Write Biot-Savart law.
Write the path of motion of an electron when it enters in magnetic field at `(a)` perpendicular `(b)`an angle `theta`.
or
Define angle of dip. Find the value of dip angle of a any point for which the horizontal components and vertical component of earth's magnetic field are equal.

Explanation

Biot-savart's law (`vecB` due to a wire) ,
It is a experimental law. A current `i` flows in a wire (may be stright or curved).Due to `dl` length of the wire the magnetic field at `P` is
`dB prop idl`
`prop 1/r^(2)`
`rArr dB prop (idl sin theta)/r^(2)`
`dB=(mu_(0)/(4pi))("idl" sintheta)/r^(2) rArrvec(dB)=(mu_(0)/(4pi))(vec"(idl)"xxvecr)/r^(3)`
here `vecr`= position vector of the test point w.r.t `vec(dl) theta` = angle between `vec(dl)` and `vecr`.
The resultant `vecB=intvec(dB)`
`vecv||vecB` here `theta=0` or `theta=180^(@)`
therefore `F_(m)=0` , therefore `veca=0` , therefore `vecV=const.`
`therefore` particle will move in a straight line with constant velocity Initial velocity `vecu_|_vecB` and `vecB="uniform"`.In this case `therefore B` is in `z` direction so the magnetic force in `z`-direction will be zero `(because vecF_(m)_|_vecB)`.
Now there is no initial velocity in `z`-direction.
`therefore` particle will always move in `xy` plane.
`therefore` velocity vector is always `_|_vecB`
`therefore F_(m)=qupsilonB=`constant
now `quB=("mu"^(2))/(qB)=` constant
The particle moves in a curved path whose radius of curvature is same every where, such curve in a plane is only a circle.
`therefore` path of the particle is circular.
or
Inclination or Angle of Dip `phi` :It is the angle which the direction of resultant intensity of earth's magnetic field subtends with horizontal line in magnetic meridian at the given place.Actually it is the angle which the axis of a freely suspended magnet (up or down) subtends with the horizontal in magnetic meridian at a given place.Angle of dip at a place is measured by the instrument called Dip-Circle in which a magnetic neddle is free to rotate in a vertical plane which can be set in any vertical direction.
`B_(H)=B_(1)cos phi`
and `B_(y)=B_(1) sin phi`
For `B_(H)=B_(V) rArr phi=45^(@)`