What is Current?

Current is defined as the flow of electric charge.

When using the water tank analogy, current is represented by the flow of the water in the pipes. With water, we would measure the volume of the water flowing through a point in the pipe over a certain period of time. So with electricity, current is a measure of the amount of charged particles that pass a point per a unit of time. The unit value given to charge in regards to electric current, is 6.241*10^18 electrons (1 Coulomb) per second passing through a point in a circuit. This amount of charge is referred to as an Ampere, the unit for electric current. Amps are represented in equations by the letter I. The letter I was used by André-Marie Ampère, which originates from the French phrase intensité du courant, or current intensity, after whom the unit of electric current is named.

Here we have two pipes of the same diameter, therefore the same resistance, with water flowing through them. There is more water flowing through one pipe than the other so it has more current over time. 

The same is true when talking about a wire, the total available charge will flow through the entire cross-sectional area of a conductor and "fill up" and that area. If less charge is available for the same gauge wire, the charges will have more space to flow and not fill the entire cross-sectional area so it would have less current.

DC Current Flow

In the 1700's when Benjamin Franklin was doing experiments with the mysterious (at the time) force, called electricity, very little was known. Franklin had a convention of assigning positive charge signs to the “things” that were moving in a circuit doing work. Franklin also assigned the convention that charge flowed from positive to negative- this is called Conventional Current Flow.

It turns out that Franklin was wrong, at least about the sign of the charges that move. Later, a physicist named Joseph Thomson performed an experiment that isolated the charges. He found that they were moving in the opposite direction of conventional current, the convention that Franklin surmised earlier. Also, we know that electrons are free to move and protons are not from other experimental evidence.

The truth is that electron flow is technically correct – current flows from negative to positive in conductors. However, there are things such as plasma and certain liquids in which positive charge can flow but that is a different page. 

Conventional Current Flow

Conventional current flow is the one most often used. It states that electrons flow from positive to negative. In the Gif above, we can see the current moving from the positive terminal of the battery through the load to the negative terminal of the battery. This illustrates conventional current flow.

Electron Flow Theory

Electron flow is simply the opposite of conventional current flow. Current flows from the negative side of the battery through the load to the positive side rather than vice versa. In the Gif above, we can see the current moving from the negative terminal of the battery through the resistance to the positive terminal of the battery. This illustrates the electron flow theory.

So why do we still mostly use conventional current flow if it’s wrong?

It turns out that it really doesn’t matter which convention you use if you’re consistent. Since electrons and protons have an equal but opposite charge, an electron flowing in one direction is equal to a proton flowing in the opposite direction. Therefore, we can still use conventional current flow even though it’s technically wrong and come up with the right answer.

In fact, most formulas used in electronics such as Ohm’s Law "pretend" that current flows from positive to negative.

Annotating a Circuit

A current in a wire or circuit element can flow in either of two directions. When defining a variable I to represent the current, the direction representing positive current must be specified, usually by an arrow on the circuit schematic diagram. This is called the reference direction of the current I. When analyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown until the analysis is completed. Consequently, the reference directions of currents are often assigned arbitrarily. When the circuit is solved, a negative value for the current implies the actual direction of current through that circuit element is opposite that of the chosen reference direction.

AC Current Flow

Another type of current flow is Alternating Current. Unlike Direct current where the electrons flow in one way or another depending on what convention you are following, Alternating Current moves in both directions. Alternating current describes the flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current. Alternating current will be kept at a stable frequency and altitude to ensure stable power delivery. 

Types of Current

Beyond Alternating Current and Direct current there are still many other types of current. Looking at a graph, the absolute value of amplitude is the total voltage while its direction moving positive or negative on the y-axis is its flow while moving through time, represented by the x-axis.

Direct Current

DC, flows in one direction in a circuit and stays level at a stable voltage.

Alternating Current

AC, travels in equal and opposite directions in a circuit. Alternating current flows a stable frequency but variable voltage.

Pulsating Current

Typically Alternating current that never drops below a positive threshold. This can be done with a diode rectifier. Pulsating Current simulates a DC output with an AC input. 

Variable Current

Happens when you do not have a steady power source. This is typically bad for electronics and is most often not intentional. 

