Electromagnetism

Jul 25, 2022, 16:45 IST

Introduction of Electromagnetism

A substance which attracts small pieces of iron and points in North-South direction when suspended freely (or hanged freely) is known as a magnet. A flat piece of this ore, when suspended freely comes to rest along North-South direction. These special properties of Magnetite are termed as magnetism. This iron ore (i.e. magnetite) is an iron oxide whose chemical formula is Fe3O4. This natural magnet of dark blackish brown colour is also known as Black stone or Lodestone or leading stone or Kissing stone. Natural magnets are irregular in shape. Moreover, they are weak magnets. An iron bar can be made a magnetic by rubbing it with a natural  magnet. Such a magnet is known as man-made or artificial magnetic.

Magnetic Field and Field lines

Magnetic compass

When a bar magnet is suspended freely from its center by a thread, one end of the magnet points towards the north of the earth and the other end of the magnetic points towards the south of the earth. The end of the magnet is pointing toward north of the earth is called north seeking pole or simply north pole of the magnet. On the other hand, the end of the magnet is pointing towards south of the earth is called south seeking pole or simply south pole of the magnet.Like magnetic poles repeal each other while unlike magnetic pole attract each other.The force  with which a magnetic pole of a magnet attracts or repels another magnetic pole of another magnet is called magnetic force. The space or region around a magnetic within which magnetic force is exerted is called the magnetic field of the magnet.The space or region around a magnet (or a current carrying wire) within which its influence can be felt (or magnetic force is experienced) by another magnet is called magnetic field. It is denoted by B.

Magnetic Lines of force (or magnetic field Lines )

The path (straight or curved) along which unit north pole moves in a magnetic field (if free to do so) is called magnetic line of force or magnetic field line. Magnetic liens of force or magnetic field lines are helpful to show the direction and strength (or intensity) of a magnetic field.If magnetic field lines are very close to each other in a particular region. Then the strength of the magnetic field in that region is very large. On the other hand, if the magnetic field liens are far from each other in a particular region. Then the strength of the magnetic field in that region is very small.

Properties of magnetic liens of force or field lines

Properties of lines of force

(i) They are closed, continuous curves.

(ii) Magnetic lines of force start at the north pole and end at the south pole outside the magnet. Inside the magnet, the lines of force move from the south pole to the north pole.

(iii) Two lines of force can never intersect each other.

(iv) They are crowded near the poles.

(v) The strength of the field is more where the lines of forces are crowded.

(vi) The lines of force have longitudinal tension and lateral pressure.

(vii) Number of magnetic lines of force per unit area gives the intensity of the magnetic field at a point.

Type of Magnetic fields

Uniform magnetic field 

Magnetic field is said to be uniform if its magnitude is equal and direction is same at very point in the space.

Non-Uniform magnetic field 

Magnetic field is said to be non-uniform magnetic field if its magnitude is not equal or direction is not same at very point in the space.

Magnetic field due to a current through a straight conductor 

When a straight conductor carries electric current (dc) a magnetic field is set up around the conductor. This magnetic field causes the deflection in the needle of the magnetic compass.

1.Any current carrying wire produces a magnetic field around it, as it can deflect a magnetic needle placed near it.

2.The intensity of the magnetic field is proportional to the magnitude of the current passing through the wire.

3.The magnetic field setup acts at right angles to the direction of the flow of current.

4.The direction of the magnetic field depends upon the direction of the flow of current.

The direction of the magnetic field produced due to a current carrying wire may be determined using any one of the following rules.

Ampere’s swimming rule

Imagine a man swimming along the conductor in the direction of current, facing a magnetic compass kept near and below the conductor. Then the north pole of the needle will be deflected towards his left hand.

Ampere’s right hand thumb rule

Imagine you are holding the current carrying wire with your right hand, with the thumb pointing the direction of current. Then the direction of fingers encircling the wire show the direction of magnetic field.

Max well screw rule.

Imagine a right handed cork screw held by your hand. Rotate the screw's head such that its tip advances in the direction of the current. Then the direction in which the head rotates gives the direction of the magnetic field.

 Direction of current

Factors on which the intensity (or strength) of the magnetic field due to a current carrying conductor depends. Magnetic field due to a current through a circular wire or loop.The magnetic field around a straight current carrying conductor or wire can be increased by bending the wire into a circular loop. A circular wire is made up of large number of very-very small straight wire. 

Direction of Magnetic field due to a current carrying circular wire or loop

Right Hand Thumb Rule is used to determine the direction of the magnetic field produced due to a current carrying circular wire.The direction of the magnetic field at the center of the circular wire is perpendicular to the plane of the circular wire.

Uses of electromagnets

1.They are used to lift heavy iron places, They are fitted on cranes lifting heavy masses of scrap iron.

2.They are used in many devices like electric bell, electric horn, telephone receiver, electric relay, microphones, radio set, television, loudspeakers etc.

Force on a current carrying conductor placed in a magnetic field.

When a current carrying conductor placed in a magnetic field, the conductor experiences a force. Direction of force experienced by a current carrying conductor placed in a magnetic field is given by Fleming’s left hand rule.

