Electric Motors
Electric motor is a device which converts electrical energy into mechanicalenergy. It works on the principle that when a current carrying conductor isplaced in the magnetic field it exerts a force on it and the coil rotatescontinuously as long as the current flows through it.
It consists of a rectangular coil PQRS. It is placed between the poles of the magnetic field in such a way that the arms PQ and RS are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves A and B, of the split ring. The inner sides of these split ring are insulated and attached to an axle. The external conducting edges of A and B touches two conducting stationary brushes U and V, as shown in the figure given below.
Current in the coil PQRS enters from the source through conducting brush U and flows back to the source through brush V. The current in the coil flows from P to Q in the arm PQ and in the arm RS, it flows from R to S, which is opposite to the direction in the arm PQ. The force acting on the arm PQ pushes it downwards, while the force acting on arm RS pushes it upwards. This makes the coil and the axle M mounted free to turn about an axis rotate anticlockwise. At half rotation, B makes contacts with the brushes U and A makes contacts with brush V .Thus, the direction of current in the coil gets reversed and flows along the path SRQP. The device, which helps in changing the direction of the current is called commutator half ring and here the split ring acts as a commutator. The reversal of direction of current also reverses the direction of forces acting on the two arm PQ and RS. The arm PQ which was earlier pushed downward is now pushed upward and vice - versa. Therefore, coil and the axle rotates in the same direction and this process of rotation continuous till the current flows in the coil.
Force on Current Carrying Conductor
So far we have learnt about the current flowing through a conductor produces magnetic effect. The reverse of this is also true. A magnet exerts a mechanical force on a current carrying conductors when it is placed in the magnetic field and produces motion in the conductor. The various devices like motors, generators, fan etc. works on this principle. We can demonstrate this with the help of the following activity given below:
Electromagnet
Electromagnet is the magnet which is produced by passing electric current through the magnetic material. It consists of long coil of insulated copper wire wound on a soft iron core. The core of the electromagnet is a soft iron core, which gets strongly magnetized when the electric current is passed through it and easily get demagnetized if the current is switched off. But if other material is used for this purpose, it will not loose its magnetism instantly when we switch off the electric current. There are some factors which affects the strength of the electromagnet.
The factors are:
Introduction
There are various effects of electric current. One of such effect we have studiedin the previous chapter of electricity. One of the other effect of electric currentis the magnetic effect of electric current. We will study about the magneticeffect produced by the electric current flowing through the wire. Let us take astraight copper wire and place it between the point A and B in an electriccircuit, as shown in the figure given below.
The wire AB is placed perpendicular to the plane of the paper. Place a magnetic compass near the wire and see the position of the needles. Pass the current through the circuit and observe the change in position of the needle. This change in position of the needle shows that, as the current passes through the wire magnetic effect is produced. This magnetic effect produced by electric current is called electromagnetism. The concept of electromagnetism was introduced by Hans Christian Oersted in 1820.
A magnet is an object which attracts pieces of iron like material towards itself. It is found in various shapes and size. The most commonly used magnetic is the bar magnetic and horseshoe magnetic.
A bar magnetic is a rectangular bar of uniform cross section which attracts piece of iron, steel, nickel and cobalt etc. It has two pole near its end, north pole and south pole. If we suspend a bar magnetic freely with the help of a thread it always comes to rest in geographical north- south direction. The other properties of bar magnet is that like pole repel each other and unlike pole attract each other. It is used in many electronic devices like radio, TV, MRI, etc.
Magnetic Field
The magnet create a field around itself where magnetic effect can be felt. It is known as magnetic field. We can also define it as the space around the magnet where magnetic effect can be felt. The magnetic field has both magnitude and direction. The direction of magnetic field at a point is the direction of the resultant force acting on a hypothetical north pole, placed at that point. The north end of the needle of a compass indicates the direction of magnetic field at a point where it is placed.
Magnetic Field Lines
Magnetic field lines are the lines which is drawn around the magnetic fieldstarting from North Pole and ending at South Pole. It also gives the direction ofthe magnetic field. Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is one in which a north pole of the compass needle moves inside. Therefore, it is taken by convention that the field lines emerges more... 
Electrical Power
When electric current flows through the circuit it use up the electrical energy to do certain amount of work. The rate of doing work by the electric current is called the electrical power. It is given by
\[\text{Power }=\frac{\text{Electrical Work Done}}{\text{Time Taken}}\]
Or, \[P=\frac{W}{T}\]
The SI unit of power is watt or joule per second. The power of one watt is defined as the rate of doing work of one joule per second. One kilowatt is equal to the 1000 watt. We can also define the electrical power as the rate of consuming electrical energy.
