A domain in a ferromagnetic substance is in the form of a cube of side length 2 μm. If it contains 4×10¹⁰ atoms and each atomic dipole has a dipole moment of 16×10⁻²⁴ A m², then the magnetisation of the domain is A m⁻¹.

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(1) 8×10⁴. Given: cube of side a=2 μm=2×10⁻⁶ m, N=4×10¹⁰ atoms, each of moment 16×10⁻²⁴ A m². Idea: magnetisation is net dipole moment per unit volume, M=(mₙₑₜ)/V. Inside one domain every atomic dipole points the same way, so the moments simply add. Total moment: mₙₑₜ=4×10¹⁰×16×10⁻²⁴=6.4×10⁻¹³ A m². Volume: V=a³=(2×10⁻⁶)³=8×10⁻¹⁸ m³. M=(6.4×10⁻¹³)/(8×10⁻¹⁸)=8×10⁴ A m⁻¹.

A domain in a ferromagnetic substance is in the form of a cube of side length 2 μm. If it contains 4×10¹⁰ atoms and each atomic dipole has a dipole moment of 16×10⁻²⁴ A m², then the magnetisation of the domain is ______ A m⁻¹.

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