Contents
Physics Topics such as mechanics, thermodynamics, and electromagnetism are fundamental to many other scientific fields.
What are Magnetic Lines of Force?
Oersted’s Experiment
In 1820, scientist Hans Christian Oersted discovered that a mag-netic field is generated around a current carrying conductor. He inferred this through the following experiment.
A magnetic needle is kept just below a conducting wire stretched along north-south direction. The magnetic needle should be free to rotate about its vertical axis [Fig.(a)]. Now if current is passed through the straight conductor with the help of an external source, the magnetic needle gets deflected, i.e., the nee-dle undergoes an angular deflection θ [Fig.(b)].
Observations related to deflection:
i) As soon as the current stops flowing through the wire, the magnetic needle rotates back to its initial position.
ii) With increase in current through the conducting wire, the angular deflection of the needle increases.
iii) If the direction of current in the conductor is reversed, the magnetic needle deflects in the opposite direction.
iv) If the conducting wire is rotated slowly from its north-south direction while carrying current, the deflection of the mag-netic needle gradually decreases. Ultimately when the direction of current is along east-west, there will be no deflection of the magnetic needle. So, when the conducting wire is placed normal to the axis of the magnetic needle and current is passed through the wire, no deflection of the magnetic needle is observed.
The direction of deflection of the magnetic needle due to flow of electric current through the conductor can be determined with the help of any one of the following two rules :
i) Ampere’s swimming rule: If a man is imagined to be swimming along the direction of current facing the mag-netic needle with his arms outstretched, the north pole of the needle will be deflected towards his left hand [Fig.],
ii) Right hand thumb rule: The right hand, with its thumb sticking out, is held in. Such a way that the conducting wire is in between the palm and the magnetic needle [Fig.]. If the other fingers point the direction of the current then the thumb will indicate the direction of deflection of the north pole of the needle.
The direction of deflection of the magnetic needle is shown in the following table:
Position of Magnetic Needle | Direction of Current | Direction of Deflection of the north pole of magnetic needle |
Below the current carrying wire | From south to north[fig(a)] | towards west |
from north to south [fig.(b)] | towards east | |
from east to west or from west to east [fig(c) and (d)] | No deflection occurs | |
Above the current carrying wire | From south to north [fig.(a)] | Towards east |
From north to south [fig.(b)] | Towards west | |
From east to west or from west to east [fig.(c) and (d)] | No deflection occurs |
Discussions:
- Dependence of magnetic field: At any adjacent point of a current carrying conductor, the magnitude of magnetic field depends on the magnitude of current and the direction of the magnetic field depends on the direction of current and on the position of the point with respect to the current carrying conductor.
- Presence of Insulating material: The magnetic field is not affected if the current carrying conductor is covered with an insulating material.
- Current carrying material: The current carrying conductor itself is not magnetised. If some iron filings are brought in contact with the conductor no attraction is observed.
- Magnetic field due to a moving charged particle: Motion of charged particles is the cause of electric current. Hence, a moving charge can produce a magnetic field around it. Obviously when a charged particle is at rest, it cannot produce a magnetic field.
Mapping of Magnetic Lines of Force due to an Electric Current
Any magnetic field can be represented by magnetic lines of force. The direction of the magnetic field at any point is denoted by the direction of the magnetic line of force at that point.
To determine the direction of the magnetic field at any point around a current carrying conductor, either of the following two rules can be used.
i) Maxwells corkscrew rule: 1f we imagine a right handed corkscrew to be driven along the direction of current in a conductor, then the direction in which it rotates, gives the direction of the magnetic field [Fig.]
ii) Right hand grip rule: If a current carrying conductor is imagined to be held within the grip of the right hand and if the direction of current through the conductor is indicated by the thumb, direction then the other fingers will curl in the direction of the magnetic field [Fig.].
Long straight conductor: A long straight conducting wire carrying current ¡s passed through the centre of a cardboard and the cardboard is held normally to the length of the wire [Fig.]. Some light iron-filings are scattered over the card board. Now if the cardboard is slightly tapped, the iron filings arrange themselves in some concentric circles around the conducting wire.
These concentric circles indicate the magnetic lines of force on a plane perpendicular to the current carrying long straight conductor. With the help of the corkscrew rule, the direction of the lines of force can also be determined. For an upward current, the directions of the magnetic lines of force are shown in the Fig. If the direction of current flow be reversed, i.e., for a downward current, the direction of the lines of force will also be reversed.
In the laboratory, generally a magnetic needle is used instead of iron filings for plotting of magnetic lines of force.
Circular conductor: A circular current carrying conductor is shown in Fig., penetrating a cardboard plate kept perpendicular to the plane of the circular conductor. With the help of iron filings or a magnetic needle, if the lines of force of the magnetic field be drawn on the cardboard, their nature will be just like that in the given figure. The directions of the magnetic lines of force may be determined by the corkscrew rule. If the direction of current in the circular conductor be reversed, the directions of the lines of force will also get reversed. It is to be noted here that, at the centre of the circular conductor the lines of force are almost parallel to the axis going through the centre.