# Basic RC/RL circuit question

Discussion in 'Homework Help' started by agroom, Feb 7, 2011.

1. ### agroom Thread Starter Active Member

Oct 15, 2010
60
1
I'm pretty new to electronics, I'm now in AC/DC 2 at the local tech school and we're going over RL and RC circuits. I have the math down to calculate reactance and impedance, I'm just having a difficult time conceptualizing how this actually works though.

The biggest issue is the lead/lag of voltage and current and the phasor or wave diagrams that represent this. I can't really grasp the concept of how current and voltage can lag or lead in a circuit. From AC1 you learn that current is constant through the circuit, so I would think current couldn't do that. I think since it's AC Current though, this is what's throwing me off.

For capacitors, I can kind of understand how voltage lags through them. When voltage enters the component, it takes some amount of time to charge it before it continues on its way. If that's even correct?

I've read through the AC Resistor and inductor circuits pages here, but it's the same confusing language I get in most places. Is there somewhere that describes this in easier to understand terms?

2. ### bob smith112 New Member

Nov 14, 2010
8
0
AC quantities like voltage and current. Alternating current has no set "polarity" as direct current does. In DC circuits, we denote the polarity of voltage sources using "+" and "-" symbols.

In AC circuits we don't deal with "negative" quantities of voltage. Instead, voltage aids or opposes another by phase: the time-shift between two waveforms. We never describe an AC voltage as being negative in sign, because the facility of polar notation allows for vectors pointing in an opposite direction. If one AC voltage directly opposes another AC voltage, we simply say that one is 180o out of phase with the other.

When it comes to AC circuits we are dealing with Dynamic quantities because they are alternating in direction and amplitude as the rotor spins in the stator of the generator in the power station. Frequency, phase shift and amplitude are dimensions that are taken into account.
AC circuits can be analysed using phasor diagrams incorporating vectors and complex numbers. When drawing phasor diagrams
· The greater the amplitude of the waveform, the greater the length of its corresponding vector.

also http://en.wikipedia.org/wiki/Three-phase_electric_power theres a moving diagram that shows how ac flows back and forth through the cables

hope this helps