Since we have, \[P=\frac{W}{T}\]
But work done is equal to the
\[B=V\times I\times T\]
Therefore, \[P=\frac{V\times I\times T}{T}\]
\[\Rightarrow \,\,P=V\times I\]
Where V is potential difference and I is the current flowing through the conductor.
But, \[V=I\times R\]
Putting in the above equation we get,
\[P=I\times R\times I\]
\[\Rightarrow \,P={{I}^{2}}\times R\]
Again, \[I=\frac{V}{R}\]
Therefore, \[P=\frac{{{V}^{2}}}{R}\]
Where R is the resistance of the conductor.
Electrical Energy
Electrical energy is defined as the product of power and time. The unit of electrical energy is kilo watt hour.
\[E=P\times t\]
One kilowatt hour is the amount of electrical energy consumed when an electrical appliance of one watt power is used for one hour.
1 Kilo Watt = 1000 watt
Therefore, 1 kilowatt hour = 1000 x 3600
\[=3.6\times {{10}^{6}}\,\text{joules}\] .
Heating Effect of Electric Current
The heat is produced when an electric current is passed through the wire of high resistance. The resistance in the wire offers resistance to the flow of current. Hence, work must be done by the current to keep itself flowing. If W be the work done by the current I flowing through the conductor for time t and Q be the total charge flow during this time against the potential difference V, then
\[W=Q\times V\]
But, \[Q=I\times t\] and \[V=I\times R\]
Therefore, \[W=I\times t\times I\times R\]
\[\Rightarrow \,W={{I}^{2}}\times t\times R\]
Assuming that all the electrical energy consumed during this work done is converted into heat we have,
Heat produced \[(H)={{I}^{2}}\times t\times R\] Joules
Thus, according to joules law of heating:
Some of the Heating Effect of the Current are
Domestic Circuit
The electric circuit which we use in our houses, is called the domestic circuit. In domestic circuit we normally use parallel connection, so that each appliance works independently and the fault at any point does not disrupt the working of other points. We use separate switches for each points and hence can operate independently.
We have normally two types of circuit, i.e. parallel and series circuit. The main disadvantage of series circuit in the house hold is that, if one electrical point’s stops working due to some defects then all the other points will also stop working. Entire points works with one switch only and it is not possible to switch off any one point separately, at any instant. Apart from this all the points do not get same voltage of the power supply. The overall resistance of the circuit also increases. As a result power supply from the mains decreases.
In domestic circuit, we normally use parallel connection because of its advantages. The main advantages of parallel connections is that in parallel connection each electrical appliance work independently. Defect in any switch does not disrupt the working of other switches. Each appliances gets same 220 V as that of the power supply. The overall resistance of the household circuit is reduced due to which the current from the power supply is high. In this circuit, each appliances gets the required amount of current for their operation. 
Combination of Resistance
There are two ways of connecting the resistance that is, the series combination and is parallel combination.
Series Combination
The combination, in which the resistance are connected end to end with each other, is called series combination.
Let \[{{R}_{1}},\,\,{{R}_{2}}\] and \[{{R}_{3}}\] be three resistance, connected in series across a battery of potential V volt, as shown in the figure above. Now suppose \[{{V}_{1}}\] be the potential difference across the resistance \[{{R}_{1}};\,\,{{V}_{2}}\] be the potential difference across \[{{R}_{2}};\] and \[{{V}_{3}}\] be the potential difference across \[{{R}_{3}}\]. The total potential across the three resistance is given by
\[\mathbf{V=}{{\mathbf{V}}_{\mathbf{1}}}\mathbf{+}{{\mathbf{V}}_{\mathbf{2}}}\mathbf{+}{{\mathbf{V}}_{\mathbf{3}}}\]
But by Ohms law, \[V=I\times R\]
Since the same current I flows through the three resistance \[{{R}_{1}},\,\,{{R}_{2}}\]and \[{{R}_{3}}\], so by ohms law
\[{{V}_{1}}=I\times {{R}_{1}},\,\,{{V}_{2}}=I\times {{R}_{2}}\]and\[{{V}_{3}}=I\times {{R}_{3}}\]
Therefore, \[I\times R=I\times {{R}_{1}}+I\times {{R}_{2}}+I\times {{R}_{3}}\]
\[\Rightarrow \,\,I\times R=I\times \,({{R}_{1}}+{{R}_{2}}+{{R}_{3}})\,\Rightarrow \,\,R={{R}_{1}}+{{R}_{2}}+{{R}_{3}}\]
Hence, the resultant resistance is equivalent to the sum of all individual resistance connected in series.
