# AC Circuit: Finding I(t) general question

1. Jan 29, 2013

### charlies1902

When you're asked to find I(t) in an AC circuit, what form does I(t) need to be in?
I remember from a long time ago that when the '(t)' is included you have to have it in a specific form.

2. Jan 29, 2013

### Simon Bridge

The correct form is the one which gives a predictions consistent with measurements.

Usually, when the "(t)" (means "function of time") is included, the current is called i(t) and not I(t). Caps are reserved for constants as in $i(t)=I\cos(\omega t)$.

The exact form that the function i(t) has will depend on the circuit and the driving function ... which, in context, would be given as v(t). The function i(t) will be the solution to the differential equation modelling the system.

For sinusoidal driving functions, you'd have solutions in the form of complex exponentials which may be convenient to write down as trig functions. Impulses, though, produce transients - and, with feedback, the functions can be quite complicated.

3. Jan 29, 2013

### charlies1902

Thank you.

In the book I'm using, they used capital I(t).

4. Jan 30, 2013

### Simon Bridge

Yeh well - it's not compulsory.
Technically you can use any letters you like, as long as you are consistent (or signal your inconsistencies).

The first sentence is the bottom line though.
With the caveats that (a) other people have to be able to understand it, and (b) the examiner has to recognize it as the correct answer for you to get the marks in an exam.