Hi.
I'm doing some reading on fundamentals of electricity..
DC is straightforward but AC. I have a question.
If I meter my AC outlet at home, I see 120 V from the 'hot' to ground.
If I measure the other to ground.. 0 V...
So my question is, if AC is constantly changing direction... what exactly does that mean?
Is it polarity within the 'hot' wire?

Thanks,
Regards,
David

Thank you.. I did more reading to refresh my memory.. I understand voltage now..
When th mention of direction is made (I guess) they are referring to the wave going from positive to negative.
This is occurs within 1 cycle.. and as such the polarity reverses itself. The direction is really positive to negative.

I just read about current.. and again.. they are referring to it in a 'back and forth' manner.
What are they talking about?

The AC voltage can be expressed as v(t)=120*sqrt(2)*sin(wt)

The sin function is periodic and w (is omega) is a radian frequency = 2*PI*f; PI = 3.14159 and f is 60 Hz for the US.

The SINE function has a minimum of -1 and a maximum of +1, thus the AC voltage goes from about -170 to +170 volts and is periodic. The 120 comes about because of something called the RMS value or root of the mean (Average) of the square of the voltage.

The RMS value also makes ohms law works for resistive loads.
120 VDC applied to a 1 ohm resistor will generate 120 Watts and 120 VrmsAC will also generate 120 Watts.

So between two measurement points, Hot and neutral you have 120 VAC. In the US, we distribute power for residences as a center-tapped secondary of a transformer. The center tap is grounded (connected to earth) and you end up with two 120 V legs that are 180 degrees out of phase, thus they sum to 240 volts.

So, between hot and neutral = 120 V
between hot and ground = 120 V
between hot and neutral ~ 0V

Businesses generally get 3 phase power.

Amplitude - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Amplitude)

Not sure this is too much to comprehend or not. It is a special case. To measure something, you need a reference voltage.

Simple meters are called average responding True Rms reading. They rectify the AC voltage creating a pulsating sine wave. They average with a small capacitor and scale such that when a sine wave is applied, you read 120V.

When something else is, all bets are off and the voltage has an innacurately measured RMS value. Meters that usually measure correctly are called TRMS or True RMS meters.

In a resistive load Voltage and Current are "in phase". When the voltage is going up, the current is going up. In other words the waveform has the same shape just different amplitude and units.