Fleming’s left hand rule. 

Stretch the left hand such that the thumb, first finger and the central figure are mutually perpendicular to each other. If the first finger points in the direction of the magnetic field and he central finger points in the direction of current, then the thumb will point in the direction of Motion (or Force) as shown fig. above.Factors on which the force acting on the current carrying conductor depend. The force acting on a current carrying conductor placed in the magnetic field depends upon (i)The strength of the magnetic field

(ii)The strength of the electric current.

(iii)The length of the conductor.

F The force acting on a current carrying conductor placed perpendicular to the magnetic field depends upon (i) the strength of the magnetic field (b), (ii) the amount of current (I) flowing through the conductor and (iii) the length (I) of the conductor.

Magnetic Effects of Current

(i) H.C. Oersted: He first proposed experimentally that a magnetic field is developed around a current carrying conductor. He showed it by deflection of a magnetic needle placed near a closed circuit, having current.

(ii) Maxwell cork-screw rule or right hand thumb rule:  He gave the rule for finding the direction of the magnetic field around a current carrying conductor. He stated that when you hold a conductor in your right hand and if the thumb of your hand indicates the direction of flow of current through a conductor then curvature of rest of the fingers indicates the orientation and direction of magnetic field developed due to the flow of current.

(iii) Fleming Left hand Rule: Stretch the forefinger, the central finger and the thumb of your left hand mutually perpendicular to each other. If the forefinger shows the direction of the magnetic field and the central finger that of the current, then the thumb will point towards the direction of motion of the conductor.

(iv) Fleming’s Right Hand Rule: Stretch out the forefinger, the middle finger and the thumb of the right hand such that these are mutually perpendicular to each other. If the forefinger shows the direction of the magnetic field, thumb shows the direction of motion of the conductor then the middle finger gives the direction of current produced in the conductor (Induced current).

ELECTRIC MOTOR AND ELECTROMAGNETIC INDUCTION

An electric motor is a device for converting electric energy into mechanical energy. Thus, an electric motor is the reverse of an electric generator. There are two types of electric motors:

(i) AC motor 

(ii) DC motor.

Principle of Electric motor: When a conductor or rectangular coil carrying current is placed in between a magnet, the conductor experiences a force and moves. Thus, its converts electric energy to mechanical energy.

Electric Generator: A device which converts mechanical energy into electrical energy is called an electric generator.

Principle of Electric generator – Electromagnetic induction: When a conductor/rectangular coil is kept in between a magnet and when there is a relative motion between them, there is a change in magnetic flux and thus current is induced. Thus, a generator changes mechanical energy to electrical energy.

Reasons of short-circuiting

Short-circuiting happens due to :

(a) Damage to the insulation of the power-lines

(b) A fault in an electric appliance due to which current does not pass through it.

Consequences of short-circuiting

On account of short-circuiting, resistance of the circuit decreases to a very small value and consequently the current becomes very large. This large current results in heating of live wires, which produces sparking at the point of short-circuiting. This sparking sometimes causes fire in a building. (Apart from short-circuiting, the increase in current in the circuit and consequent heating may also be due to overloading of the circuit).

Electric Fuse: A Safety Device

An electric fuse is a device, which is used in series to limit the current in an electric circuit so that it easily melts due to overheating when excessive current passes through it. A fuse is a wire of a material with very low melting point.

Electric fuse can avoid incidents like electric shock, fire, damage to an electric appliance due to :

Short-circuiting or

Overloading (withdrawing current beyond a specified limit) in a circuit.

Earthing

Many electric appliances of daily use like electric press, toaster, refrigerator, table fan etc. have a metallic body. If the insulation of any of these appliances melts and makes contact with the metallic casing, the person touching it is likely to receive a severe electric shock. This is due to the reason that the metallic casing will be at the same potential as the applied one. Obviously, the electric current will flow through the body of the person who touches the appliance. To avoid such serious accidents, the metal casing of the electric appliance is earthed. Since the earth does not offer any resistance, the current flows to the earth through the earth wire instead of flowing through the body of the person. More over, due to very low resistance (almost nil) offered by the earth wire, the current in the circuit rises to a very high value, thereby melting fuse in that circuit and cutting off its electric supply.

Role of Magnetism in Medicine and Organisms

In our body, small electric current travels along the nerve cells due to ions. This current produces a very weak magnetic field (about one billionth time weaker than the Earth's magnetic field) in our body. Heart and brain are the two main organs in our body where this magnetic field is quite significant. The magnetic field in our body enables us to obtain the images of its different parts by using a technique called MRI (Magnetic Resonance Imaging). On analysing the images obtained through MRI, we are able to make a medical diagnosis, e.g., location and size of a tumour in brain etc. Thus, magnetism plays an important role in modern medical science.Apart from this, there are certain organisms, which have the ability to sense Earth's magnetic field and travel from one place to another. For example, some type of fishes are able to detect magnetic field by using special receptors whereas in certain organisms, crystals of magnetite enable to move along the Earth's magnetic field.