Characteristics
Parallel Combination
When all the resistance are connected between the two common ends of the circuits, it is called a parallel connection. Let \[{{R}_{1}},\,\,{{R}_{2}}\] and \[{{R}_{3}}\] be the three resistance connected in parallel, as shown in the circuit diagram given above, across a potential difference of V volt. In this case, the potential difference across the ends of all the three resistance will remain same. However the current flowing through each resistance will not be the same. Suppose \[{{I}_{1}},\,\,{{I}_{2}}\] and \[{{I}_{3}}\] be the current flowing through the three resistance \[{{R}_{1}},\,\,{{R}_{2}}\] and \[{{R}_{3}}\] respectively. Then the total current flowing in the circuit is
\[I={{I}_{1}}+{{I}_{2}}+{{I}_{3}}\]
But by Ohm's law, \[I=\frac{V}{R}\]
Therefore, \[{{I}_{1}}=\frac{V}{{{R}_{1}}},\,\,{{I}_{2}}=\frac{V}{{{R}_{2}}}\] and \[{{I}_{3}}=\frac{V}{{{R}_{3}}}\]
Putting these values in the above equation we get,
\[\frac{V}{R}=\frac{V}{{{R}_{1}}}+\frac{V}{{{R}_{2}}}+\frac{V}{{{R}_{3}}}\]
\[\Rightarrow \,\frac{V}{R}=V\,\left( \frac{1}{{{R}_{1}}}+\frac{1}{{{R}_{2}}}+\frac{1}{{{R}_{3}}} \right)\] \[\Rightarrow \,\,\frac{1}{R}=\frac{1}{{{R}_{1}}}+\frac{1}{{{R}_{2}}}+\frac{1}{{{R}_{3}}}\]
Hence the equivalent resistance is equal to the sum of reciprocal of individual resistance.
Characteristics
Electric Potential
What makes the charge flow through the conductors? Is it that the chargeflow on their own through the conductors? The answer to this question isno. The charge do not flow on their own through the conductors. The chargeflow through the conductors due to the difference of electric pressure which iscalled the potential difference. This difference of pressure may be generatedby a cell or a battery or any other sources. Thus, electric potential may bedefined as the amount of work done in taking a unit positive charge from infinityto a particular point.
Therefore, \[V=\frac{W}{Q},\]
Where,
W is the work done
Q is the total charge
V is the potential difference
The SI unit of potential difference is volt and is named after Alessandro Volta and Italian Physicist. One volt is defined as the potential difference between two points in a current carrying conductor, when one joule of work is done in moving a charge of one coulomb, from one point of another. It can be measured with the help of an instrument called voltmeter which is always connected in parallel across the points, between which the potential difference is to be measured.
Potential Difference
The potential difference between any two point may be defined as the amount of work done in taking a unit positive charge from one point to another; The SI unit of potential difference is Volt. The potential difference between any two point is said to be of one volt, if one joule of work is done in moving one coulomb of electric charge from one point to another.
Electric Current
When two charge bodies, at different potentials, are connected by a metal wire, the electric charge will flow from the body at higher potential to the one at lower potential. This flow of charges in the metal conductor will constitute the electric current through the conductor. Thus, the flow of electron through -the conductor is called the electric current. The SI unit of electric current is ampere. It is expressed as
\[I=\frac{Q}{t}\]
One ampere current is defined as the current produced by the flow of onecoulomb of charges through a cross sectional area in one second. It can be measure with the help of an instrument called ammeter. It is always connected in series with the circuit in which the current is to be measured.
Ohm’s Law
According to ohms law, at constant temperature the current flowing through a conductor is directly proportional to the potential difference across its end. It gives the relationship between the current and the potential difference.
If I is the current flowing through the conductor and V is the potential difference across more... 
Introduction
One of the important sources of energy in modern time is the electricity. Modern life depends on electricity to a very large extent. All the modern development has been made possible due to the electricity. It is used everywhere, in house, in industry, transportation, and in what not?. Now the most important question is 'how is electricity produced?' and 'what is electricity'?
