Electricity Fundamentals

Before we start the circuit basics series, let's first take a look at the fundamentals and the history of electricity.

Fundamentals

For understanding circuits it is important to know what electricity is and what different terms mean. At the beginning of this series I want to take a quick look on the history electricity and then cover basic terms as charge, voltage, current and power.

Electricity

What is electricity? That is not a question you could answer in one sentence. At the beginning, electricity fascinated only few. Most people were scared. Today we could not live without electricity anymore. One thing that's for sure, is that we did not invent electricity. Electricity describes different phenomenons, that all have one thing in common: They are caused by electric charge. Some of these phenomenons are known for ages like lightning or the electric eel.

In the early days, scientist focused mainly on electrostatics. We all know the felling of unexpectedly getting an electric shock when touching something or someone. The reason for this is that either we or the object were electrically charged. We feel pain, when electric current flows through our body, while the object is discharging. But let's not get ahead of our time. Let's start at the beginning.

The word electricity and also the term electron root in the Latin word 'electricus' or in English 'like amber'. You now probably ask yourself why? The term was established by William Gilbert a physician who lived from 1544 to 1603 and researched in the area of electricity and magnetism. The reason for the term 'like amber', lays in the fact that amber gets charged up and attracts dust particles when rubbed with a cloth. But amber is not the only material that behaves this way. Different materials show the same behavior. They act 'like amber'.

More frightening are experiments with electricity and formerly living creatures like Luigi Galvani's twitching frog legs. For the people of his time this was some sort of scary magic. Today we know, that our neural system uses electric signals to transmit information and that an external electrical stimulation can cause muscle contractions. This is what happens with the frog legs. Well, only if the frog legs are fresh and the frog hasn't been dead for too long, of course. But let's skip the ugly details. One thing to keep in mind is that, even if electricity is used everywhere it is still dangerous for us and for this reason I will use a voltage below 24 V wherever possible in my tutorials.

Let's return to a more pleasant topic: electric charges and electrostatics.

Electrical Charge and Electrostatics

We are already familiar with the phenomenon of electrostatic attraction form William Gilbert's experiment with amber. A more common variant of this experiment is using a balloon as shown in the picture below. To charge it you can use a soft cloth or just your hairs, if you don't mind ruining your hairstyle. Once charged the balloon magically attracts the confetti. In physics, we differentiate between positive and negative electrical charges. Different electric charges attract each other. If you used your hairs to charge up the balloon, you are more than likely able to see the effect of same charges repelling each other, as well. Your hair will stand out in all directions. Pictures where people touch the charged sphere of a Van de Graaff generator are undoubtedly a bit more impressive, however. Feel free to search for some.

The force we see in this experiment is also described as the electromagnetic force. It's one of the forces we observe in nature. Another example would the attractive force between masses. Sometimes a different form of expressing the effect of forces is used: fields. Fields like the electric field just another way to express, that there is a force influencing the particles. Whenever there are different charges there is an electric field in between them. In case of the attraction between masses, it is common practice to use this way of expressing it. We often speak of the gravitational field of the earth in this context.

Charge (\(Q\)) is measured in coulomb. At subatomic level electrons have negative charge while protons have a positive charge. This charge is also called elementary charge \(e\). Protons have a charge of \(+ 1 e\) and electrons a charge of \(- 1 e\). As I said charge is measured in coulombs. The elementary charge is only \(1.602176634 \cdot {10 ^ {−19}} C\). When rubbing the balloon in our hairs, electrons are forced to move from out hairs into the balloon, which is the charged negatively. Our hair is now missing electrons and it is thus charged positively.

Uncharged atoms have the same amount of protons and electrons. In reality, we are unlikely to deal with single atoms anyway. We have charged molecules, which are also called Ions. You might know them from your chemistry class. In electronics, we focus on electrons, as electrons can freely move into materials like metals. This is a special case for conductive materials, we will have a more detailed look at this in the next tutorial on resistance. Liquids are one example where whole charged molecules are able to move freely. A famous example for this is salt or \(Na Cl\). In water, it will split up into an \(Na^+\) and a \(Cl^-\) ion. We don't really care about this in electronics as long as we don't look at battery chemistry or the question of why water is conductive.

Till now, we only talked about electrostatics. In electronic circuits everything is about moving charges. We know that being charged up is not an optimal state for objects. The balloon will lose its charge over time, if it touches other objects. If we get an electric shock or see lightning, we do witness not static but moving charges. In this case as part of the discharge process of charged objects. This brings us right to the next topic: the electric current.

Electric Current

Electric current (\(I\)) is the flow of electric charges. To be a bit more exact, it is the flow rate. We measure electric current in ampere. A current of one ampere means that a charge of one coulomb is moves through a point of our circuit in one second.
\(1 A = 1 \frac{C}{s}\)

In theory, we could calculate the electric current by dividing the moved amount of charge with the time it took.
\(I = \frac{Q}{t}\)
We won't need this formula in electronics. To use it we would need to measure the amount of moved charge and this is not something that can be simply done with a multi meter.

