Iron has positive susceptibility copper has negative susceptibility negative susceptibility of a substance signifies that the substance will be repelled by a strong magnet such a substance is diamagnetic.

One ampere is defined as that constant current when it is passed through each of the two infinitely long parallel straight conductors kept side by side parallely at a distance of one meter apart in vacuum causes each conductor to experience a force of 2 x 10^-7 newton per meter length of conductor.

If we hold the current carrying conductor in our right hand such that the thumb points in the direction of current flow, then the fingers encircling the wire points in the direction of the magnetic field lines produced.

Magnetic field is the region or space around every magnet within which its influence will be felt by keeping another magnet in that region.
\(\vec { B } =\frac{1}{q_m}\vec{E}\)
Its unit is N A^-1m^-1

The angle between magnetic meridian at a point and geographical meridian is called the declination or magnetic declination (D).

There are three quantities required to specify the magnetic field of the earth on its surface, which are often called as the elements of the earth's magnetic field. they are
(a) Magnetic declination (D)
(b) Magnetic dip or inclination (I)
(c) the horizontal component of the Earth's magnetic field (B_H)

Hysteresis is the phenomenon of lagging of magnetic induction behind the magnetising field.

Biot-Savart's law states that, the magnitude of magnetic field \(d\vec { B } \) at a point P at a distance of r from the small elemental length taken on a conductor carrying current varies
(i) directly as the strength of the current I
(ii) directly as the magnitude of the length of element \(\vec { dl } \)
(iii) directly as the sine of the angle θ between \(\vec { dl } \) and \(\hat { r } \).
(iv) inversely as the square of the distance r between the point P and length of element \(\vec { dl } \).
\(d\vec { B } =\frac { { \mu }_{ 0 } }{ 4\pi } \frac { I\vec { dl } \times \hat { r } }{ { r }^{ 2 } } \)

Coulomb's inverse square law states that the force of attraction or repulsion between two magnetic poles is directly proportional to the product of their pole strengths and inversely proportional to the square of the distance between them.
\(\vec { F } =k \frac { { q }_{ m_{A} }{ q }_{ m_{B} } }{ { r }^{ 2 } } \hat { r } \)

Magnetic flux is defined as the number of magnetic field lines crossing per unit area kept normal to the direction of lines of force.

(i) When a substance is placed in a uniform magnetising field, the substance gets magnetised.
(ii) The total magnetic field inside the specimen is equal to the sum of the magnetic field produced in vacuum due to the magnetising field and the magnetic field due to the induced magnetism of the substance.

It is defined as the deflection produced per unit voltage applied across the galvanometer.
V_s =\(\frac { \theta }{ V } \)
V_s = \(\frac { \theta }{ IR_{ g } } =\frac { NAB }{ KR_{ g } } =V_{ s }=\frac { 1 }{ GR_{ g } } =\frac { { I }_{ s } }{ { R }_{ g } } \)

(i) Obey inverse square law, so they are long-range fields.
(ii) obey the principle of superposition and are linear with respect to source in magnitude,
E ∝ q
B ∝ Idl

The magnitude of the reverse magnetising field for which the residual magnetism of the material vanishes is called its coercivity.