Answer
The given question is to determine earth's magnetic field.
When a current I amp. flows through the wire a magnetic field B
is developed.
This magnetic field is perpendicular to horizontal component of
earth's magnetic field H.
If a compass needle is placed in the external magnetic field B ,
it experiences a resultant force and lies at an angle
with the horizontal component of earth's magnetic field.
According to Tangent law B = Htan
And H = B / tan.
Calculating the external magnetic
field B .
We can use Biot Savartt law to calculate magnetic field due to
the current.
According to Biot Savartt law , the magnetic field B =
0 I / 2π R.
Where
0 = permeability of vacuum.
0
= 1.256 * 10-6
I = current , the given Value = 1.98 A.
And R = distance between the wire and the point where the field
is calculated.
R = 7.0 cm., Given in the problem.
R = 0.07 m.
Substituting the given values for calculating magnetic
field,
B = 1.256 * 10-6 x 1.98 / 2* 3.14 * 0.07
B = 5.6569 * 10-6
Calculating earth's magnetic
field.
Applying Tangent law B = H tan
The angle at which the compass needle makes with horizontal
component of earth's magnetic field , the given Value =
450
Hence the earth's magnetic field H = B/ tan
H = 5.6569 * 10-6 / tan45 .
The value of tan45 = 1.
Hence the earth's magnetic field
H= 5.6569 * 10-6 T.