NEET Physics Notes Magnetism-The Earth’s Magnetism
The Earth’s Magnetism
The Earth’s Magnetism
Angle of Declination (α)
At a given place, the acute angle between the magnetic meridian and the geographical meridian is called the angle of declination (or magnetic declination) (α) at that place.
Angle of Inclination or Dip (δ)
The angle of dip (δ) at a place is the angle which the direction of the earth’s total magnetic field BE subtends with the horizontal direction.
Horizontal Component of the Earth’s Magnetic Field (Bh)
As earth’s magnetic field, in general, is inclined at an angle (δ) with the horizontal direction, it may be resolved into horizontal component BH and a vertical component
Neutral Points
- A neutral point is a point at which the resultant magnetic field is zero. Following two cases are of special interest.
- When a bar magnet is placed along the magnetic meridian with its North pole pointing towards geographic North, two neutral points are obtained on either side of the magnet along its equatorial line. If r be the distance of the neutral
point, then
- When a bar magnet is placed along the magnetic meridian, with its North pole pointing towards the geographic South, two neutral points are obtained on either side of the magnet
along its axial line. Hence, we have
Tangent Galvanometer
It is an instrument to measure electric current. The essential parts are a vertical coil of conducting wire and a small compass needle pivoted at centre of coil. The deflection, θ of needle is given by 
Important Terms Used in Magnetism
- Magnetic Induction or Magnetic Flux Density (B)
Whenever a piece of magnetic substance is placed in an external magnetising field, the substance becomes magnetised. If B0 is the magnetic field in free space, then on placing the given magnetic substance at that place, the magnetic field changes from B0 to B, where 
is magnetic flux which is equal to fi0 minside- where minside is the net pole strength inside a close surface.
For a dipole, Gauss’s law for magnetism is 
for a closed surface.
- Magnetic Permeability (µ)
It is the degree or extent to which the magnetic lines of induction may pass through a given distance.
Magnetic permeability of free space (µ) has a value of 
. However, for a material substance, absolute permeability (µ) has a value, different than (µ)0. For any magnetic substance 
relative magnetic permeability of that substance. Relative magnetic permeability (µ) is a unitless and dimensionless term. For magnetic substance like iron (µ)r > 1000 less than 1 (µ)r < 1, but it can never be negative. Thus, 0 ≤ (µ)r < 1 for a diamagnetic material.
- Intensity of Magnetisation (I)
Intensity of magnetisation of a substance is defined as the magnetic moment induced in the substance per unit volume, when placed in the magnetising field.
Thus 
It is a vector and its SI unit is is Am -1.
- Intensity of Magnetising Field or Magnetic Intensity (H)
It is a measure of the capability of external magnetising field to magnetise the given substance and is mathematically defined as
Magnetic intensity H is a vector and its SI unit is Am -1.
- Magnetic Susceptibility
Magnetic susceptibility of a substance is the ratio of the intensity of magnetisation I induced in the substance to the magnetic intensity H.
Thus, It is a scalar term and has no units or dimensions.
Relation 