We can harness electric charges and their potentials by artificially creating networks of paths for them in which they can equalize. We call these networks "circuits" and we can use them to create light, among other things.

This is a circuit:

Source: https://www.caretxdigital.com/split-ac-outdoor-contactor-wiring-diagram-2/ (2026-09-02)

This is also a circuit:

Source: https://stock.adobe.com/in/images/a-complex-electrical-wiring-diagram-with-colorful-wires-circuits-and-components-on-a-white-background-illustrating-a-detailed-schematic-for-an-electronic-system/938846171 (2026-09-02)

In the previous post, we saw how objects or materials can become negatively or positively charged by friction, such as when you - wearing slippers - walk over a carpet: electrons from the carpet are "smeared" onto you, making you negatively charged until you discharge/equalize your potential by touching something that is less negatively charged.

There are canisters/containers that hold opposite charges artificially: batteries. How nifty, that we can carry around canisters of electric potentinal in our pockets!!!

Various batteries:

Source: https://www.popsci.com/technology/why-so-many-batteries/ (2026-09-02)

In simple terms, batteries contain two chambers with opposite charges, one negative and one positive. To access the charges inside the chambers, we add a gate to each chamber called nodes: one negative and one positive. If the two nodes are in some way connected to each other, a path is created for the charges in the chambers to equalize. That which connects the two nodes is called a "conductor" and it is the potential difference - the voltage - in the charges that drives the equalization. Where there is a potential difference, charges always want to equalize. Connecting two opposite charges this way is one way of creating an electric circuit. When the circuit is closed - when we connect the two opposite charges - a current of these charges in the form of electrons is allowed to flow through the conductor from the negative chamber to the positive chamber. The flow of electrons in a closed circuit, "carrying" charges, is called an electric current and it goes on until the potential difference has been equalized.

Electrons do not move with the speed of light. That which we perceive as an instantaneous effect of closing a circuit, for instance flicking a light switch, is the effect of the flow of electrons. When an electric circuit is closed, an electromagnetic wave propagates all over the circuit. This wave pushes the electrons in the direction of the potential difference - negative to positive - so that the electrons closest to the negative "source" pushes the electrons closest to the positive "end". This propagation of the electromagnetic wave occurs almost at the speed of light. Saying that "electrons move at the speed of light" is just a simplification.

Open circuit:

Source: Petzold, Charles. Code: The Hidden Language of Computer Hardware and Software: Second Edition. Pearson Education, Inc. 2023. p22.

Closed circtui:

Source: ibid.

That which we used to connect the two nodes of the battery is usually made of copper, but different materials are differently good - or bad - at allowing electric current to flow. Electrons orbit the atomic nucleus at various distances, simplified as "shells". The number of electrons that orbit an atom differs by what basic element that specific atom constitutes. Some elements - or materials - have electrons that are not strongly bound to shells. These materials have "free electrons" or "valence electrons". Valence electrons in conductors move even when there is no voltage applied, but they move randomly and very slowly. By applying a voltage, we can influence the movement of the electrons.

Valence electrons:

Source: https://valenceelectrons.com/valence-electrons-of-copper/ (2026-09-01)

Materials that have many valence electrons - often metals - are called conductors and materials that have no valence electrons - such as glass, plastic and ceramic - are called insulators. The ability of a material to allow current is called conductivity and its inability to allow current is called resistivity. One way to picture resistivity is to imagine electrons colliding with other particles, creating friction. High conductivity = low resistivity and vice versa. The resistivity of a material is decided by what material we are talking about, its temperature, length and thickness. Temperature plays a role because heat increases the random movement of electrons. Therefore, resistivity must be given alongside temperature. A short, thick, cool conductor has generally lower resistivity while a long, thin, hot conductor has generally higher resistivity.

Finally, returning to creating light, let's add a lightbulb between the two nodes of the batteries as in the images above. The filament in traditional lightbulbs is a long, thin piece of material called tungsten, which endures high temperatures. Because the current in a closed circuit - the flow of electrons - continues with the same pressure (voltage) but encounters higher resistance in the filament, only a few electrons are going to be able to pass through it at any given time. The potential is going to equalize at a slower pace. The resistance that the flow of electrons encounters creates heat. Heat increases the resistivity of the filament. This circle continues until the lightbulb reaches its target temperature and emits light. A component in an electric circuit that does "useful work" this way, such as a lightbulb - or its filament - is called a "load".

