Question

HOW Did they get answer to part b???? Having trouble understanding what is a circular band.

D5.1. Given the vector current density J = 10ρ2zaρ − 4ρ cos2 φ aφ mA/m2:
(a) find the current density at P(ρ = 3, φ = 30◦, z = 2); (b) determine the
total current flowing outward through the circular band ρ = 3, 0 < φ < 2π,
2 < z < 2.8.
Ans. 180aρ − 9aφ mA/m2; 3.26 AD5.1. Given the vector current density J = 10ρ2zaρ − 4ρ cos2 φ aφ mA/m2:
(a) find the current density at P(ρ = 3, φ = 30◦, z = 2); (b) determine the
total current flowing outward through the circular band ρ = 3, 0 < φ < 2π,
2 < z < 2.8.
Ans. 180aρ − 9aφ mA/m2; 3.26 A

Answer 1

Here goes the solution, I hope on time
The secret is to understand what the differential of area is: dS

Answer 2

o.o