Final answer:
The magnitude of the current density across a1 is 0.165 A/mm².
Step-by-step explanation:
The magnitude of the current density across a1 can be calculated using the formula J = I/A, where J is the current density, I is the current, and A is the cross-sectional area. In this case, the current is given as 0.87 A and the cross-sectional area is A = 5.26 mm². Plugging in these values, we get:
J = 0.87 A / 5.26 mm² = 0.165 A/mm²
Therefore, the magnitude of the current density across a1 is 0.165 A/mm².