Moving Charges And Magnetism
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The branch of physics which deals with the magnetism due to electric current or moving charge (i.e. electric
current is equivalent to the charges or electrons in motion) is called electromagnetism.
Magnetic Field
A region of space near a magnet, electric current or moving charged particle in which magnetic effects
are exerted on any other magnet, electric current, or moving charged particle. It is also known as magnetic
flux density or magnetic induction or magnetic field.
Unit : SI : weber/(meter)2 or tesla (T) C.G.S. : gauss (1 Tesla = 104 gauss) Dimensions : [MT–2A–1]
OERSTED'S DISCOVERY AND BIOT-SAVART LAW
* Oersted Discovery
The relation between electricity and magnetism was discovered by Oersted
in 1820. Oersted showed that the electric current through the conducting
wire deflects the magnetic needle held near the wire.
*When the direction of current in conductor is reversed then deflection of
magnetic needle is also reversed.
*On increasing the current in conductor or bringing the needle closer to the
conductor, the deflection of magnetic needle increases. North
S N
Oersted discovered a magnetic field around a conductor carrying electric current. Other related facts are as follows:
Oersted's experiment. Current in
the wire deflects the compass needle
electric charge at rest produce an electric field around it.
(b) A current carrying conductor has a magnetic field and not
an electric field around it. On the other hand, a charge
moving with a uniform velocity has an electric as well as a
magnetic field around it.
(c) All oscillating or an accelerated charge produces E.M. waves
also in additions to electric and magnetic fields.
Current Element dB
A very small element ab of length dl of a thin conductor carrying
current is called current element. Current element is a vector
quantity whose magnitude is equal to the product of current and
length of small element having the direction of the flow of current
* Biot – Savart's Law
With the help of experimental results, Biot and Savart arrived at a mathematical expression that gives the
magnetic field at some point in space in terms of the current that produces the field. That expression is based
on the following experimental observations for the magnetic field dB
Magnetic field lines (By Michael Faraday)
In order to visualise a magnetic field graphically, Michael faraday introduced the concept of field lines.
Field lines of magnetic field are imaginary lines which represents direction of magnetic field continuously
* Magnetic field lines emanate from or enters in the surface of a magnetic material at any angle.
* Magnetic field lines exist inside every magnetised material.
* Magnetic field lines can be mapped by using iron dust or using compass needle.
AMPERE'S CIRCUITAL LAW
Ampere's circuital law states that line integral of the magnetic field
SI=(I -I +I ) 1 2 3
around any closed path in free space or vacuum is equal to m0 times
of the net current or the total current which is crossing through the area
bounded by the closed path. Mathematically ò = m S
r r
Ñ l 0 B.d I
This law is independent of size and shape of the closed path.
Any current outside the closed path is not included in writing the right hand side of the law.
Note : This law is suitable for infinite long and symmetrical current distribution.
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