Current in general is the flow of any material, like water, from one point to another. Assume there are two containers connected together with a channel and they are filled with water. When one container is higher than the other one, the water in it will have a higher potential energy. Therefore the potential energy of the water turns into the kinetic energy through the open channel, as shown in the figure below:
The intensity of the current is simply known as the volume of the flowing material in time, the more material flows in a amount of specific time, the more is the current intensity. These also apply to the electrical current. The electrical current is made of electrons flowing in a closed circuit from a point with a higher electrical potential (voltage) to the other. The volume of electrons is measured by their electrical charge. Therefore we can rephrase the definition, saying that the electrical current is made of electrical charges flowing in a closed circuit.
The value of current or the current intensity is equal to the volume of the flowing electrical charge in the unit of time, second, as shown in the formula below:
were I is the electrical current, which we refer to as Current, q is the positive electrical charge in Coulomb (the electrical charge unit) passing in the time t with the unit of seconds. If any two of the parameters are known, the other one can be calculated using the formula above.
The direction of current is assigned to the flow of positive charges in the circuit. But the only flowing particles are electrons that generate current having a negative charge. You may wonder why scientists use the flow of the positive electrical charges while they are not moving at all! It's quite simple: years ago when scientists were developing formulas for their experiments, they didn't know about the electrons. They only knew about charges and therefore they randomly assigned the current flow direction to the positive charges. Afterwards when they knew about electrons, it didn't really matter in the final results which direction they used. Because they only required to change the parameter sign in every formula. Therefore to be compatible with the previous formulas, which were already settled in the scientific world, they didn't change the direction. You can simply assume that when the negative charge is flowing in one direction, the positive charge is virtually flowing in the reverse direction.
AC and DC
You often hear about AC and DC when discussing about electronics. They stand for: Alternative Current (AC) and Direct Current (DC). But what do they mean? Assume the containers in the above figure, with an infinity amount of water in the top one. What happens is that the water keeps running continuously with a constant intensity. This is called a Direct Current (DC). Same way in electronics, when the constant amount of charge flows in one second, it results in a constant electrical current, which is called a Direct Current (DC).
But assume that the position of the containers is not constant. Assume we put them on two ends of a seesaw and they are swinging up and down. What happens is that the current would flow in both directions alternatively depending on which container is higher, generating an Alternative Current (AC) as shown in the figure below. In the same fashion, when the electrical charges go back and forth in a circuit they produce Alternative Current (AC). For more information about the waveforms and signals please read the section about Signals.
Although the AC and DC terms are based on current changes, they are used for any kind of signal as all signals basically have one of these components or both. I said both because they can be both present at the same time. Back to our containers, assume that the containers are still swinging, yet one of them is always or most of the time higher than the other one. Therefore although the current magnitude is going up and down, the current is always flowing to one side, or more of the current is flowing to one side and less to the other side. This is were the AC and DC terms are added together or, the AC signal is riding on a constant DC level. Diagram below shows a sinusoidal AC signal and a DC signal and their sum.
A battery is a voltage source and has a DC voltage, meaning that its output voltage level is constant. Yet a battery is not an ideal source because it loses its charge and its output voltage drops after a while of usage. But the voltage coming from a power plug is a very good source of AC voltage. Don't touch it!
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