An ac source of period t and maximum voltage v is connected to a single unknown ideal element that is either a resistor, and inductor, or a capacitor. at time t = 0 the voltage is zero. at time t = t/4 the current in the unknown element is equal to zero, and at time t = t/2 the current is i = -imax, where imax is the current amplitude. what is the unknown element?

This satisfy the above given condition so we can say that this capacitor.

Explanation:

Let's take one by one option and check whether is wrong or right

For inductor:

Given that at t=T/4 ,I=0 and we know that

So at T/4

I=0 A

At t=T/2

It means that this not a indutor.

For capacitor:

At T/4, I=0

At t=T/2

This satisfy the above given condition so we can say that this capacitor.

this could be newton#'s law of cooling. the cooling curve is exponential

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.