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MAGNETISM

Magnet was first discovered some 5,000 years ago in Magnesia. It could attract small pieces of iron towards it and had directional property i.e. when suspended freely, it always points in north-south direction. There is a true story behind its discovery.

(A) Discovery of Magnet: There was a shepherd boy named Magnaus in the town Magnesia in Greece. He had wooden stuff attached to iron sole. One day he left it in a mine. When he came back, the iron sole was firmly attacked to the roof. He got terrified and thought it to be work of some evil spirit or ghost. The roof was actually an iron ore (Fe3O4) magnetite. This first discovered magnet was called magnetite or natural magnet. It points in a particular (N–S) direction. It is also called loading stone which has now been changed to lode stone. Gilbert made detailed study of magnets and their properties. Magnets are essential parts of all generators used for the production of electricity. They also form essential parts of electric motors, TV., radio, stereos and large number of instruments.

(B) Properties of Magnets
(i) It attracts small pieces of iron towards it i.e. it has attractive property.
(ii) When suspended freely, it always points in north south direction. Thus magnet possesses directional property.

(C) Properties of magnet: William Gilbert of England was the first person to study and record the properties of a magnet in a book titled “The magnet”. Let us study some important properties of magnets.

(D) Poles of Magnet: The two ends of a magnet where the magnetic force is greatest are called the poles of the magnet. Each magnet has two poles magnetic north pole and magnetic south pole.

(E) Directional Property: The end of the magnet that points towards the North is called the North Pole (N-Pole) and the other end of the magnet pointing towards the South is called the South Pole (S-Pole), A magnet always points in the north-south direction when suspended freely.

(F) Like poles repel each other:

(G) Magnetic poles always exist in pairs: If a bar magnet broken into two pieces you will see that each piece behaves as a whole magnet. This shows that new poles are formed at the broken ends as shown in the figure. If these pieces are broken again, each smaller piece still remains a whole magnet with two opposite poles. Even a very small piece of a magnet is a whole magnet. Thus, we see that even the smallest piece of a magnet has north and south poles and we cannot separate the two poles.

We, therefore, conclude that the poles of a magnet cannot be separated. Magnetic poles always exist in pairs.

MAGNETIC FIELD LINES

Magnetic field line is an imaginary line such that tangent to it at any paint gives the direction of magnetic field at that point in space. Magnetic field lines are drawn to represent magnetic field. Magnetic field lines can be drawn with the help of magnetic compass. Magnetic field lines are also called as magnetic lines of force.

Properties of Magnetic field lines:
(i) Magnetic field lines are close curves that start from north pole and end on south pole outside magnet.        Inside magnet the field lines start from south pole and end on north pole.
(ii) No two magnetic field lines ever intersect because if it is so, there will be two directions of magnetic field at that point which is not possible.
(iii) Magnetic field lines come closer to one another near the pole of a magnet but they are widely separated at other places.

Plotting magnetic field lines of a magnet:
To trace the magnetic field lines, place the bar magnet NS on a sheet of paper and mark its boundary. Mark a point A near the north pole of the given magnet. Place the compass needle (Figure) such that one of its ends (south) lies exactly over point A . Mark point B on the paper at the opposite (north) end of the compass needle. Move the compass needle so that the south end of the compass needle lies over B and mark point C at the north end of the needle and so on. Go on doing so till a point is reached near the south pole of the given magnet. Join all these points with a free hand curve so as to form a smooth dotted curve.

Mark an arrow head to show the direction of magnetic field line, which will be north pole to south pole outside the magnet. This dotted curve marked with an arrow head represents a magnetic field line. Similarly starting from other points near the north pole of the magnet, draw other magnetic field lines. Magnetic field lines plotted for a bar magnet are as shown in figure.

We can visualise the magnetic field around a bar magnet by sprinking some iron filings near a bar magnet and tapping the sheet on which the magnet is placed. The iron filings will orient themselves according to figure.

