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

cant without the picture! ?

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.