This is what a traditional incandescent looks like in slow motion while using Alternating Current. Note how the filament is flickering- that is the variable voltage that happens with AC. This is because according to the graph above, for a brief moment voltage is actually at zero. Remember, current is the flow of electric charge which means the electrons need to be moving. When the electron switches directions to head the other way, it passes its origin location and therefore hasn't "moved."  Usually this happens so fast that the human eye never notices this. The same thing happens with all the things that run off Alternating Current but we never notice because the devices never turn off long enough before they turn back on. 

The Current Wars: Thomas Edison and Nikola Tesla

With the invention of the electric lightbulb, America raced to build the infrastructure to send power to our homes and businesses to power it. There are many types of current but when the United States was accepting bids to build their electric grid, there were two front runners: DC pioneered by Thomas Edison and AC pioneered by Nikola Tesla and George Westinghouse. 

Direct Current: Thomas Edison

In the late 1800s, DC could not be easily converted to high voltages. As a result, Edison proposed a system of small, local power plants that would power individual neighborhoods or city sections. Power plants would need to be located within 1 mile of the end-user. This limitation meant that you would need a small power plant at the end of every city street and this made power distribution in rural areas extremely difficult, if not impossible. Now we see DC in portable electronics and modern personal home grids through the use of Solar Panels.

Alternating Current: Nikola Tesla

With Tesla’s patents, Westinghouse worked to perfect the AC distribution system. Transformers provided an inexpensive method to step up the voltage of AC to several thousand volts and back down to usable levels. At higher voltages, the same power could be transmitted at much lower current, which meant less power lost due to resistance in the wires. Too high of current in long transmission lines will also result in the power lines heating up and melting at some point. This is why AC made perfect sense for large power plants far away from communities with high voltage transmission lines dispersing it to homes and businesses. As a result, large power plants could be located many miles away and service a greater number of people and buildings.

Ways to Generate Direct Current


Batteries generate a level voltage of direct current using chemical reactions. Batteries can be built to have different voltages, current production, and power densities. 

When graphing DC, there should be a straight line showing a steady flow of current over time. The voltage is represented by the amplitude which is distance away from the origin line.

DC Generator

In a direct current electric generator, there are permanent magnets on the outside casing that pick up a magnetic field as the center inductor is spinning. The stator is the part of the generator that does not move and the rotor is the part of the generator that spins (or rotates.) In the example to the left, the stator are the permanent magnets and the rotor is the spinning inductor. The Rotor rotates and as it spins between the north and south poles the polarity of the current flow flips. However, the contacts that create a with the load, are spinning with the rotor so they swap at the same time the current flips which keeps the current flowing in only one direction.


Rectifiers are an extremely common electronic device that we use daily but don't even know it. Rectifiers are devices that convert low voltage AC into low voltage DC. These are in our cellphone chargers to convert the AC from the wall to the DC our phone batteries need to charge. 

Ways to Generate Alternating Current

Alternating Current Generator

In an alternating current electric generator, there are inductors on the outside casing that pick up a magnetic field as the center magnet is spinning. The stator is the part of the generator that does not move and the rotor is the part of the generator that spins (or rotates.) In the example to the left, the stator are the inductors and the rotor is the spinning magnet. 

When generating current, the rotor is spun at a specific RPM where the inductor generates the AC Sine wave. As the north end of the magnet approaches the inductor the sine wave increases in the positive direction and as it moves away in decreases towards zero. Then as the south end of the magnet approaches the indictor, the sine wave continues to decrease into the negative direction until the south pole moves away and moves back towards zero. This repeats over and over creating the frequency of the sine wave. 

To get more consistent power output, you can add inductors to the generator to add phases. This keeps the average amplitude high keeping the average voltage higher and increasing power. 




Current Flow Circle - AC vs. DC

Objective: This activity is designed to help participants understand the difference between Alternating Current (AC) and Direct Current (DC) flow through a fun and interactive circle game.

What you'll need:

Sheets of Paper crumpled into balls up to number of people participating


Set Up:

Introduce AC and DC:

Hand Out the Paper Balls:

The Activity:

For AC Flow:

For DC Flow:

Discuss and Wrap Up:

This activity allows participants to experience and visualize the concepts of AC and DC current flow, making it a memorable and engaging learning experience.