Magnetic Field of Earth

A freely suspended magnet always points in the north south direction even in the absence of any other magnet. This suggests that the earth itself behaves as a magnet which causes a freely suspended magnet or magnetic needle to point always in particular direction; north and south. The shape of the earth’s magnetic field resembles that an imaginary bar magnet.  The axis of earth magnetic field is inclined at an angle of about 15° with the geographical axis. Due to this a freely suspended magnet (or magnetic needle) makes an angle of about 15° with the geographical axis and points only approximately in the north south directions at a place.

Magnetic Effect of Current (or Electromagnetism) :

The magnetic effect of current was discovered by Oersted in 1820. Oersted found that a wire carrying a current was able to deflect a compass needle. Now, the compass needle is a tiny magnet, which can be deflected only by a magnetic field. Since a current carrying wire was able to deflect a compass needle, it was concluded that a current flowing in a wire always gives rise to a magnetic field around it. The importances of magnetic field of current lies in the fact that gives rise to mechanical forces.

(i) Magnetic field patterns produced by current carrying conductors having different shapes:  The pattern of magnetic field (or shape of magnetic field lines) produced by a current carrying conductor depends on its shape.  Different magnetic field patterns are produced by current carrying conductors having different shapes.
(A) A straight conductor (or straight wire) carrying current.
(B) A circular loop (or circular wire) carrying current.
(C) A solenoid (long coil of wire) carrying current.

(A) Magnetic field pattern due to straight current carrying conductor (Straight current carrying wire):
The magnetic field lines around a straight conductor (straight line) carrying current are concentric circles whose centres lie on the wire. The magnetic field lines are circular in nature. The magnitude of magnetic field produced by a straight current carrying wire at a given point is:
(a) directly proportional to the current passing in the wire.
(b) inversely proportional to the distance of that point from the wire.

Direction of Magnetic Field Produced by current carrying conductor :
(a) According to Maxwell’s right hand thumb rule:  Imagine that you are grasping (or holding) the current carrying wire in your right hand so that your thumb points in the direction of current, then the direction in which your fingers encircle the wire will give the direction of magnetic field lines around the wire.

Maxwell’s right hand thumb rule is also known as Maxwell’s corkscrew rule (Corkscrew is a device for pulling corks from bottles, and consists of a spiral metal rod and a handle).

(b) According to Maxwell’s corkscrew rule :  Imagine driving a corkscrew in the direction of current, then the direction in which we turn its handle is the direction of magnetic field (or magnetic field lines).

(B) Magnetic field pattern due to a circular loop (or circular wire) Carrying current :

When a current is passed through the circular loop of wire, a magnetic field is produced around it.

The magnitude of magnetic field produced by a current carrying circular loop (or circular wire) at its centre is :
(i) Directly proportional to the current passing through the circular loop (or circular wire), and
(ii) Inversely proportional to the radius of circular loop (or circular wire). The strength of magnetic field produced by a circular coil carrying current is directly proportional to both, number of turns (n), and current
(i) ; but inversely proportional to its radius (r).  The strength of magnetic field produced by a current carrying circular coil can be increased –

(i) by increasing the number of turns of wire in the coil
(ii) by increasing the current flowing through the coil and,
(iii) by decreasing the radius of the coil.

(C) Magnetic field due to a solenoid: The solenoid is a long coil containing a large number of close turns of insulated copper wire.  The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet.

The strength of magnetic field produced by a current carrying solenoid depends on :

(i) The number of turns in the solenoid: Larger the number of turns in the solenoid greater will be the magnetism produced.
(ii) The strength of current in the solenoid: Larger the current passed through solenoid, stronger will be the magnetic field produced.
(iii) The nature of core material used in making solenoid :
The use of soft iron rod as core in a solenoid produces the strongest magnetism.

Electromagnet

An electric current can be used for making temporary magnet known as electromagnets. An electromagnet works on the magnetic effect of current. An electromagnet consists of a long coil of insulated copper wire would on a soft iron core.