That's all? Well, there is a somewhat confusing point left: the direction of electrical current. Take a look at the picture below. The conventional direction of current is \(+\) to \(-\). The electrons, however, move in the opposite direction from \(-\) to \(+\).

Defining the conventional direction of current the opposite way around, seems to be an odd decision at first. The reason for this is a historical one. When the conventional direction of current was defined, it was not known that negative charges (electrons) move inside the circuit. Moving positive charges from \(+\) to \(-\) is somewhat equivalent to moving negative charges from \(-\) to \(+\), even if we know that the first is not what truly happens. For understanding electrical circuits it doesn't really matter. It is common to stick to the conventional direction.

Voltage

Voltage is a bit more difficult to explain. Voltage is the potential energy per charge and measured in volts. A voltage of one volt means that one coulomb of charge carry an energy of one Joule.
\(1 V = 1 \frac{J}{C}\)

The electrical charges are only the carrier for the electrical energy. No electrons are consumed by the circuit. The same amount of electrons that get into circuit will exit it. But were does the energy come from, that makes a lamp light up?

Potential energy is the energy held by and object because of its position or configuration. It is released by moving the object into a lower energetic state. Let my give you a simple example of potential energy in a different area of physics.

If a ball is held high, it has a higher potential energy than a ball laying on the ground. It has this potential energy because of its position in the gravitational field of the earth or simply speaking because of its height. If you let it fall this potential energy is transformed into kinetic energy. The ball will accelerate and fall onto the ground. On impact on the ground the kinetic energy will be transformed into thermal energy. Well, the ball will likely bounce few times, because it is elastic, but that's a different topic. What is important, is the fact that the ball is still there after it fell down. The ball hasn't changed.

Another example would be a hydroelectric dam. The potential energy the water has because of its height can be converted into electric energy. Neither will the water change nor disappear during this process. It only carries the energy.

The same holds true for the electrons in electric circuits. To understand it let's look at a simple plate capacitor. We have two plates one charged positively and one charged negatively. When we connect both plates with a wire, the electrons from the negative plate will move to the positive plate. The capacitor is instantaneously discharging. We get a spark and some heat. We know for sure that energy has been released here. But were does it come from?

Well, someone has previously charged the capacitor and put in energy to force the electrons move to one of both plate. This is similar to placing the ball at a higher spot from where it can fall down. By moving the electrons to one plate we increase their potential energy, just like we did with the ball. We create an electric field in between the plates. If we connect both plates with a wire they will discharge. Once all surplus electrons reached the positive plate and the electric filed collapsed, there is no more energy to release. This is analogue to the ball laying on the floor.

Without the separated charges, there is no energy that can be released and no current flows when connecting both plates. We have a voltage of 0 V. If the plates are charged we can measure a voltage. This tells us there is potential energy that can be released.

Where exactly is this potential energy? Well, that's hard to define. The fact that we can't easily locate potential energy makes it so hard to understand it. The energy is there due to the state of separated charges and the attractive force between them. In other words, the energy is stored in the electric field.

Of course, there are not only plate capacitors and in other cases the source of energy can be different one. In batteries, it is a chemical reaction. In power plants it is usually a turbine, where the kinetic energy of steam or water is converted into electrical energy. The important takeaway is that there is no energy created our of nowhere, we can only converted one type into another. It's actually worse, we will always loose some energy when doing so. Don't get me wrong this energy is not really gone, it is just in a form we can't use it anymore. In most case this form of useless energy is thermal energy. An example: In a lot of devices, we have no use for the heat produced by the electric components, we need to get rid of it to prevent the device from overheating.

An important fact about voltage and potential energy is, that it is always measured relative to a reference point. In case of the ball this was the ground. In case of our capacitor it is one of the plates. In circuits, we measure voltage relative to ground or in case of batteries relative to the negative terminal. However, we don't have to. In circuits with multiple components we can measure the voltage between different points. This is analogue to a river with multiple barrages. We can look at the energy we can get out of the whole system or look at a single barrage. We can measure the voltage drop over a single resistor or load. In combination with the current flowing through the circuit we can use this to calculate the amount energy that is converted by this specific component. Which brings us to the last topic: electric power.

Electric Power

Power (\(P\)) is the physical size to express the amount of energy converted per time. The power can be calculated by multiplying the electric current (flow rate of electric charges) with the voltage (amount of energy carried per charge):
\(P = U \cdot I\)

Power is measured in the unit watt (W). A power of one watt means that one joule of energy is converted per second:
\(1 W = 1 V A = 1 \frac{C}{s}\frac{J}{C} = 1 \frac{J}{s}\)