformer, with fewer windings of thicker wire on the primary or input coil. 

A transformer takes in electricity at a higher voltage and lets it run through lots of coils wounded around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Also around the core, there is an output wire with fewer coils. The magnetism changing back and forth creates a current in the wire. Having fewer coils means less voltage. So the voltage is "stepped-down."

A transformer consists of two coils - the primary coil and the secondary coil both wound on a soft iron core. If an ac voltage is applied to the primary coil then this will produce a changing magnetic field in the iron core. This induces an alternating voltage in the secondary coil according to electromagnetic induction.
A voltage in the secondary coil occurs only if the magnetic field is changing. Therefore, transformers will only work with ac current only. The transformer is used in alternating current circuits.

A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical, electro-mechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, inductors, magnetic recording heads, and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrites.
The magnetic field is often created by a coil of wire around the core that carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core. The use of a magnetic core can enormously concentrate the strength and increase the effect of magnetic fields produced by electric currents and permanent magnets. 
The properties of a device will depend on the following factors:

• The geometry of the magnetic core.
• The amount of air gap in the magnetic circuit.
• The properties of the core material like permeability and hysteresis.
• The operating temperature of the core.
• Whether the core is laminated to reduce eddy currents.

A transformer is normally used to converts high voltage into low voltage. 

Transformers work on the principle of voltage induction by the linkage of changing magnetic flux between between the two cores. Changing magnetic flux requires changing/alternating current. 
Transformers are used only in AC circuits. In DC circuits, no alternating/changing magnetic flux is produced to induce voltage in secondary from the primary terminals of the transformers.

A step down transformer has more turns of wire on the primary coil and less turns of wire on the secondary coil. This makes a smaller induced voltage in the secondary coil. 

Core of transformer is made up of Soft Iron.

A transformer is used to change the voltage low to high or vice versa depending upon the efficiency for power transfer and usage.

In step down transformers, voltage in secondary coil decreases. and we know that 

A transformers changes an alternating potential difference from one value to another of greater or smaller value using the principle of mutual induction. so, in transformer current and voltage changes but frequency remains same.

Transformer is used to increase voltage and decrease current in the secondary coil with respect to primary coil or vice versa.
Energy in the coil is always stored in the form of magnetic field.

Transformers are used only in A.C. Circuits because D.C. current could not produce changing flux which in turn not able to produce induced emf in the secondary coil of the transformer but A.C. Current produces changing flux.

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.

Answer is A.

A transformer takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit. A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
If we change the number of turns in the coils we change the induced emf. This allows us to change (transform) the voltage from the primary to the secondary coil.
The Turns Rule is:
Ns/Np=Vs/Vp
Where,
Ns = number of turns on the secondary coil
Np = number of turns on the primary coil
Vs = voltage across the secondary coil
Vp = voltage across the primary coil
So if number of turns on the secondary coil is more than on the primary coil, the output voltage will be higher than the input voltage. This is called a step up transformer.
Hence, The secondary voltage as per the given question can be obtained as follows:
Let input voltage be 6V and the output voltage be x. Let 5000 be the number of turns of secondary coil and 500 be the number of turns of primary coil.
Ns/Np=Vs/Vp; 
That is, 5000/500=x/6 
$x=\frac { 5000\times 6 }{ 500 } =60\quad V$.
Therefore, the secondary voltage is 60 V.

Answer is D.

The efficiency of a transformer can be calculated using Equation,
$Efficiency\quad =\quad \frac { Output\quad power\quad of\quad the\quad transformer }{ Input\quad power\quad of\quad the\quad transformer } \quad =\quad \frac { { P } _{ s } }{ { P } _{ p } } \times 100$
In this case, the output power is calculated as follows.
The output voltage is 11 V and current is 90 A
$Power\quad =\quad VI\quad =\quad 11\times 90=990W$.
The input power is calculated as follows.
The input voltage is 220 V and current is 5 A
$Power\quad =\quad VI\quad =\quad 220\times 5=1100W$.
So, $\eta =\quad \frac { 990 }{ 1100 } \times 100\quad =\quad 90%$.
The efficiency of the transformer is 90%.

A

An electric motor is an electrical machine that corrects electrical energy into mechanical energy. Working of electric motor mainly depends upon the interaction of magnetic field with current.

Answer is A.
Mutual inductance is where the magnetic field generated by a coil of wire induces voltage in an adjacent coil of wire. A transformer is a device constructed of two or more coils in close proximity to each other, with the  purpose of creating a condition of mutual inductance between the coils.

Answer is A.

