Answer:
The current in the filament is 0.81 A
Explanation:
Given :
Diameter of wire
m
Diameter of filament
m
Current density 
From the formula of current density,

Where
area =
Current in the filament is given by,

Area of filament is given by 
Put value of A in above equation,

A
Therefore, current in the filament is 0.81 A
Answer:
The answer is 'D' none
Explanation:
In the figure shown we have

From the figure we can see that

Thus value of tension be will be

Answer:
R = 5.20 Ω, L = 6.24 10⁻⁴ H
Explanation:
The current in an RL circuit is
I = 
τ = L / R
In the problem they indicate the value of the voltage, the current and the time constant, for which the resistance must be found
The stable current is when enough time has passed (t »τ) after closing the circuit, therefore the exponential term is very small and we can neglect it.
I = E / R
R = E / I
let's calculate
R = 13.0 / 2.50
R = 5.20 Ω
now with this value we can find the inductance of the coil
τ = L / R
L = τ R
L = 1.20 10⁻⁴ 5.2
L = 6.24 10⁻⁴ H