Magnetism

Magnetic Effect Of Electric Current

• A current carrying conductor produces a magnetic field around it. In other words, electric current can produce magnetism. This Phenomenon is called 'magnetic effect of electric current.

Experiments:

• Take long insulated copper wire and wind it around  the large  iron nail to form many closed turns .

•  connect the ends of the wire to a battery. The large iron nail  now become magnet and attract tiny iron nails towards it . This has happened because an electric current flowing in the wire has produced a magnetic field which has turned the large iron nail into a magnet.

• The current-carrying straight electric wires (like an electric iron connecting cable) do not attract the nearby iron objects towards them because the strength of magnetic field produced by them is quite weak.

Magnet

• A magnet is an object which attracts pieces of magnetic materials like iron, steel, nickel and cobalt.

• Magnets come in various shapes and sizes depending on their intended use. One of the most common magnets is the bar magnet.

Bar Magnet

• A bar magnet is a long, rectangular bar of uniform cross-section which attracts pieces of iron, steel, nickel and cobalt.

Characterstics  Of  Magnet

• A magnet has two poles near its ends : north pole  and South pole .  

• The end of a freely suspended magnet which points towards North  direction of earth is called the north pole of the magnet .

• The end of a freely suspended magnet  which points towards the south direction of earth  is called the south pole of the magnet.

• like magnetic poles repel each other whereas unlike magnetic poles attract each other.

Uses Of Magnet :

• Magnets are used for a variety of purposes.

• Magnets are used in radio, television, and stereo speakers.

• Magnets are used in refrigerator doors, in audio and video cassette tapes.

•  Magnets are used in hard discs and floppies for computers, and in children's toys.

• Magnets are used in motors , Alternator.

• The Magnetic Resonance Imaging (MK) technique which is used to scan inner human body parts in hospitals also uses magnets .

• Magnets are used  for making electric generators and electric motor.

Magnetic Field

• The region around a magnet in which magnetic force is exerted on piece of magnetic material , is called a magnetic field.

• When A compass needle placed near a magnet , it gets deflected due to the magnetic force exerted by the magnet, and the iron filings also cling to the magnet due to magnetic force.

Magnetic Field Lines:

• The curve drawn around the magnet along which a hypothetical north or south  magnetic pole move is called Magnetic Field Line.

• The magnetic field lines are also known as magnetic lines of force. It is vector quantity.

Properties of Magnetic Field Lines:

• Outside the magnet ,magnetic field lines always emerge from the N-pole and end on the S-pole of the magnet . Inside the magnet the direction of magnetic field lines are from the S-pole  to the N-pole of the magnet. Ie Each  magnetic field line form closed loop .

• The magnetic field lines come closer to one another near the poles of a magnet but they are widely separated at other places.
• When  magnetic field lines are closer together, it indicates a stronger magnetic field. On the other hand, when magnetic field lines are widely separated, then it indicates a weak magnetic field.

•  The direction of magnetic force on a north pole placed  at any point around the magnet show  the direction of  magnetic field line at that point.

• No two field lines intersect each other. If they intersect then at point of intersection magnetic field show two direction which is not possible.

Plot Magnetic Field Lines Pattern of Bar Magnet:

• To Plot the Magnetic Field Pattern Due to a Bar Magnet by Using Iron Filings, place a card (thick, stiff paper) over a strong bar magnet .

• Sprinkle a thin layer of iron filings over the card with the help of a sprinkler, and then tap the card gently.

• The iron filings arrange themselves in a regular pattern.

This happens as follows :

• The bar magnet produce a magnetic field all around it. The iron filings experience the force of magnetic field of the bar magnet. The force of magnetic field of bar magnet makes the iron filings to arrange themselves in a particular pattern.

•  Under the influence of the magnetic field of the bar magnet, the iron filings behave like tiny magnets and align themselves along the directions of magnetic field lines.

Earth Magnetism :

• A freely suspended magnet or magnetic needle always points in the north-south direction .

• It indicate that  earth itself behaves as a magnet .