The rate at which alternating current reverses direction is called its frequency, expressed in hertz. Thus, standard household current in India is 50 Hz. AC electricity alternates back-and-forth in direction 50 times per second, according to the electrical system in the country. The charge at the ends of the wire alternates between negative (-) and positive (+). If the charge is negative (-), that pushes the negatively charged electrons away from that terminal. If the charge is positive (+), the electrons are attracted in that direction. Thus the electricity in the wire moves in one direction for a short while and then reverses its direction when the generator armature is in a different position. The direction changes twice in each cycle so that change of direction takes place after every 1/100 second.

AC voltage, It also helps in regulation the level of AC voltage

Answer is C.

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. So the current is sent through step up transformers to increase the voltage and to push the power over long distances.

Answer is C.

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. So the current is sent through step up transformers to increase the voltage and to push the power over long distances.
Hence, transformer is employed to obtain a suitable A.C. voltage.

 A transformer is a device that is used to either raise or lower voltages and currents in an electrical circuit. In modern electrical distribution systems, transformers are used to boost voltage levels so as to decrease line losses during transmission

Answer is D.

Soft iron are ferromagnetic materials made of iron which can be magnetized and demagnetized easily. This is an advantage in transformers because in transformers, we're constantly varying magnetism of the core. This is important because without the change of magnetism of the core, there wouldn't be any voltage induced in the second circuit (the output)! Hence, using a soft iron would allow the magnetism of the core to be changed easily, lessening the amount of energy lost and increasing the efficiency of the transformer. 
Hence, soft iron is used to make core of the transformer.

To obtain suitable AC voltage so that energy loss is avoided.

Soft iron provides the best material for the core of a transformer as its permeability $(\mu)$ is very high. Its hysteresis curve is of small area and its coercivity is very low.

Transformer works only for alternating current because there will be no flux linked with the coil of transformer as the voltage is constant.

A transformer consists of a primary coil and the secondary coil. The current from the primary coil is induced into the secondary coil. Since there is the involvement of two coils mutually, it is termed as mutual induction.

C

Transformer is a static device which transforms electrical energy from one circuit to another and with the help of mutual inductor between two windings. The transformer may be step up or step down. A transformer that increases voltage from primary to secondary (more secondary windings turns than primary winding turns) is called step up transformer. Conversely a transformer designed to do just the opposite is called step down transformer. 

A transformer is based on the principle of mutual inductance , and it is used to change the alternating voltage . The transformer which increases the voltage is called step-up transformer , and which decreases is step-down transformer .

The transformer works on the principle of mutual induction. The voltage in one coil changes to change the magnetic field that it creates. This changing magnetic field causes a flux change in the secondary coil, which would create an induced emf in the coil. Thus it is necessary to have a changing magnetic field in a transformer to cause induction in secondary coil.

Working of transformer is based on the conservation of power i.e no power is lost inside the transformer.

Transformer works on the basis of electromagnetic induction, which is produced by changing magnetic flux. In a transformer both the coils are fixed and not moving thus, the change in magnetic flux is because of change in magnetic field.

In a transformer the change in the current in primary coil produces the change in the current in secondary coil because of electromagnetic induction. 

iron core is basically used to minimise flux leakage so that energy losses can be minimise.

Power= $VI$

efficiency of a transformer $=\frac { Power _{ output } }{ { Power } _{ input } } $ for having efficiency >1 is not allowed by law of conservation of energy and e$=$1 is only for cases.
so e<1.

A transformer is used to change the voltage low to high or vice versa depending upon the efficiency for power transfer and usage and since it is an application of the changing emf due to change in magnetic flux which is created by changing current it can be only applied to a.c. .

A transformer changes Voltages and current but the power remains constant but that is case only for ideal transformer, in general there is loss in power in form of heat , sound , vibrations, eddy currents , etc .Only the frequency remains constant.

In a transformer, source of alternating current is applied to the primary coil. Due to this, the current in the primary coil (called as magnetizing current) produces alternating flux in the core of transformer. This alternating flux gets linked with the secondary coil, and because of the phenomenon of mutual induction an emf gets induced in the secondary coil. 
Mutual induction:
An alternating current flowing in a coil produces alternating magnetic field around it. When two or more coils are magnetically linked to each other, then an alternating current flowing through one coil causes an induced emf across the other linked coils. This phenomenon is called as mutual induction.
From these principles, it can be derived that emf divided by number of turns is same for both primary and secondary winding.

Transformers are found everywhere alternating current is used. This includes both large power stations to the power cord for our laptop computer. A transformer is an electrical device that trades voltage for current in a circuit, while not affecting the total electrical power. This means it takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice-versa. That is, step up or step down the voltage in a circuit.
A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
For a transformer to work, the current in one coil has to somehow make current flow in the other coil and the circuit it's connected to. A DC current in one coil will make a magnetic field on the other coil, but a magnetic field by itself won't drive any electrons around. A changing magnetic field, however, does create an electric force which will accelerate those electrons in the other coil into carrying a current. This process is described by Faraday's law of induction. We get a changing field from an AC current, since the current which makes the field is changing. Hence, DC current cannot be used as a source for a generator.

