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# Torque on current loop in a uniform magnetic field

**Torque on current loop in a uniform magnetic field:**

As shown in fig. Consider a rectangular coil PQRS suspended in a uniform magnetic field , with its axis perpendicular to the field.

Let I = current flowing through the coil PQRS

a, b = sides of the coil PQRS

A = ab, area of the coil

θ = angle between the direction of B and that of the vector drawn normal to the plane of the coil.

Now, a current carrying conductor experiences a force in a magnetic field.

therefore force on side PQ

Its magnitude is

f_{1} = laB sin(90-0) = IaB cosθ

therefore force on side QR

Its magnitude is

f_{2} = lbB sin90 = IaB

therefore force on side RS

Its magnitude is

F_{3} = laB sin(90+0) = IaB cosθ

therefore force on side SP

Its magnitude is

F_{4} = lbB sin90 = IbB

The magnitude of the torque is

= lbB * a sinθ = IBA sinθ

Where m = lA = magnitude of the magnetic dipole moment.

In vector form

In rectangular loop has N turns, the torque increases N times

τ=NBIAsinθ