1. Resistance
Let’s discuss the concept of resistance. Resistance is how much an object or material resists the flow of charged particles. In a wire, there is typically very little resistance, and electrons can flow quite easily. As an analogy, imagine the water flowing in a garden hose. The water itself is electric charge, and the pressure of the water makes it flow just as voltage makes charged particles flow. The actual flow is the current. Imagine the hose as completely unobstructed. The flow will not see much resistance, and it will come out the end with ease to water the flowers. If you put a kink in the hose by bending it, you are introducing resistance to the flow of the water. It is not too far off from mechanical friction either.
What makes up resistance? There a few different factors that affect resistance. One important factor is the material itself. Copper is such a good conductor because of the all the electrons floating around in clouds , orbiting the nucleus of the atoms, but fairly loosely attached and maybe not even faithful to one atom at all. These electrons are willing to flow if given the proper motivation to do so. In other materials, the electrons may be significantly more uptight about how much they move around. They may be quite comfortable attached to the atom that they are currently with, and unwilling to flow. A greater force is required to get it moving. For example, the conductivity of teflon, the type of material that you find in a non-stick frying pan, is massively lower than that of copper. In fact, the conductivity is 1-followed-by-30-zeros lower. Another factor is the shape of the object: the length, and its thickness. Thicker material has lower resistance and greater conductivity, as there are more electrons in the conductor and generally more room for a greater number of electrons to flow. So the resistance is inversely proportional to the cross-sectional area. A larger cross-section, the lower the resistance offered by that object. Conversely, it is directly proportional to the length of the object. A really long copper wire is going to have higher resistance than an equivalent wire of shorter length. Thick and short objects have less resistance than long and narrow. A side note: this issue becomes more significant for extreme cases, such electrical transmission lines, which extend from generation stations to distribution stations and carry electricity for many hundreds or thousands of kilometres. The length of the conductor (the overhead wires) introduces significant resistance and results in losses of power over great distances. This can be partly mitigated by making those lines large in diameter and carefully picking the material to lower the resistance.
2. How Lightbulbs Work
A great example of resistance is the everyday lightbulb. How does the bulb generate light and let us joyfully study until early morning? A combination of the three things we have learned so far: current, voltage, and resistance, the three amigos. But let’s discuss something else briefly: ever watch car races and look closely at the disc brakes during heavy breaking? A great example of this is the 24 hour Le Mans race. The brakes have to dissipate the tremendous energy of the car moving forward at 320 km/h, and they do this by frantically grabbing onto the spinning disks which are attached to the wheel. This seems like a poor decision by the brake pads, because it generates a disturbing amount of friction and therefore heat; rub your hands together quickly and you can get a tiny taste of the amount of heat generated. The incredible speed of the mass of steel is converted into heat energy by the brakes, and the car slows down. What happens when an object, particularly a metal, becomes very hot? It glows. So if you look at the disc brakes, you’ll see them glowing red-hot immediately after heavy breaking.
That’s great and all, what does this have to do with lightbulbs? Well, the lightbulb operates on mostly the same principles. Except that having a wheel spin at Le Mans level speeds with disc brakes to light up your home during braking would be fantastically inefficient, although probably a real conversation starter if you have trouble with small talk. No, instead, the light is generated by mechanical friction’s close friend: electrical resistance. The lightbulb, if you look closely (when it is off is probably a good idea), is nothing more than two wires with a very thing piece of filament connecting them. We’ve seen that electrical resistance is inversely proportional to increasing cross-sectional area, so that little piece of wire (poor guy) is going to have a large amount of resistance. The electrons have trouble getting through the filament (although they do eventually), compared to their normal travels in the regular wire. They push and shove and whine and generally make a great big deal about squeezing through the filament. This friction among the electrons is similar to good old-fashioned mechanical friction in that it creates extreme heat, enough to make the filament glow white-hot. This is where the light comes from: the glowing hot filament. It makes sense why lightbulbs are so hot then! Lightbulbs aren’t all that complicated then, although they weren’t invented until 1835, and the scientists played around with the filament and the bulb’s vacuum for about 40 years. That’s dedication, but even then lightbulbs were only good for a very limited amount of time, minutes or hours at most. Thomas Edison improved it dramatically in 1879 and 1880. The carbon filament produced a lightbulb that could burn for 14.5 hours – and we get frustrated when lightbulbs burn out at 1000 hours. Interestingly enough, Edison’s further experiments revealed that a bamboo filament result in an astonishing 1200 hours, which was good enough for commercialization. Other great contributions to the field of lighting and electricity including him demonstrating the ability to distribute electricity from a generator. His company also developed the first power utility in Manhattan, the Pearl Street Station. A smart individual, although some people question whether or not his patents on lightbulbs infringed on existing patents. Regardless, Edison did some amazing work.
How efficient are filament lightbulbs nowadays? About 5% of the energy that is emitted from the electron’s struggle through the filament is converted to light; the remaining 95% is converted to heat. A thankless job for the electrons. Later we’ll talk about the compact fluorescent and LEDs, because fuck it why not? Lightbulbs are interesting.
3. Resistors (The Passive Component)
We’ve discussed resistance in general, but haven’t yet talked about what resistors are. Resistors may seem a bit odd at first – it seems as if they don’t quite have a real purpose. Essentially, they are resistance for the sake of resistance in that they don’t really do anything at all. They don’t produce light or sound, they aren’t motors, they aren’t loads that do work. But they are so widespread and the probably the most familiar electrical component and the simplest – and immensely useful. Resistors are passive two-terminal electrical components. Passive refers to the fact that resistors consume energy but do not produce energy. They are small, beige looking cylindrical devices that have usually 4 coloured stripes on them, which indicates the amount of resistance that it provides (you can easily search for resistor tables online to learn how to interpret the colours).
Resistors are a way to adjust the voltage and current in the circuit. If you have a power supply and need a certain current for some application, adding resistors can allow you to achieve the current. If you need some particular voltage, resistors can help to achieve that as well. You can think of them as components to fine tune the circuit or conductor to the desired current and voltage. Remember that they use energy, so they will reduce current flow or lower voltage levels – they aren’t adding anything to the circuit in terms of energy (passive).
Resistors come in fixed resistance or variable resistance, meaning that the resistor has the ability to be adjusted. One example of a variable resistor is a potentiometer, which is a three terminal resistor with a small dial that can be physically turned to adjust resistance. These variable resistance devices will be discussed later on. Fixed resistors, or commonly called axial resistors, can ranged in magnitude from milli-ohm (0.001 ohm) to megohm (1,000,000 ohms) although resistors in the range of 10-100,000 ohms are the most common.
the first year engineer 