This question is already present in ID: 177624.

Transformers are found everywhere alternating current is used. This includes both large power stations to the power cord for our laptop computer. A transformer is an electrical device that trades voltage for current in a circuit, while not affecting the total electrical power. This means it takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice-versa. That is, step up or step down the voltage in a circuit. 
A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity. 
For a transformer to work, the current in one coil has to somehow make current flow in the other coil and the circuit it's connected to. A DC current in one coil will make a magnetic field on the other coil, but a magnetic field by itself won't drive any electrons around. A changing magnetic field, however, does create an electric force which will accelerate those electrons in the other coil into carrying a current. This process is described by Faraday's law of induction. We get a changing field from an AC current, since the current which makes the field is changing. Hence, DC current cannot be used as a source for a generator.

A transformer takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit. A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
If we change the number of turns in the coils we change the induced emf. This allows you to change (transform) the voltage from the primary to the secondary coil.

$\text{Power source}=EI=240\times 0.7=166$
$\Rightarrow \eta =\dfrac{\text{Output}}{\text{Input}}=\cfrac{140}{166}$ $\Rightarrow \eta=83.3%$

A transformer can't  work on dc current because in dc current, neither magnitude nor direction change, so, there is no change in flux, so, no emf is induced in secondary coil hence no voltage or current is there in secondary coil.

Step up transformer increases the voltage in secondary coil and decreases current in secondary coil. Transformer works only on ac current because in DC current, there is no change of flux associated with the coil

The efficiency of the transformer is defined as ratio of useful power output to the input power. The two being measured is the same unit. Its unit is either watts or kw. Transformer efficiency is denoted by $\eta$

$\eta = \dfrac{\text{output power}}{\text{input power}} = =\dfrac{\text{output power}}{\text{input power + losses}}$

$\eta  = \dfrac{\text{output power}}{\text{input power + iron losses of copper + losses}}$

$\eta = \dfrac{V _2I _2 \cos \phi _2}{V _2 I _2 \cos \phi _2 + P _i + P _c}$

Where

$V _2  = $ secondary terminal voltage

$I _2 = $ Full load secondary current.

$\cos \phi _2 = $ Power factor of the load

$P _i = $ Iron losses = constant loss = exactly current loss + hysteresis loss

$P _c = $ full load copper loss

The efficiency is a function of load i.e. lead current $I _2$ assuming $\cos \phi$ constant. The secondary terminal voltage $V _2$ is also assumed constant.

$\dfrac{d \eta}{d I _2} = 0$

Now $\eta = \dfrac{V _2I _2\cos \phi _2}{ V _2I _2\cos \phi _2 + P _i + I _2^2 R _{2e} }$

$\dfrac{d \eta}{d I _2} = \dfrac{d}{d I _2}(\dfrac{V _2I _2\cos \phi _2}{ V _2I _2\cos \phi _2 + P _i + I _2^2 R _{2e} }) = 0$

$( V _2I _2\cos \phi _2 + P _i + I _2^2 R _{2e}) \dfrac{d}{d I _2}( V _2I _2\cos \phi _2) – (V _2I _2\cos \phi _2) \dfrac{d}{d I _2}( V _2I _2\cos \phi _2 + P _i + I _2^2 R _{2e}) = 0$

$( V _2I _2\cos \phi _2 + P _i + I _2^2 R _{2e})(V _2 \cos \phi _2) – (V _2 I _2 \cos \phi _2)(V _2 \cos \phi _2 + 2 I _2 R _{2e}) = 0$

Cancelling $V _2 \cos \phi _ 2$ from both the terms we get

$ V _2I _2\cos \phi _2 + P _i + I _2^2 R _{2e} – V _2I _2\cos \phi _2 – 2I^2 R _{2e} = 0 $

$P _i – I^2 R _{2e} = 0$

$P _i = I^2 R _{2e} = P _{Cu}$

$P _i = P _{Cu}$

 So, condition to achieve maximum efficiency is that

Copper Loss = Iron Loss

$\mu =\frac { { V } _{ s }{ I } _{ s } }{ { V } _{ p }{ I } _{ p } } =0.8\Rightarrow { I } _{ p }=\frac { { V } _{ s }{ I } _{ s } }{ { V } _{ p }\mu  } =\frac { 440\times 2 }{ 220\times .8 } =5A$

Power = Voltage x Current
As voltages are high so currents are lower. This is done to reduce the resistive losses.
Resistance Loss = ${ { I } _{ rms }^{ 2 } }R$
So, lower current implies less power loss. 
As resistance is inversely proportional to their diameter; hence decreasing their diameter by x will increase resistance by $x^2$ and hence increasing power loss hence not done.
As the voltage required for domestic and commercial uses are pretty less then they can be easily brought down by step-down transformers. 

