DC stands for Direct current (DC), a kind of electrical current characterized by the uni-directional flow of electrical current. In direct current, the flow of electricity is caused by electrons moving in one direction, from positive to negative poles. Direct current is opposed to alternating current (AC), a kind of electric current where the direction of electric current periodically alternates. Both AC and DC are used in our everyday electrical appliances (and are also the inspiration for the name of Australian rock band ACDC).
Direct current is useful because it can be used to store large amounts of electrical energy for later use. Most batteries, fuel cells, and solar panels produce DC electricity. DC currents are also mainly used to charge devices because most batteries are built to charge and store direct currents. The first kind of commercially available electricity was direct current, spearheaded by inventor Thomas Edison. Direct currents can also be used to send high amounts of electrical energy far distances with comparatively little energy loss.
Basics Of Electrical Currents
All objects are made out of protons, neutrons, and electrons. Protons have a positive charge, electrons have a negative charge, and neutrons have a neutral charge. Atoms that have an equal amount of protons and electrons are electrically neutral because the positive and negative charges of the protons and electrons cancel each other out. Electrons can be removed from atoms, creating a difference in charge and an electrically charged atom. In some cases, electrons can be removed from one atom and join another, displacing another electron which moves to another atom.
At its core, electricity is this flow of electric charges across the atoms of a conductive material. Electrical currents are the result of the movement of electric charges and the accompanying energy from that movement. The movement of electric charges in conductive materials is due to the delocalized position of electrons. Strictly speaking, electricity is NOT the actual flow of electrons as understood in the ordinary sense of the term, but the flow of charges between electrons.
A simple electric circuit with voltage, current and resistance labeled. Credit: A. Mundt via WikiCommons CC BY-SA 3.0
Any electrical circuit can be characterized and understood in terms of three main quantities: voltage, current, and resistance.
The voltage (V) is the potential energy stored in a circuit in virtue of a concentration of electric charge. Voltage can be understood as the force that pushes electric charges along a wire. When there is a greater concentration of electric charges on one side of the circuit than another, then there is a voltage between those two points. Voltage can be understood as analogous to the potential energy that is the result of the gravitational interaction of spatially separated bodies. When there is a large difference in electric charge between two points, there is said to be a large voltage between those points. 1 volt is defined as the amount of electric potential required to push 1 amp of current through 1 ohm of resistance. In direct current, voltage sources emit a constant voltage source.
The current (I) corresponds to the magnitude of flow of electric charge through a point over a given time. The SI unit for electric current is the ampere. Conventional understanding dictates that the direction of current is from the positive to negative ends. In reality, electric charges flow from the negative end to the positive end, but for ease of notation, it is assumed that current flows from positive to negative ends. The current is then understood as the amount of charge flowing through one point in the circuit per unit time. 1 ampere of current is equal to 1 coulomb of charge per second (1C/s). One coulomb of charge is equal to the electric charge contained in 6.242×1018 electrons, so 1 amp corresponds to the movement of 6.242×1018 elementary electric charges per second. Current can be either positive or negative, corresponding to the direction of the current. Direct currents always have either a positive or negative current. Alternating currents switch between positive and negative.
The resistance (R) of a circuit is a measure of how much that circuit resists the flow of electric charges. Resistance is measured in Ohms (Ω) where 1 ohm equals the resistance between two points on a conductor when 1 volt of difference puts through 1 amp of current through that conductor. Every circuit resists the flow of electricity, and so every circuit has some resistance. “Good” conductors of electricity are materials that have low resistance and so allow electrical flow while “bad conductors have a high resistance and do not allow a good flow of electric charge. Resistors resist the flow of electricity by absorbing the flow of electric charge. This results in a temperature increase in the resistor called Joule heating. The inverse of resistance is called conductance and is a measure of how well a material conducts electricity.
Ohm’s Law
Ohm’s law is a mathematical expression that describes the relationship between the quantities of voltage, current, and resistance in an electrical circuit. In a nutshell, Ohm’s law (named after German physicist Georg Ohm) states that the current passing through two points is directly proportional to the voltage between those two points and inversely proportional to the resistance between those two points. Mathematically, this can be expressed as
I=V/R
Where I is current, measured in amps, V is voltage, measured in volts, and R is resistance measured in ohms. Ohm’s law allows us to extrapolate to unknown details of a circuit if the two other values are known. Rearranging Ohm’s law gives us the equations for finding the voltage and resistance of a circuit:
V = IR
R = V/I
The relationship between the parameters can be remembered with this triangle. Credit: WikiCommons CC0 1.0
Ohm’s law and its accompanying derivative laws can be used to find out missing pieces of circuits. Say we have a 24-volt circuit with 2 ohms of resistance. What is the current in such a circuit? Ohm’s law tells us:
I = V/R
I = 24v/2Ω
I = 12A
A 24-volt circuit with 2 ohms of resistance would have 12 amps of electric current flowing through it. Here is another one: Say we have a 120-volt circuit that is pushing 40 amps of current. What is the resistance of the circuit? Once again, checking Ohm’s law tells us:
R = V/I
R = 120v/40A
R = 3Ω
Such a circuit has 3 ohms of resistance. Any material that follows Ohm’s law is called ohmic. Materials that do not follow Ohm’s law are called non-ohmic.
Lastly, there is the relationship between current and power. Electrical power is the amount of energy a circuit produces and is the product of voltage and current. Electric power tells you how much work the energy produced by circuit can do and is measured watts (joules per second). In general, the power of a circuit can be calculated as:
P = I×V
Since V is equal to I×R, the formula for electrical power can be expressed as:
P = I2R
Essentially this formula tells us that the power a circuit generates is proportional to the square of the current and the resistance. This relationship is the reason we want circuits with small amounts of resistance. All other things being equal, a circuit with less resistance can put out more power than a comparable circuit with more resistance.
Differences Between DC And AC
Now that we have a grasp on the basics of electrical circuits, we can appreciate more fully the difference between direct and alternating currents. In direct currents, electricity is always flowing in one direction, from the positive end of the circuit to the negative end of the circuit. When a magnet is introduced in an electric field, electrons are pushed in one direction by the magnet; this is the basis for the flow seen in a direct current. DC circuits tend to have constant voltages, currents, and resistances that do not change over time. initially, DC current was used as the primary source of electricity as it can send large amounts of electrical energy short distances without much energy loss. DC electricity can also be stored in batteries, so it is the electricity of choice for rechargeable devices like phones, laptops, and other handheld electronics. A graph of the electrical output of a DC circuit looks like a straight line, as the direction of the current stays in a constant direction.
Alternating current, in contrast, is characterized by the direction of current rapidly changing direction. In alternating currents, a rotating magnet is used to change the direction of current through the wires as rotating the magnet changes the orientation of the poles relative to the charged body. The benefit of AC currents is that they can be used to change voltages easily. Devices like transformers on power line change the extremely high voltage AC electricity from the power lines into 240-120v electricity used in most homes. AC power cannot be stored as a charge as DC power can. A graph of the electricity generated by an AC circuit looks like a sine way, as the current periodically reverses direction. AC electricity historically has been more efficient than DC at transporting electrical energy far distances as AC circuits can large voltages in small packets of current. However, recent technological advances have made long-range DC power transmission a viable and possibly more effective alternative to AC transmission.