SHORT NOTE ON AC DC

Alternating Current

The electrons flow in both directions in a cyclic manner - first one way, then the other.  The rate of change of direction determines the frequency, measured in Hertz (cycles per second).

In electricity, alternating current (AC) occurs when charge carriers in a conductor or semiconductor periodically reverse their direction of movement of current. The voltage in AC circuits also periodically reverses because the current changes direction. Household utility current in most countries is AC with a frequency of 60 hertz (60 complete cycl es per second), In our country it is 230V & 50Hz is the grid range. 

 

AC can be produced using a device called an alternator. This device is a special type of electrical generator designed to produce alternating current. A loop of wire is spun inside of a magnetic field, which induces a current along the wire.

When the voltage reverses, so does the direction of the current flow. The electrons in an AC circuit don't really move along with the current flow. Instead, they sort of sit and wiggle back and forth. They move one direction for 1/60th of a second, and then turn around and go the other direction for 1/60th of a second.

 

Direct Current 

In direct current (DC), the electric charge (current) only flows in one direction.

 

The electrons flow in one direction only.  Current flow is from negative to positive, although it is often more convenient to think of it as from positive to negative.  This is sometimes referred to as "conventional" current as opposed to electron flow. 

 

AC/DC is dangerous

One of the reasons that AC might be considered more dangerous is that it arguably has more ways of getting into your body. Since the voltage alternates, it can cause current to enter and exit your body even without a closed loop, since your body (and what ground it's attached to) has capacitance. DC cannot do that

Voltage: Unit is Volts, Symbol is V or U, old symbol was E . Voltage is the "pressure" of electricity, or "electromotive force" (hence the old term E). 

 

A 9V battery has a voltage of 9V DC, and may be positive or negative depending on the terminal that is used as the reference.  The mains has a voltage of 220, 240 or 110V depending where you live - this is AC, and alternates between positive and negative values.  Voltage is also commonly measured in millivolts (mV), and 1,000 mV is 1V.  Microvolts (uV) and nanovolts (nV) are also used.

Current: Unit is Amperes (Amps), Symbol is I . Current is the flow of electricity (electrons).  No current flows between the terminals of a battery or other voltage supply unless a load is connected. 

 

 

The magnitude of the current is determined by the available voltage, and the resistance (or impedance) of the load and the power source.  Current can be AC or DC, positive or negative, depending upon the reference.  For electronics, current may also be measured in mA (milliamps) - 1,000 mA is 1A.  Nanoamps (nA) are also used in some cases.