Answer is C.

Slip-ring commutator merely maintains a connection between the moving rotor and the stationary stator.
Slip rings are useful for DC motors where the current has to change direction every half revolution whereas split rings are used to make constant current connection to a commutator, which is what we want for an AC motor.
Hence, if the slip rings are not present, then the armature stops its rotation.

A transformer can not step up a d.c. input so output potential here will be zero. No potential will be induced in the secondary coil.

Heat loss in a magnetic circuit is due to two reasons one is hysteresis and  other is due to ohmic loss due to eddy current .Laminations and circulatingoil are some of methods to avoid heating. Hysteresis loss is entirelymaterial property while eddy current loss depends on geometry of core and amount of current. 

Coefficient coupling  $\eta = 0.85$
Input voltage in primary coil   $V _p = 2 \ V$
Turn ratio  $\dfrac{N _s}{N _p} = 1$
Thus output voltage in secondary coil   $V _s = \eta \dfrac{N _s}{N _p}V _p$
$\implies \ V _s = 0.85\times 1\times 2 = 1.7 \ V$

Input voltage in primary coil  $V _p = 120 \ V$
Output voltage in secondary coil   $V _s = 900 \ V$
Turn ratio    $  \dfrac{N _s}{N _p} = \dfrac{V _s}{V _p} = \dfrac{900}{120} = 7.5$

When the secondary load is overloaded, it draws maximum current in the primary side due to which damaging the primary side as well as appliances. Therefore primary winding of power transformer is fused. Suppose peak voltage of power source is varying then current will also fluctuate so also to limit the current it is fused.

Transformer does not work on DC. Direct current have a variable magnetic field induced if fed through the primary winding of transformer. Thus, only a constant magnetic field will act o secondary and shall not induce any voltage across the terminals of the same. So, it works on AC circuits only.

A transformer consists of only single core and it has windings wound on the core which provides necessary magnetic field. The transformer may contain two or more windings.

An auto transformer is an electrical transformer with only one winding. The portion of the same winding acts both primary and secondary side of the transformer. 

In a transformer, we know that 
$\frac{V _2}{V _1}=\frac{N _2}{N _1}$
where $V _1$ and $V _2$ are voltage across primary and secondary coils respectively and $N _1$ and $N _2$ are respective number of turns. 

Transformer cores are made up of soft iron because soft iron has less losses due to hysteresis.

Transformer does not change the frequency of input AC.

Transformer works on the principle of Electromagnetic induction i.e an emf is induced in the coil if the magnetic flux linked with the coil changes. Magnetic flux in the coil can change only in case of AC voltage supply. In case of DC voltage supply, magnetic flux in the secondary coil remains constant and thus no emf is induced in the secondary coil and so transformer cannot work in case of DC supply.
That's why, transformer can work on AC only.

C

A transformer is a device that is used to either raise or lower voltage, and currents in an electric circuit. Thereby, we can suitably obtain the voltage and current.

Transformer is an apparatus for reducing or increasing the voltage of an alternating current.

C

Electromagnetic Induction is the production of an emf across an electrical conductor in aa changing magnetic field in a transformer the alternating current through the primary winding produces a continually changing flux and die to this alternating flux on emf is induced in the secondary winding.

False

A transformer
consists of two electrically isolated coils and operates on Faraday’s principle
of mutual induction, in which an EMF is induced in the transformer secondary
coil due to the magnetic flux generated by the voltages and currents flowing in
the primary coil working. In self induction there is only one coil and emf is induced in that coil due to the change in flux linked with that coil. 

False

Transformer does not work on DC. Direct current doesn’t have a variable magnetic field induced if fed through the primary winding of a transformer. Thus, only a constant magnetic field will act on secondary and shall not induce any voltage across the terminals of the same.

Transformer used in the transmission of electrical power because they help to distribute electrical energy efficiently and can reduce transmission loss.

Alternating current does not flow in the secondary coil unless electrical load is connected to its terminals.
In step-up transformer current in secondary coil is less than the current in primary coil.
No e.m.f. is induced in the secondary coil if the primary coil is connected to a d.c. supply.

For the step up transformer -

Transformers operate on the principle of mutual induction, which requires a changing magnetic flux. This only occurs with alternating current (AC), not direct current (DC).

The basic principle of MRI is protons in different types of human tissues have specific magnetic field environment.