# Homework Help: Alternating Current (multiple CHoice)

1. Dec 2, 2011

### BSElectrician

1. The problem statement, all variables and given/known data
An alternating voltage is given by $$v = 30\sin(314t)$$.The time taken by the voltage to reach 30V for the first time is:
A. 0.02s
B. 0.03s
C. 0.1s
D. 0.015s

2. Relevant equations
google for equation for instantaneaous current and voltage

3. The attempt at a solution
30 = 30sin(314t)
1 = sin(314t)
pi/2 = 314t
t = 0.005s
not in the choices

EDIT
30 = 30sin(314t - pi)
pi/2 = (314t - pi)
t = 0.015 from the choices

why do i need to subtract 180 degrees inside the sine eqn instead of adding
( Formula for n repeating cycles is v(t + nT) )

Last edited: Dec 2, 2011
2. Dec 2, 2011

### Staff: Mentor

It would appear that the question is incorrect. Starting from time t = 0, the first positive peak occurs at approximately 5ms (milliseconds), which is what you first calculated.

By the way, is that the complete statement of the question in its original form?