Magnetic Field and neutral points

[prasannab13]

If Earth's magnetic field is considered as "B" ,then it has two components along horizontal and vertical.But while determining null or neutral points of a bar magnet only horizontal component of magnetic field is taken into consideration but not vertical component of magnetic field.Why?

 

[springBreeze]

If Earth's magnetic field is considered as "B" ,then it has two components along horizontal and vertical.But while determining null or neutral points of a bar magnet only horizontal component of magnetic field is taken into consideration but not vertical component of magnetic field.Why?

The reason Earth's magnetic field has a horizontal and vertical component is because it is oriented at angle with regard to the bar magnet in most places on Earth. Generally, you choose the reference frame to be that of the plane of the lab bar magnet. Of course you can also do it the other way and say that the lab bar magnet has a horizontal and vertical component while Earth magnet is purely in one direction but that makes things more complicated.

In the UK, the field lines lie about 70 degrees with regard to the horizontal. So when you conduct an experiment to measure the Earth's magnetic field by finding the null points between bar magnet and Earth magnet, you are only finding the horizontal component of the Earth's magnetic field.

This brings up another question of why you do not consider the magnetic fields that "leap out" of the flat plane of the bar magnet. The reason for that is because in most cases you can consider the bar magnet to be sufficiently flat that its magnetic field lies only in one plane.