Problem solving tips
3 min read

## Magnetism And Matter

- Want to score better marks in exams? Have a look at some problem solving tips.
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Tip 1: Field due to a bar magnet
What will be the field around a bar magnet of pole strength m and length 2l?
At an axial point the field due to a bar magnet is given by the resultant of field due to North Pole, and field due to South Pole,
For a small bar magnet, l<
where is the magnetic moment()
At an equatorial point, the field is given by,
for small bar magnet, l<
Now, How will you find the field die to a bar magnet at any arbitrary point?
Let's simplify the steps to find the field due to a bar magnet.
• Determine the distance r at which the field is supposed to be calculated. is the vector between the point P and the center of the bar magnet making an angle with the magnet.
• Resolve magnet N-S into N'-S' and N"-S" so that we can assume P to be in axial position for N'-S' and in equatorial position for N"-S".
• The magnetic moment can be resolved as along the axial line, N'-S'. Similarly, magnetic moment can be resolved as along the equatorial line of N"-S"
• Field due to N'-S' can be written as,
Field due to N"-S" can be written as,
• Net field can then be found as a resultant of and
A bar magnet has pole strength and is placed as shown in Fig. (a). Find the magnetic field at .
A
B
C
D
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Tip 2: Earth's Magnetism
To find the resultant magnetic field at a region on Earth, one needs to specify three elements, also called the elements of magnetic field.
At a place the value of and are  and respectively. The resultant earths magnetic field is :
A
B
C
D
Horizontal component of the earth's field is   and of dip is . Magnetic field of the earth at that place is
A
B
C
D
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Tip 3: Neutral Points for a bar magnet
A neutral point for a bar magnet is a point where the magnetic field vanishes. The location of neutral point for a magnet depends on the orientation of the magnet as at neutral points the field due to the magnet and the horizontal component of earth's field are equal and opposite. Check out these tips to determine the neutral points of a magnet.

Two short bar magnets of magnetic moments and are placed with their like poles facing each other. If the distance between the centres of the magnet is . The distance of neutral point from the weaker magnet is

A
B
C
D

A short bar Magnet with its north pole facing North forms a neutral point P in the horizontal plane. If the magnet is rotated by 90 in the horizontal plane, the net magnetic induction at P is:

(Horizontal component of earths magnetic field is )

A
0
B
C
D