Electricity
Atom consists of charge particles, such as, proton and electron. Protons are the positive charge particles and electrons are the negative charge particles, present in an atom. Electric current is generated by flow of charges through the conductors. It depends on the amount of charge flowing through the conductor, through a particular area and in a unit time. The rate of the flow of electric charge through the conductor is called the electric current. Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges. If \[{{q}_{1}},\,\,{{q}_{2}},\,\,{{q}_{3}},\,\,{{q}_{4}}--,{{q}_{n}}\] are the n charges flowing through the conductor in a given time t, then the current flowing through the conductor of given cross sectional area is given by
\[I=\frac{{{q}_{1}}+{{q}_{2}}+{{q}_{3}}+{{q}_{4}}+----+{{q}_{n}}}{t}\]
\[\Rightarrow \,\,I=\frac{Q}{t},\]where Q is the net charges given by,
\[Q={{q}_{1}}+{{q}_{2}}+{{q}_{3}}+{{q}_{4}}+----+{{q}_{n}}\]
I is the current produced
t is the time for which the charge flows
The SI unit of electric charges is coulomb (C), which is equivalent to the charge contained in nearly \[6\times {{10}^{8}}\] electrons. The unit of electric current is ampere which is named after the French scientist Marie Ampere. One ampere current is equivalent to the current produced by flow of one coulomb of charge in one second. We can measure the current with the help of an instrument called Ammeter by connecting it in series in the electric circuit.
If we rub an ebonite rod with woolen cloth, the charge acquired is negative and if we rub glass rod with silk cloth the charge acquired is positive. There are some substances through which electric charge can flow easily and hence are called conductors. But there are some substance through which the charge cannot flow easily, are called semi-conductors. Some substance through which electric current cannot flow at all are called insulators. The metals like copper, Aluminium are good conductor of electricity. Some of the alloy such as nichrom, manganin, and constantan are also the good conductor of electricity.
In fact, human body is also the good conductor of electricity. On the other hand substance like plastics, rubber, glass, ebonite, dry wood, etc do not allow the electric current to flow through it and are the example of insulators. It is actually the presence of free electron, which are loosely bound with the atoms, are responsible for constituting electric current through the conductors. In case of insulators, the electrons are strongly held by the atoms more... 
Human Eyes
It is one of the important-organ of human beings. It works on the’ refraction of light as the other optical instruments, like camera, telescope, microscope etc. In this part we discuss about the structure and various defects of human eyes. The main parts of human eyes are Cornea, iris, pupil, ciliary muscles, lens, retina, aqueous humour, vitreous humour, and optic nerves.
Cornea is the outermost layer of the human eyes which is made up of a transparent substance and bulging outwards. The light enters into our eyes through cornea.
In front of the eye, the choroid coat forms the iris. This may be pigmented and is responsible for the different color of the eye. An opening, the pupil, is present in the center of the iris. The size of this opening is variable and under automatic control. In dim light the pupil enlarges, letting more light into the eye. In bright light, the pupil closes down. This not only protects the interior of the eye from excessive illumination, but improves its image-forming ability and depth of field.
The inner coat of the eye is the retina. It contains the visual sensing apparatus such as, the actual light receptors, the rod cells and cone cells. The exterior of the cornea is bathed by tears, while the interior is bathed by the aqueous humor. It is an isosmotic fluid containing salts, albumin, globulin, glucose,and other constituents. The aqueous humor brings nutrients to the cornea and to the lens and removes end products of metabolism from these tissues. The vitreous humor is a collagenous or gelatinous like mass that not only helps to maintain the shape of the eye, but also allows it to retain some pliability. The lens of the eye is located just behind the iris. It is held in position by ligaments. Ordinarily, these are kept under tension and the lens is correspondingly flattened. However, contraction of muscles attached to these ligaments relaxes them and permits the lens to take on a more nearly spherical shape. These changes in lens shape enable the eye to shift its focus from far objects to near objects and vice versa. The lens of the eye is bathed, on one side by the aqueous humor, and supported on the other side by the vitreous humor. The lens has no blood
supply, but it is an active metabolizing tissue. The lens is mostly water and protein. The proteins are synthesized within the lens, occurring mostly in an epithelial layer around the edge of the lens. The center area of the lens, the core, consists of the lens cells that are present at birth. The lens grows from the periphery. The human lens increases in weight and thickness with age and becomes less elastic. On an average the lens may increase threefold in size and approximately 1.5-fold in thickness from birth to about the age of 80.
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