Voltage and current: two fundamental principles of the physics of electricity. Since the fathers of electrical science discovered current electricity, man has sought ways to explain its properties. In this article, we will explore the concepts of electric current and electric potential (voltage), as well as draw analogies in order to facilitate understanding.
First of all, let's define electric current. The electric current is the rate at which charge flows. Current is due to the flow of electrons through a conductor, thus current is proportional to the number of electrons that flow past a point on the wire in a given amount of time. However, quite counter-intuitively, the direction of current is defined to be opposite in direction to the direction of electron flow! This is due to a long-held hypothesis that electricity was the flow of positive charge from a positively-charged source to a negatively-charged sink. By the time that the electron - a negatively charged particle - was discovered as constituting electricity, much work had been done with the old assumptions. As positive and negative are just labels, it was decided that the convention of referring to a flow of positive charge would remain.
A good way to visualize the electric current is with the water analogy. Picture water flowing through a pipe of fixed diameter. If you were to ask how much water was flowing through the pipe, the answer would be equivalent to the water current. Similarly, if you were to ask how many electrons were flowing through a wire, the answer would be related to the electric current.
Now let's move on to voltage - a much more abstract concept. Voltage is also referred to as electric potential - it is the electric potential energy per unit charge. It is not, as some students erroneously believe, a form of energy. It is, however, related to energy. Current in a circuit flows because of difference of electric potential. For instance, a 9V battery has an electric potential difference (voltage) of 9V, with the negative terminal assigned a potential of 0V and the positive terminal assigned a potential of 9V. (Remember, electrical concepts are defined in terms of positive charge, although it is electrons that comprise electricity.) Current (the flow of positive charge) then flows from the side of the 9V battery with a high potential (the positive 9V terminal) to the side with the lower potential (the 0V negative terminal). It follows that electrons do exactly the opposite: that is, they travel from the 0V terminal to the 9V terminal. Note that voltage does not depend on how many electrons actually flow, as that would refer to current. Instead, the voltage is referring to how much energy each electron has, relative to the terminals of the power source. An easy way to remember that the amount of electron flow does not affect the voltage is to consider common household 1.5V batteries. These batteries come in a variety of sizes (AA, C, D), and yet all share the same voltage.
A great way to visualize voltage is, once again, to think of the properties of flowing water. One such property is water pressure, which is analogous to voltage. Water pressure, for instance, would be greater if the source of water in a pipe system is located higher-up than the end of the pipe (which we will assume is capped). Likewise, as the top of the pipe is lowered to ground level, the water pressure would decrease. Note that the water pressure does not depend on how much water is in the pipe - we could achieve the very same pressure with less water and a narrower pipe.