Calculate the intensity of the Earth's magnetic field

[PRASHANT KUMAR]

Homework Statement

a circular coil of radius 120 mm with 10 turns is placed with its plane parallel to the Earth magnetic field . when a current of 0.45 A flows through the coil , a compass needle placed at the centre of the coil is seen to be deflected by an angle of 45 degree to the plane of the coil . the intensity of the Earth magnetic field is calculated to be?

first i calculated the magnetic moment = 10*pi*(0.12)^2*0.45 unit
then i can calculate the torque = cross product of magnetic moment with magnetic intensity of Earth magnetic field = 0.2035 B unit (where B represents the intensity of Earth magnetic field )
how can i take into account the deflection of the needle by 45 degrees and where will torque act ?

 

[kuruman]

Calculating the magnetic moment of the coil doesn't buy you much. How do you think you are going to use it? It's the needle that turns, not the coil. Instead, consider this: The needle aligns itself with the magnetic field it finds itself in. So initially it points along the Earth's field. When the current is turned on, the needle is at 45^o from its initial direction. What does this say to you about the total field at the needle and the field produced by the coil?

 

[Zachary Kempler]

Sounds like the magnetic field produced from the coil where the compass is placed, is equal to Earth's magnetic field, just perpendicular. This would explain why it would move 45 degrees from its initial position.