IB DP Physics Mock Exam HL Paper 1B Set 2 - 2025 Syllabus

(a)
The students should choose the separation \(r\). This separation produces the most uniform magnetic field between the coils, as indicated by the flat region of the field curve, ensuring a nearly constant field across the magnet.

(b)
The axis XX′ is aligned east–west so that the magnetic field produced by the coils (\(B_{\text{coils}}\)) is perpendicular to the Earth’s horizontal magnetic field (\(B_{H}\)), which lies in the north–south direction. This perpendicular arrangement allows the fields to be combined vectorially to determine \(B_{H}\).

(c)
From the vector relationship, with \(B_{\text{coils}}\) along XX′ and \(B_{H}\) perpendicular to it:
\[\tan(24^{\circ}) = \frac{B_{H}}{B_{\text{coils}}}\]
From the graph corresponding to separation \(r\), the magnetic field at the centre is approximately \(7.5 \times 10^{-5}\,\text{T}\).
\[B_{H} = (7.5 \times 10^{-5}) \times \tan(24^{\circ}) \approx 3.3 \times 10^{-5}\,\text{T}\]