Voltage and Current

Part 1: Fundamental Concepts: Voltage and Current

A collection of electrical components that does something useful is referred to as a circuit.  Why does the word ‘circuit’ refer to an electrical device?  It has to do with a fundamental principle in electronics. The word ‘circuit’ is defined as movement that starts and finishes at the same place.  In an electrical device the definition is the same; the word circuit refers to something that moves in a closed path or loop.  In an electrical circuit, the thing that is moving is an electrical charge.  Electrical charge in a circuit is carried by electrons.  So, fundamentally, we are talking about the way in which electrons move in a loop inside an electrical device.

When you turn on a light, listen to a radio, or power any electrical device, some of the electrons which are part of the atoms that physically make up the radio, flashlight, or whatever the device may be, are put into motion.  The electrons inside the device are literally flowing around inside of it, moving from atom to atom in the circuitry of the device. The term given to the moving electrons in a circuit is current. 

Knowing how much current is flowing in a circuit is useful.  Fortunately, the way a current is measured is easy to visualize.  Knowing that an electrical current is composed of electrons moving through a circuit, we can quantify, or put a value on, the amount of current flowing by counting the number of electrons that pass by a point in the circuit in a given period of time.

The figure below illustrates the concept of counting the electrons that are flowing in a circuit.  In this figure, imagine you are watching a portion of the circuit you are interested in.  If it were somehow possible to see them, you could count the number of electrons that pass a particular point in the circuit each second.

Figure 1.  Counting electrons.

Figure 2. Electrical charge field representation.

Electric fields have a direction.  The convention is to view something positively charged as having electric field lines that point outwards.  Something negatively charged has field lines pointing inwards.  When two positively charged items are brought together, they push apart.  The same is true of two negative charges, they also repel each other.  A positive charge and a negative charge attract each other.  Think about playing with a couple of magnets; like poles push apart, unlike poles pull together.  As is the case with magnets, there is a physical force between electric charges.  This is shown in the following figure.

Figure 3. Attraction and repulsion.

 Most of us are familiar with this.  Think of clothes clinging when they come out of a dryer or a comb attracting your hair.  The clothes, or your hair, feel a force because they have obtained a charge.  In this case, it is due to the rubbing motion of the clothes in the dryer and the comb through your hair which resulted in an imbalance of charges.

When you have a number of charges of one type, separated by some distance from a number of charges of the other, you can visualize the electric field between the two groups as a number of field lines all pointing the same direction.  This happens because the field lines of the individual charges combine together to produce a larger field.  If a charge, say the electron with its negative charge, is between them, it will be forced to move in the direction away from other negative charges as shown in the figure below.  The green arrow in this figure is the force felt by the electron.  The two groups of charges are said to have a potential difference between them.  This potential difference is the voltage.

Figure 4. Electron moving due to force from field.

This field provides the force to move charge in a circuit.  It is why current flows in a circuit.  Summing up the situation, the voltage is a potential difference between different points of a circuit which creates a force that causes charges to move.  These moving charges are the current in the circuit.  Voltage also carries a unit, the Volt, which is typically abbreviated as “V”.

The electrons that move through the circuit do so in conductors.  Conductors are made of a material, for instance copper, in which there are electrons that are not tightly bound to their atoms.  These electrons are able to move freely when exposed to a potential difference or voltage.

Although it is an overused analogy, the use of water pressure and water flow work reasonably well to understand the concept of voltage and current.  In this analogy, a water hose will be used in place of conductor made of copper wire.  Suppose you put your thumb over the end of a water hose so no water is coming out.  The pressure you feel on your thumb is analogous to the voltage.  When the water is flowing, the amount of water that is flowing is analogous to the current.

What this analogy is trying to make clear is that what is moving in our circuit are electrons.  Voltage does not flow, current does.  Voltage is present, and it is what drives the current to move.  If you find yourself saying something like, “the voltage flowing," stop yourself; it is wrong and will lead to mistakes.  Only current flows.  The voltage in the circuit is what causes the current to flow.  Voltage differences are what provide the force that cause the electrons (or charge) to move.  Said differently, electric potential, or voltage, is a form of potential energy that can induce motion in charged particles.

Modern electronics uses the concept of positive current flow, which is referred to as conventional current. This seems a bit odd given that the electron, with its negative charge, is the thing that is moving.  The reason for the definition of current flowing from a positive voltage to a negative one,  has to do with the signs associated with charge and the way the math works out.  Energy is released when positive charge moves to a lower (less positive) potential.  Although, physically it is the electron (with its negative charge) that is migrating through the circuit, the convention is to imagine positive charge flowing from a higher potential to a lower one.  This odd situation has to do with Benjamin Franklin’s choice of sign when he was initially performing electrical experiments.  He simply chose one (an excess of electrons) as negative and the other (an absence of electrons) as positive.  In reality, there are simply charges in nature and the assignment of “positive” and “negative” is a convention invented by humans.  Color names could just as easily have been assigned as the convention, and we would be discussing black and white charges.

The conceptual model for positive current flow is called hole flow.  Imagine that you could zoom in far enough to see the electrons in a piece of copper wire, and that piece of wire had electrons moving from left to right.  For an electron to move, it has to have a place to go (the electrons are loosely bound to the copper atoms in the wire and each atom can only support having a fixed number of them).  If the atom has space to take on another electron, it has a “hole” for one.  This is shown in the following figure.

Figure 5. Hole flow concept.

If you look at the holes, as opposed to the electrons, they appear to be moving from right to left, in the opposite direction as the electrons.  In the end, it is not really important how you visualize current flow, as long as you do so consistently.  Current is still flowing, and it is doing so because of the voltages in the circuit.

The positive current flow convention was adopted when semiconductors became prevalent.  In the days of vacuum tubes into the early days of transistor circuits, negative current flow was the convention.  If you find yourself looking at an old book (or talking to an older person who refused to make the switch), you will find that they use the convention where current flows from negative to positive, rather than the accepted convention of positive current flow.  Beyond the introductory material in this and the following section, this book follows the established convention of positive current flow.