Tungsten filament, no current:

Source: https://tdmfginc.net/product/tungsten-filament/ (2026-09-02)

Tungsten filament, with current:

Source: https://www.azom.com/article.aspx?ArticleID=1538 (2026-09-02)

If the circuit, intentionally or not, is closed before or after the load - in other words bypassing it - a short circuit occurs. Electric current always chooses the path with the least resistance. Because of the low resistance, a surge in current occurs, possibly depleting our batteries very quickly.

To be continued. Meanwhile:

1. What is the difference between "resistivity" and "resistance"?
2. "This circle continues until the lightbulb reaches its target temperature and emits light." This must be an oversimplification for the highschoolers. Couldn't you just keep increasing the voltage and reach even higher temperatures, even more light, until the filament pops/melts?
3. I have never seen temperature written on any electric product. I might have seen resistance on a pair of headphones, but are there any common appliences in our everyday lives at all that also give at what temperature it works with what resistance?
4. I'm confused about what the book says about the relationship between valence electrons and conductivity. Copper has one valence electron, right? Aren't there metals that we could use that have more valence electrons and thus better conductivity? Or is copper wire just that much cheaper to produce?

2] I think the filament is designed to reach an equilibrium where, at the projected current/voltage/amperage, the resistance emits a specific amount of lumens. If the strength is increased eg. a surge on the circuit, that's when the filament pops. But don't take my word for it!

3] Not sure if this is what you're asking, but toasters are another common appliance that uses resistance to produce temperature.

4] Gold is a much better conductor but much more expensive. You see it in high-end audio equipment. Another virtue of both copper and gold is that they are ductile, able to be easily drawn into wires.

1. Resistivity is material property and resistance is value for particular component
2. It's not. Resistance is lowest when filament is cold, but as it warms up it goes up to a design temperature. Most of metals have this property. This is also why when tungsten light bulbs burn out, most often this happens when they are turned on or just after. You can increase voltage to get higher power, but this causes tungsten to evaporate faster (that's black residue on old light bulbs) and so lasts shorter time
3. That value is not resistance, it's impedance and while it's expresses in ohms, it's only specified for AC and not DC, and often also only for some range of frequencies. There are thermistors, that is resistors that change resistance with temperature, and these are included with every modern Li-ion battery
4. it's complicated, but if there's only one then the mobility of that electron is greatest, sort of. copper is second most conductive metal, silver is slightly better, then aluminum is a bit worse but it's also much lighter so it beats copper on conductivity per weight. gold is slightly more conductive than aluminum but all other metals are less, sometimes requirements like corrosion resistance or mechanical strength is more important

gold is only used as a plating for corrosion protection

Thanks for your answers! Much appreciated! 😊

This is also why when tungsten light bulbs burn out, most often this happens when they are turned on or just after.
This was such a clever way explaining the relationship between increased heat and resistance.
if there’s only one then the mobility of that electron is greatest
I can almost FEEL how this subject goes deep. 🤣 But I take with me, that not only conductivity but also corrosion resistance or mechanical strength can be factors taken into account when designing circuits.

Thanks again! <3

Thanks for your comment!

2] Right! In my next post, I'm summarizing how we can regulate current by controlling resistance, so I'm inclined to agree!
3] Yeah, I was wondering why the book says that resistance - or perhaps impedance, as another commented pointed out - is given (i.e. labeled/written) on electric appliances together with the temperature at which that specific resistance applies, since it changes with heat.
4] This reminds me that I once bought gold/maybe gold plated? cables for my studio monitors. They promised better noise reduction.

I don’t have a good intuition as to why the random movement of electrons increases resistance, as you say

Temperature plays a role because heat increases the random movement of electrons. Therefore, resistivity must be given alongside temperature. A short, thick, cool conductor has generally lower resistivity while a long, thin, hot conductor has generally higher resistivity.

Like, is it that at higher temperatures the electrons move randomly, so they are less likely to move productively towards their end goal, effectively increasing the distance they have to travel to reach their goal? Or is it interference between the electrons in the conducting medium move randomly and interfere with the electrons trying to move through the conducting wire? Like, why does the property of resitivity arise from increased random motion of electrons?

Very good question! I too wonder: do the electrons that are being pushed by the voltage actually slam/bump into randomly moshing electrons in the conductor, resulting in kinetic energy being released/converted from their movement, resulting in heat?