Direct Current:

When there is movement of charge carriers in a substance, there is an electric current. The current is measured in terms of number of charge carriers, or particles having a unit electric charge, passing single point in 1 second.

Charge carriers come in two major forms: electrons, that have a unit negative charge, and holes, which are electron absences in atoms and which carry a unit positive charge. The ions can act as charge carriers, and in some situations, atomic nuclei can too. These kinds of particles carry whole number multiples of a unit electric charge. The ions can be positive or negative in polarity, though atomic nuclei are always positive.

Generally, most of the charge carriers go past any given point in 1 second, even when the current is little. In a household electric circuit, a 100 W light bulb draws a current of around 6 quintillion (6 x 10¹⁸) charge carriers per second. Even the smallest mini bulb carries a huge number of charge carriers every second. It is absurd to speak of a current in terms of charge carriers per second; therefore generally it is measured in coulombs per second rather. A coulomb (represented by C) is equivalent to around 6.24 x 10¹⁸ electrons or holes. A current of 1 coulomb per second (1 C/s) is known as an ampere (represented by A), and this is the standard unit of electric current. A 60-W bulb in an ordinary table lamp draws around 0.5 A of current.

Whenever a current flows through a resistance-and this is always the situation, as even the best conductors have resistance-heat is produced. Sometimes visible light and other forms of energy are emitted also. A light bulb is purposely designed so that the resistance causes visible light to be produced. Though, even the best incandescent lamp is inefficient, creating more heat than light energy. The fluorescent lamps are better; they produce more light for a given quantity of current. To put another way, they require less current to give off a certain quantity of light.

In physics, electric current is theoretically considered to flow from the positive to the negative pole. This is known as conventional current. When you connect a light bulb to a battery, thus, the conventional current flows out of the positive terminal and into the negative terminal. Though, the electrons, that are the primary kind of charge carrier in the wire and the bulb, flow in the reverse direction, from negative to positive. This is the way engineers generally think about current.