10 The magnitude J of the current density in a certain lab wire with a circular cross section of radius R = 2.00 mm is given by J = (3.00 × 108 )r 2 , with J in amperes per square meter and radial distance r in meters. What is the current through the outer section bounded by r = 0.900R and r = R?

I = 0.002593 A = 2.593 mA

Explanation:

Current density = J = (3.00 × 10⁸)r² = Br²

B = (3.00 × 10⁸) (for ease of calculations)

The current through outer section is given by

I = ∫ J dA

The elemental Area for the wire,

dA = 2πr dr

I = ∫ Br² (2πr dr)

I = ∫ 2Bπ r³ dr

I = 2Bπ ∫ r³ dr

I = 2Bπ [r⁴/4] (evaluating this integral from r = 0.900R to r = R]

I = (Bπ/2) [R⁴ - (0.9R)⁴]

I = (Bπ/2) [R⁴ - 0.6561R⁴]

I = (Bπ/2) (0.3439R⁴)

I = (Bπ) (0.17195R⁴)

Recall B = (3.00 × 10⁸)

R = 2.00 mm = 0.002 m

I = (3.00 × 10⁸ × π) [0.17195 × (0.002⁴)]

I = 0.0025929449 A = 0.002593 A = 2.593 mA

Hope this Helps!!!

I'm not too sure about this but unit is definitely one of the answers. If "number" refers to (for example) the "6" in 6 inches then it should also count. I think "symbol" could also just be unit so I'm not sure about that.