When most people think about electricity, they think about direct current or DC, which is electricity flowing in a constant direction with a constant voltage and a constant polarity. Batteries, for example, with definite positive and negative terminals, supply direct current. Electrons flow from the negative side of the battery to the positive side of the battery and perform some kind of work on their way there that makes the battery-powered appliance worthwhile.
Direct currents are very impossible to the functioning of our electricity-powered society today, but direct currents aren’t the only currents that we use; not by a long show. Many sources of electricity such as rotary electro-mechanical generators naturally produce voltages that alternate in polarity, meaning that over time they switch periodically between positive and negative. Whether the voltage switches polarity or the current switches back and forth, this type of electricity is called Alternating Current or AC.
Let’s back up for a minute and define some important key terms.
An electric current is a flow of electric charge. This often happens when electrons move through a wire carrying a charge.
Voltage is an electromotive force or potential difference expressed in volts. the greater the voltage, the greater the flow of electrical current, i.e. the the greater the quantity of charge carriers that pass a fixed point per unit of time.
Electric polarity (which can either be positive or negative) is present in every electrical circuit. Electrons flow from a negative pole to a positive pole. In a direct current, one pole is always positive and the other pole is always negative. With an alternating circuit, the polarity of the poles alternates.
The battery symbol is generally used to delineate a direct current, while a circle with a wavy line inside of it usually means AC.
So why use AC instead of DC? It sounds more complicated, what with the changing poles. In some cases, it doesn’t even make a difference. For example, if you’re using electricity to create heat, the polarity of direction of the electric current simply doesn’t make a difference; there jus needs to be enough voltage and current to the load to produce the desired head you want. It’s a matter of power dissipation.
However, if you want to create electric generators, motors and power distribution systems, you can make far more efficient ones with AC than with DC.
To explain this, I’ll first have to give some background on how motors can be built.
With AC, you can create a machine in which a magnetic field is rotated around a set of stationary wire coals by turning a shaft. This then produces AC voltage across the wire coils. This is the basic principle behind creating an AC generator also known as an alternator.
DC generators work in a less efficient and stable way in which the coil itself spins and carbon bushes contact copper strips on the rotating shaft. The need to make and break electrical contact with a moving coil creates sparking and heat, which makes DC motors much less reliable.