Given B₁ = 4 mT.

Exercice I: Magnétisme

Step 1.a: Determine the direction and magnitude of $\vec{B_1}$.
Magnet (1) has its North pole (red) to the right. Point M is to the right of magnet (1). The magnetic field lines exit the North pole. Therefore, $\vec{B_1}$ is directed horizontally to the right.
Given $B_1 = 4 \text{ mT}$.
Scale: $2 \text{ mT} \to 1 \text{ cm}$.
The length of $\vec{B_1}$ is $4 \text{ mT} \times \frac{1 \text{ cm}}{2 \text{ mT}} = 2 \text{ cm}$.

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