• The shape of the earth's magnetic fields resembles that of an imaginary bar magnet buried at its center whose  length is one-fifth of earth's diameter.

• The south pole of earth's magnet is in the geographical north because it attracts the north pole of the suspended magnet. Similarly, the north pole of earth's magnet is in the geographical south because it attracts the south pole of the suspended magnet.

• The axis of earth's magnet and the geographical axis do not coincide with each other. The axis of earth's magnetic field is inclined at an angle of 15⁰  with the geographical axis.

• It is  believed that the earth's magnetism is due to the magnetic effect of current (which is flowing in the liquid core at the centre of the earth). Thus, earth is a huge electromagnet.

MAGNETIC  EFFECT  OF  CURRENT  (OR ELECTROMAGNETISM)

• The magnetic effect of current was discovered by Oersted in 1820.

• Oersted found that a  current carrying conductor  was able to deflect a compass needle. 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 produce magnetic field around it.

•  The electric motor, electric generator, telephone and radio, all utilize the magnetic effect of current.

• The magnetic effect of current is also called electromagnetism which means electricity produces magnetism.

Experiment to Demonstrate the Magnetic Effect of Current

• Take a thick insulated copper wire and fix it in such a way that the portion AB of the wire is in the north-south direction . A plotting compass M is placed under the wire AB. The two ends of the wire are connected to a battery through a switch. When no current is flowing the wire AB, the compass needle is parallel to the wire AB .

• On passing the current that compass needle is deflected from its north-south position. And when the current is switched off, the compass needle returns to its original position.

• If we reverse the direction of electric current flowing in the wire AB by battery connections t the compass needle is deflected in the opposite direction. This shows that when we reverse the direction of electric current, then the direction of magnetic field produced by it is also reversed.

Magnetic 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.

• current-carrying conductors having different-shapes produce Different magnetic field patterns .

Magnetic Field Pattern due to Straight Current-Carrying Conductor:

• The magnetic field lines around a straight current carrying conductor are concentric circles whose centers lie on the conductor.

• When current in the wire flows in the upward direction then the lines of magnetic field are in the anticlockwise direction.

•  When current in the wire flows in the downward direction then the lines of magnetic field are in the clockwise direction

• the magnitude of magnetic field produced by a straight current- carrying conductor  at a given point is :

(i)           Directly proportional to the current passing in the wire

(ii)          Inversely proportional to the distance of that point from the wire.

• So, greater the current in the wire, stronger will be the magnetic field produced. And greater the distance of a point from the current-carrying wire, weaker will be the magnetic field produced at that point.

• Direction of magnetic field produced by a straight current- carrying conductor  can be known by Maxwell right hand rules.

Maxwell's right- hand thumb rule

• According to Maxwell's right- hand thumb rule : grasp  the current-carrying conductor  in right hand so that  thumb points in the direction of current, then the direction in which  fingers encircle the conductor  will give the direction of magnetic field lines around the conductor  .

• Maxwell's right-hand thumb rule is also known as Maxwell's corkscrew rule .

• 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.

2. Magnetie Field Pattern due to a Circular Loop (or Circular Wire) Current Carrying  conductor :   

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

• The magnetic field lines are circular near the current-carrying loop.

•  As we move toward centre of circular loop  the concentric circles representing magnetic field lines become bigger and bigger.

• At the centre of the circular loop, the magnetic field lines are nearly straight .

• Each segment of circular loop carrying current produces magnetic field lines in the same direction near the centre of loop.

• At the centre of the circular loop, all the magnetic field lines are in the same direction and aid each other, due to which the strength of magnetic field increases .

• The magnitude of magnetic field produced by current-carrying circular loop at its centre is :

•         directly proportional to the current passing through the circular loop

•         inversely proportional to the radius of circular loop wire .

• The strength of magnetic field can be increased by taking a circular coil consisting of a number of turns of insulated copper wire closely wound together.

• Thus, if there is a circular coil having n turns, the magnetic field produced by this current-carrying circular coil will n times as large as that produced by a circular loop of a single turn of wire.