When the two ends of the copper coil are connected to a battery, an electromagnet is formed. The core of an electromagnet must be of soft iron because soft iron loses all of its magnetism when current in the coil is switched off. If steel is used for making the core of an electromagnet, the steel does not lose all its magnetism when the current is stopped and it becomes a permanent magnet so steel isnot used for making electromagnets.

(a) Factors Affecting the Strength of an Electromagnet :

(i) The number of turns in the coil: If we increase the number of turns in the coil, the strength of electromagnet increases.
(ii) The current flowing in the coil:  If the current in the coil is increased, the strength of electromagnet increases.
(iii)The length of air gap between its poles: If we reduce the length of air gap between the poles of an electromagnet, then its strength increases.

Magnetism in Human beings

Extremely weak electric current are produced in the human body by the movement of charged particles called ions. These are called ionic currents. When the weak ionic currents flow along the nerve cells, they produce magnetic field in our body. The two main organs of the human body where the magnetic field produced is quite significant are the heart and the brain. The magnetism produced inside the human body (by the flow of ionic currents) forms the basis of a technique called Magnetic Resonance Imaging (MRI) which is used to obtain images (or pictures) of the internal parts of our body.

Fleming’s Left hand Rule for the direction of Force

According to Fleming’s left hand rule:

Hold the forefinger the centre finger and the thumb of your left hand at right angles to one another. Adjust your hand in such a way that the forefinger points in the direction of magnetic field and the centre finger points in the direction of current, then the direction in which thumb points, gives the direction of force acting on the conductor.

Electric Motor

A motor is a device which converts electrical energy into mechanical energy. A common electric motor works on direct current, called D.C. motor, which means a ‘Direct Current Motor’.

(i) Principle of a Motor: A motor works on the principle that when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously.

(ii) Construction of a Motor :

An electric motor consists of a rectangular coil ABCD of insulated copper wire, which is mounted between the curved poles of a horseshoe – type permanent magnet M. The sides AB and CD of the coil are kept perpendicular to the direction of magnetic field between the poles of the magnet. A device which reverses the direction of current through a circuit is called a commutator. The two ends of the coil are soldered permanently to the two half rings X and Y of a commutator. A commutator is a copper ring split into two parts X and Y, these two parts are insulated from one another and mounted on the shaft of the motor.
The commutator rings are mounted on the shaft of the coil and they also rotate when the coil rotates. The function of commutator rings is to reverse the direction of current flowing through the coil every time the coil just passes the vertical position during a revolution. The carbon brushed P and Q are fixed to the base of the motor and they press lightly against the two half rings of the commutator. The battery to supply current to the coil is connected to the two carbon brushes P and Q through a switch.  The function of carbon brushes is to make contact with the rotating rings of the commutator and through them to supply current to the coil.

(iii) Working of a Motor: When an electric current is passed into the rectangular coil, produces a magnetic field around the coil. Suppose that initially the coil ABCD is in the horizontal position.  The current flows in the direction ABCD and leaves via ring Y and  carbon brush Q.

(a) In the side AB of the rectangular coil ABCD, the direction of current is from A to B. And in the side CD of the coil, the direction of current is from C to D. By applying fleming’s left hand rule to sides AB and CD of the coil we find that the force on side AB of the coil is in the downward direction whereas the force on side CD of the coil is in the upward direction. Due to this the side AB of the coil is pushed down and side CD of the coil pushed up. The coil ABCD rotate in the anti clockwise direction.

(b) While rotating, when the coil reaches vertical position, then the brushes P and Q will touch the gap between the two commutator rings and current to the coil is cut off. The coil does not stop rotating because it has already gained momentum due to which it goes beyond the vertical position.

