Question

The peak value of an AC passing through a 13.0 W device is 3.9 A. What is the rms voltage across it?

Answer 1

Answer: rms voltage is $T_{rms}=2,33 V$

P = power

T = voltage

I = current

In a circuit you have that:

$P_{peak} = T_{peak} * I_{peak}$   (eq 1)

The rms value of a signal X is:

$X_{rms}=X_{peak}/\sqrt(2)$   (eq 2)

From eq 1, $T_{peak} = 13 W / 3.9 A = 3,33 V$

Then, from eq 2,  $T_{rms} = 3,33 V / \sqrt(2) = 2,33 V$