2. A constant voltage is applied to a series RL circuit by closing a switch. The voltage across L is 30 volts at t = 0 and drops to 6 volts at t = .0025 sec. If L = 0.2 H, what must be the value of R?

Explanation:

Resistance R= ?

For RL series circuit

let the voltage across inductance L in the RL circuit be

V₁ = 30 volts at time t₁= 0

the voltage drops to

V₂= 6 volts at time t₂ = 0.0025 sec

the equation to calculate R is

V₂ - V₁ = exp(-(t₂ - t₁ ) R/L)

or

L ln( V₂ - V₁ )= - (t₂ - t₁)R

R = L ln( V₂ - V₁ )/ (t₁ - t₂)

R= 0.2 x ln(30- 6)/ 0- 0.0025 = -2542.44 Ω

attach the statements so i can answer : )