(c) After half rotation, when the coil goes beyond vertical position, the side CD of the coil comes on the left side whereas side AB of the coil comes to the right side, and the two commutator half rings automatically change contact from one brush to the other. So after half rotation of the coil, the commutator half ring Y makes contact with brush P whereas the commutator half ring X makes contact with brush Q. This reverses the direction of current in the coil. Due to this side CD of the coil is pushed down and the side AB of coil is pushed up. This makes the coil rotates anticlockwise by another half rotation.

(d) The reversing of current in the coil is repeated after every half rotation due to which the coil continue to rotate as  long as current from the battery is passed through it. The rotating shaft of electric motor can drive a large number of machines which are connected to it.

Electromagnetic Induction

(i) Electricity From Magnetism:  The production of electricity from magnetism is called electromagnetic induction. The current produced by moving a straight wire in a magnetic field (or by moving a magnet in a coil) is called induced current.  The Phenomenon of electromagnetic induction was discovered by a British Scientist Michael Faraday and an American scientist Joseph Henry independently in 1831. The process of electromagnetic induction has led to the construction of generators for producing electricity at power stations. A galvanometer is an instrument which can detect the presence of electric current in a circuit.

(ii) To demonstrate electromagnetic induction by using a coil and a bar magnet:  We have fixed coil of wire AB. The two ends of the coil are connected to a current detecting instrument called galvanometer.  When a bar magnet is held standstill inside the hollow coil of wire, there is no deflection in the galvanometer pointer showing that no electric current is produced in the coil. When a bar magnet is moved quickly into a fixed coil of wire AB, then a current is produced in the coil. This current causes a deflection in the galvanometer pointer. The production of electric current by moving a magnet inside a fixed coil of wire is also a case of electromagnetic induction. The condition necessary for the production of electric current by electromagnetic induction is that there must be a relative motion between the coil of wire and a magnet.

(a) A current is induced in a coil when it is moved (or rotated) relative to a fixed magnet.
(b) A current is also induced in a fixed coil when a magnet is moved (or rotated) relative to the fixed coil.
(c) No current is induced in a coil when the coil and magnet both are stationary relative to one another.
(d) When the direction of motion of coil (or magnet) is reversed, the direction of current induced in the coil also gets reversed.
(e) The magnitude of current induced in the coil can be increased:

• By winding the coil on a soft iron core,
• By increasing the number of turns in the coil,
• By increasing the strength of magnet, and
• By increasing the speed of rotation of coil (or magnet).

Fleming’s Right Hand Rule for the direction of induced current

(i) According to fleming’s right hand rule:
Hold the thumb, the forefinger and the centre finger of your right hand at right angles to one another. Adjust your hand in such a way that forefinger points in the direction of magnetic field and thumb point s in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.

Electric Generator

The electric generator converts mechanical energy into electrical energy. A small generator is called a dynamo.

(i) Principle of Electric Generator:  The electric generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor.

(ii) Electric generators are of two types :
(a) Alternating current generator (A.C. generator)
(b) Direct Current generator (or D.C. generator)

(a) A.C. Generator:
• Construction of an A.C. Generator:  A simple A.C. generator consists of a rectangular coil ABCD which can be rotated rapidly between the poles N and S of a strong horseshoe type permanent magnet M. The coil is made of a large number of turns of insulated copper wire.  The two ends A and D of the rectangular coil are connected to two circular pieces of copper metal called slip rings R1 and R2. As the slip rings R1 and R2 rotate with the coil, the two fixed pieces of carbon called carbon brushes, B1 and B2 keep contact with them.  The outer ends of carbon brushes are connected to a galvanometer to show the flow of current in the external circuit.
• Working of an A.C. generator :

Suppose that the generator coil ABCD is initially in the horizontal position.

(i) As the coil rotates in the anticlockwise direction, the side AB of the coil moves down cutting the magnetic field lines near the N pole of the magnet, and side CD moves up, cutting the magnetic field lines near the S-pole of the magnet. Due to this, induced current is produced in the sides AB and CD of the coil. On applying fleming’s right hand rule to the sides. AB and CD of the coil, we find that the currents are in the directions B to A and D to C. Thus, the induced currents in the two sides of the coil are in the same direction. Thus, in the first half revolution of coil, the current in the external circuit flows from brush B1 to B2.

