Required Practicals 10–11: Force and Flux Linkage

Place a magnet assembly on a top-pan balance and arrange a supported straight wire in the approximately uniform gap, perpendicular to the field. The force on the wire has an equal and opposite reaction on the magnets, changing the balance reading. Convert the change in mass reading to force with Δmg.

Vary current for fixed field and wire length in the field, then plot force against current: gradient is BL. Vary effective wire length at fixed current for a second test; compare calibrated field strengths at fixed I and L where apparatus permits. Zero the balance before each set and keep leads mechanically separate from the balance. Reverse current to check that the force reverses. Limit heating and keep magnetic objects clear.

A coil of N turns and area A in a uniform field has flux linkage NBA cosθ, where θ is between the field and the normal to the coil. Rotating the coil or changing the field induces an emf detected by an oscilloscope. When the coil is stationary in a steady field, there can be flux linkage but no induced emf.

One practical approach records the emf pulse while the coil is moved from a chosen angle to a reference orientation outside the field. The signed time integral of emf gives the change in flux linkage. Repeat the same geometry for several starting angles and compare the inferred flux linkage with cosθ. If only pulse height is used, motion speed must be reproducible and its limitation explained.

Set voltage and time scales so the entire transient is recorded without clipping, note the zero level and estimate pulse area using a numerical sum or geometric approximation. Reversing the movement reverses pulse polarity. Leads should not form a large unintended pickup loop.

If a rotating coil is used instead, a sinusoidal emf has amplitude NBAω. At fixed ω the signal can be compared with angle and the expected sine variation of emf, while the underlying flux linkage varies as cosine. State whether the angle refers to the coil plane or normal; those definitions differ by 90°.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.