In the previous post, we were talking about creating light by leveraging the properties of electricity: voltage, current and resistance. Voltage occurs when two opposite charges consitute a potential difference in a closed circuit. This voltage in turn causes a current - a flow of electrons - in our circuit, flowing from the negative node of the power source to the positive node of the power source. When the current passes through the load of our circuit - a filament made of tungsten in our case with the lightbulb - it encounters high resistance. As current is forced through this high resistance, heat is generated. The increased heat increases the resistivity of the tungsten filament, and vice versa, which at a certain point starts emitting light.

Tungsten filament emitting light:

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

We can - to some extent - control how much light is emitted by increasing or decreasing the amount of current that flows through the filament. There are two methods for achieving this:

1. Voltage causes current to flow, which means that if we increase the voltage - and the resistance stays constant - the current also increases, and vice versa.
2. Resistance hinders the flow of current, which means that if we increase the resistance - and the voltage stays constant - the current decreases, and vice versa. There are - as we discussed earlier - at least three ways to manipulate resistance:

• choose a material for our conductor/load with greater or lesser resistivity
• change the length of the conductor/load
• change the thickness of the conductor/load

To summarize, we can regulate current by changing voltage if resistance stays the same or by changing resistance if voltage stays the same. This relationship between these three characteristics of electricity is expressed and explained by Ohm's Law. One takeaway from Ohm's Law gives us a means to calculate any one of these three properties if two of them are known. This relationship can be represented with the following triangle, which also gives us what arithmetics should be performed on the properties and in what order to find the value of which property.

Ohm's Law expressed with a triangle:

Source: https://www.inchcalculator.com/ohms-law-calculator/ (2026-09-02)

A few notes:

• Voltage/V is sometimes expressed as "E", which is the first letter in "electromotive force", which helps us visualize how voltage actually drives, "motivates" or gives "motion" to current. It can also be represented by "U".
• Current is represented by "I", which comes from the French phrase intensité du courant (current intensity).

To be continued. Meanwhile:

1. Where does the "U" come from? It seems to be an international standardization for symbolizing voltage.

I found this answer on an ancient stack exchange that quotes a broken link with this explanation for where the “U” comes from

Germans took freedoms and started calling voltage "U", probably since that letter was largely unused and so couldn't be confused with anything else. They also came up with etymology: U is for Unterschied, which is German and means "difference"; very fitting since voltage is obviously the same as potential difference.

Totally awesome that you dug this up. Thanks! I myself was thinking that it slowly "evolved" from some poorly hand written "V". 😅