Force on a current-carrying conductor
Section: Magnetic Fields | Syllabus: Cambridge AS Level Physics 9702
The Motor Effect A current-carrying conductor placed in an external magnetic field experiences a force, provided the current is not parallel to the field. Fleming's Left-Hand Rule Thumb: Motion (Force) First Finger: Field (N to S) Second Finger: Current (Conventional, + to -) Magnetic Flux Density (B) Magnetic flux density is a measure of the strength of the magnetic field.
F = BIL θ Where θ is the angle between the conductor and the magnetic field lines. Definition of B Magnetic flux density (B) is the force acting per unit current per unit length on a wire placed at right angles to the magnetic field.
Unit: Tesla (T). 1 T = 1 N A^-1 m^-1. Special Cases θ = 90^: Max force, F = BIL. θ = 0^ (Parallel): Zero force, F = 0. Experimental Skill: Current Balance This experiment demonstrates the relationship between Force and Current (F I).
A stiff wire is held fixed between the poles of a magnet which rests on a top-pan balance. When current flows, the wire experiences a force (e.g., Upwards). By Newton's 3rd Law, the magnet experiences an equal and opposite force (Downwards), causing the balance reading to increase .
Diagram: Current Balance Show a wire passing between magnet poles on a balance. Current I, Force on wire F_wire, Force on magnet F_magnet. Worked Examples Worked Example Question: A 5.0 cm wire carries 3.0 A at 30^ to a 0.1 T field.
Calculate the force. Solution: F = BIL θ = 0.1 × 3.0 × 0.05 × (30^) = 7.5 × 10^-3 N.
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