(ii) After half revolution, the sides AB and CD of the coil will interchange their positions. The side AB will come on the right hand side and side CD will come on the left side.  So, after half a revolution, side AB starts moving up and side CD starts moving down.  As a result of this, the direction of induced current in each side of the coil is reversed after half a revolution giving rise to the net induced current in the direction CDAB. The current in the external circuit now flows from brush B2 to B1. Thus, in 1 revolution of the coil, the current reverse its direction 2 times. In this way alternating current is produced in this generator.

(iii) The alternating current (A.C.) produced in India has a frequency of 50 Hz. A.C. generators are used in power stations to generates electricity.

(b) D.C. Generator:

In order to obtain direction current (which flows in one direction only), a D.C. generator is used Actually, If we replace the slip rings of an A.C. generator by a commutator, then it will become a D.C. generator. Thus, in a D.C. generator, a split ring type commutator is used (like the one used in an electric motor). When the two half rings of commutator are connected to the two ends of the generator coil, then one carbon brush is at all times in contact with the coil arm moving down in the magnetic field while the other carbon brush always remains in contact with the coil arm moving up in the magnetic field. Due to this, the current in outer circuit always flows in one direction. So, it is direct current.

Domestic Electric Circuit (or Domestic Wiring)

Electric power is usually generated at places which are very far from the places where it is consumed. At the generating station, the electric power is generated at 11,000 volt (because voltage higher than this causes) insulation difficulties, while the voltage lower than this involves high current). This voltage is alternating of frequency 50 Hz. (i.e. changing its polarity 50 times in a second). The power is transmitted over long distances at high voltage to minimise the loss of energy in the transmission line wires. For a given electric power, the current becomes low at a high voltage and therefore the loss of energy due to heating (= I2 Rt) becomes less. thus, the alternating voltage is stepped up from 11 kV to 132 kV at the generating station (called grid sub station). It is then transmitted to the main sub station. At the main sub station, this voltage is stepped down to 33 kV and is transmitted to the switching transformer station or the city sub station. At the city sub station, it is further stepped down to 220 kV for supply to the consumer as shown in figure. To supply power to a house either the overhead wires on poles are used or an underground cable is used before the electric line is connected to the meter in a house, a fuse of high rating (≈ 50 A) is connected a the pole or before the meter. This is called the company fuse. The cable used for connection has three wires (i) live (or phase) wire, (ii) neutral wire, (iii) earth wire. The neutral and the earth wires are connected together at the local sub station, so the neutral wire is at the earth potential. After the company fuse, the cable is connected to a kWh meter. From the meter, connections are made to the distributions board through a main fuse and a main switch. The main switch is a double pole switch. It has iron covering. The covering is earthed. This switch is used cut the connections of the live as well as the neutral wires simultaneously. The main switch and the meter are locally earthed (in the compound of house). From the distribution board, the wires go to the different part of the house.

Electric Fuse

The electric wires used in domestic wiring are made of copper metal because copper is a good conductor of electricity having very low resistance. If the current passing through wires exceeds this maximum value, the copper wires gets over heated any may even cause a fire. An extremely large current can flow in domestic wiring under two circumstances: short circuiting and overloading.

(i) Short circuiting: If the plastic insulation of the live wire and neutral wire gets torn, then the two wires touch each other. This touching of the live wire and neutral wire directly is known as short circuit.

(ii) Overloading: If too many electrical appliances of high power rating are switched on at the same time, they draw an extremely large current from the circuit. This is known as overloading the circuit. Overloading can also occur if too many appliances are connected to a single socket. A fuse is a safety device having a -short length of a thin, tin plated copper wire having low melting point, which melts and breaks the circuit if the current exceeds a safe value.  An electric fuse works on the heating effect of current. A fuse wire is connected in series in the electric circuit.