Magnetic Effects of Electric Current

FOREVIEW

This chapter is concerned chiefly with magnetic field and the entire syllabus is covered under the following heads.

  • Magnetic field and Magnetic field lines
  • Magnetic field around a straight conductor carrying current
  • Magnetic field due to a current carrying circular coil
  • Magnetic field due to a current in a Solenoid
  • Electromagnet and permanent magnet
  • Force on a current carrying conductor in a magnetic field
  • Electric motor
  • Electromagnetic Induction.
  • Direct and Alternating Current
  • Electric Generator
  • Overloading and Short Circuiting.

EXPOSITION OF THE SUBJECT MATTER

Magnetic Compass: It is a compact of magnetic needle which is pivoted at the centre of a small brass box with glass top. It is used to

  1. To find the magnetic north-south direction.
  2. To find the direction of magnetic field at a place and
  3. To test the polarity of a magnet.

Magnetic field: It is the space around a magnet in which the force of attraction or repulsion due to the magnet can be detected. It has both magnitude and direction.

Sources of magnetic fields :

  • Natural and artificial magnets
  • Electro magnets
  • A conductor, a coil and a solenoid carrying current.
  • Earth.

Magnetic field lines: It is the curved paths along which the iron filings arrange themselves due to the force acting on them in the magnetic field of the bar magnet.

Magnetic Flux: It is the number of magnetic lines of force passing through the given area. Properties of Magnetic field lines:

  1. They start from the north pole of a magnet and end at its south pole (outside the magnet).
  2. They are always normal to the surface of the magnet.
  3. They are closed and continuous curve.
  4. Two lines of force do not intersect one another. If they intersect at a point, it would mean that compass needle will point towards two directions at that point which are not possible.
  5. They come closer to one another near the poles of a magnet but they are widely separated at other places.

OERSTED EXPERIMENT:

Oersted observed that when a magnetic needle is brought near the current carrying conductor, he observed that it undergoes deflection and also observed that when the direction of current is reversed, direction of deflection is also reversed.

Observation: -

  • The North Pole of the needle is deflected towards east when current flows from North to South. (Fig a)
  • The North Pole of the needle is deflected towards the west when the current flows from South to North. (Fig b)
  • There is no deflection in the needle if no current is passed.

The direction of deflection is given by Ampere's swimming rule.

Ampere's swimming rule : -
Imagine a man swimming along the conductor, the direction of current is from feet to head, looking at the needle, and then the north pole of the needle is deflected towards his left hand.

Magnetic field around a straight conductor carrying current :-
The magnetic field around a current carrying straight conductor consists of concentric circles of magnetic lines of force lying in a plane, which is right angle to the current carrying conductor. The conductor acts as the centre of magnetic lines of force. These lines are crowded near the conductor and become farther apart as the distance from the conductor increases. This indicates magnetic field near the conductor is stronger and becomes weaker as the distance from the conductor increases.
The magnitude of magnetic field produced by a straight current carrying wire at a given point is:

  1. Directly proportional to the current passing in the wire, and
  2. Inversely proportional to the distance of that point from the wire.
Magnetic field (µo I) / (2 π r)
o = Permeability of free space (constant)mWhere
I = Current flowing through the wire
r = radius of the circular wire
The direction of magnetic field is given by right -Hand Thumb Rule.
* Diagram – Refer NCERT Text Book

Right hand Thumb Rule:-
If a straight conductor is held in right hand, such that thumb point along the direction of the current, then the tips of the finger show the direction of magnetic field or magnetic lines of force. This is known as the Right -Hand Thumb Rule. This rule is also called Maxwell’s Corkscrew Rule.

According to this rule, if we imagine a right handed screw placed along the current carrying conductor, be rotated such that the screw moves in the direction of flow of current, then the rotation of the thumb gives the direction of magnetic lines of force.

Magnetic field due to a current carrying circular coil :-
In order to find the magnetic field due to a coil, it is held in a vertical plane and is made to pass through a smooth cardboard in such a way that the centre (O) of the coil lies at the cardboard. A current is passed through the coil and iron fillings are sprinkled on the cardboard. These iron filings arrange themselves in a pattern similar to one shown in the figure. (REF TEXT)

Conclusion :-

  • The magnetic field lines near the coil are nearly circular and concentric.
  • The field lines are in the same direction in the space enclosed by the coil.
  • Near the centre of the coil, the field lines are nearly straight and parallel.
  • The direction of magnetic field at the centre is perpendicular to the plane of the coil.

The magnitude of magnetic field at the centre of the coil is

  1. Directly proportional to the current ( I ) flowing through it.
  2. Inversely proportional to the radius ( r )of the coil
  3. Directly proportional to the total number of turns (N) in the coil. o I ) / ( 2r )mMagnetic field, B = (N) 

The direction of magnetic field is given by right hand thumb rule.

Solenoid : It is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
Magnetic field due to a current in a Solenoid: The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet and the polarities of its ends depend upon the direction of current flowing through it.

Determination of polarities of a current carrying solenoid : Place it in a brass hook and suspend it with a long thread so that it moves freely .Bring north pole of bar magnet near one of its ends. In case the solenoid moves towards the bar magnet that end of the solenoid is a south pole and in case the solenoid moves away from the bar magnet that end of the solenoid is its north pole. The polarity of the other end of the solenoid can similarly be determined.

β The polarity of a solenoid can also be determined with the help of a Clock Rule.

The anti clockwise current in a face of the solenoid gives north polarity and clockwise current gives south polarity.
The lines of magnetic force pass through the solenoid and return to the other end as shown in figure. If a current carrying solenoid is suspended freely, it comes to rest pointing north and south acts like a suspended magnetic needle .One end of the solenoid acts like a N-pole and the other end a S-pole. Since the current in each circular turn of the solenoid flows in the same direction, the magnetic field produced by each turn of the solenoid adds up, giving a strong resultant magnetic field inside the solenoid. The strength of magnetic field produced by a current carrying solenoid depends on:

β α The number of turns per unit length in the solenoid i.e B η
β  Iα The strength of current in the solenoid i.e B
β The nature of core material used in making solenoid m ai.e B
β o η Iμ Magnetic field B =

 

CBSE Magnetic Effects of Electric Current Class X ( By Mr. Basant )
Email Id : basant.physics@gmail.com

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