Resistance - LeFonch

What is Resistance?

Before we discussed how current is the movement of electrons.
As electrons move in-between atoms, there are several factors that may impede their movement.
This is called resistance, which is the name to anything that resists a current.
It is given in the unit ohms, named after the German mathematician and physicist Georg Simon Ohm.
Ohms are written with the symbol Ω.

Ohm's law states that, at constant temperature, the current through a metallic conductor is proportional to the potential difference (voltage) across it.

I ∝ V

A resistor that follow's Ohm's law is said to be ohmic.
In ohmic resistors, current and potential difference are proportional, which means they form a straight line when graphed.

However, oftentimes the relationship isn't a straight line, as with increased current there is increased temperature.
This results in the resistance increasing.

As you can see in the graphs above, the line continues into the negatives.
This occurs if the current is in the reverse direction, which can happen if the sides of the battery which the wires are attached to are swapped.
Diodes are a special component that only lets current go in one direction.

Here the current is drastically reduced if its not going in the "forward" direction.

Imagine a wire with a potential difference.
The electrons will accelerate and start moving to the positive end of the wire.
However, as they move, they collide with the ions in the wire, causing heat to be released.
This shows us that current is doing work on the wire, leading to some energy being released as heat.
The heat leads to particles vibrating more, causing more collisions with electrons and thus more resistance.
Some inventions utilize this feature, such as filament light bulbs, where current is forced through a thin filament which starts glowing due to the heat.
While this creates light, it is very inefficient, as a lot of energy is released as heat, which is considered waste.

In many applications, we've switched to more efficient LED bulbs.
But even these can get quite hot! I wouldn't want to touch a filament light bulb...

Resistance in a wire is dependent on three factors.
These are the length of the wire, the cross-sectional area of the wire and the resistivity of the material the wire is made of.

If the length is increased, electrons travel a longer distance and collide with ions more often, leading to greater resistance.
If the cross-sectional area is increased, then there is more room for electrons to travel, and resistance is decreased.
The resistivity is the ability of a material to conduct electric currents.
Increased resistivity means it is harder for a material to conduct currents, resulting in higher resistance.

The resistivity of metals will increase with temperature because of the increased vibrations of metal ions.
The number of free electrons (charge carriers) in metals will not increase significantly at high temperatures unless the increase in temperature is extreme.
However, it can be very different for semiconductors and insulators.

The number of charge carriers (per cubic meter) in non-metals can increase significantly with rising temperatures, so that their resistance can decrease considerably as they get hotter.
For example, glass is usually described as an insulator, but at 500°C, many types of glass can become good conductors.

Sources

https://en.wikipedia.org/wiki/Georg_Ohm