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I want to understand why exactly resistors or materials with high resistivity difficult the flow of current. If I'm not mistaken, there are not many free electrons in this materials to move and contribute to the current. But now comes my question, why can't the free electrons that come from conductors (for example a wire that is connected to the resistor, and are moving because of a potential difference) just traverse the resistant material. Or is it that resistance is more than just the lack of free electrons to contribute to the current and is also related to a collision of free electrons due to the structure of the material? I am a bit confused so can someone explain me what happens when current flows through a resistor and why resistors oppose to the flow of current? Thank you!

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  • $\begingroup$ It's worth thinking about the analogous situation with water and pipes. Your question translates to: "why can't the freely flowing water in wide pipes with low resistance move just as freely in thin pipes with high resistance?" $\endgroup$
    – KF Gauss
    Commented May 18, 2020 at 3:06

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I want to understand why exactly resistors or materials with high resistivity difficult the flow of current. If I'm not mistaken, there are not many free electrons in this materials to move and contribute to the current.

Instead of talking about "resistors" it is better to talk in terms of electrical "resistance". That's because devices can be designed with specific materials so as to have a controlled amount of resistance.

Metals, such as copper, aluminum, silver, gold, etc. have many mobile electrons that can easily move in response to an electric field. This makes these materials very good conductors. At the other extreme, plastics have virtually no free electrons and conduct no current. In order to for current to flow extremely high voltages are needed to pull electrons out of their bonds. This makes plastics very good insulators.

In between these extremes, there are devices specifically designed to provide a controlled amount of free electrons and therefore a controlled amount of resistance in a circuit. Examples are carbon composition resistors, thermistors, wire wound resistors, metal film resistors, etc. You can look up how these devices are designed.

But now comes my question, why can't the free electrons that come from conductors (for example a wire that is connected to the resistor, and are moving because of a potential difference) just traverse the resistant material.

Free electrons that come from the conductors do traverse the resistant materials, but they collide with atoms in the resistor and make them vibrate. As a consequence the introduction of resistance reduces the flow of current in the circuit. Some of the electrical potential energy given the electrons by the voltage source is dissipated as heat in the resistance.

Or is it that resistance is more than just the lack of free electrons to contribute to the current and is also related to a collision of free electrons due to the structure of the material?

A combination of both, but primarily it is the result of collisions between the free electrons and the atoms of the resistance materials.

I am a bit confused so can someone explain me what happens when current flows through a resistor and why resistors oppose to the flow of current?

The electric field has to do work to move the electrons through the resistance which opposes the flow of current. The potential difference, voltage, across the resistor is the work required per unit charge to move the charge between the points.

Yes thanks now it's very clear. I just have a doubt about the potential energy that the electrons lose when they collide with the atoms of the material. My question is, why is it that they lose potential energy and not kinetic energy?

They do lose kinetic energy. The end result is potential energy is converted into heat. The progression is as follows: The electric field exerts a force on the electrons giving them kinetic energy. But each time an electron gains kinetic energy it quickly loses it due to collisions with the atoms and molecules of the material which generate heat. Then it gets kinetic energy again from the field and again loses it due to collisions. The gaining and losing of kinetic energy due to collisions averages out to an overall change in kinetic energy (change in drift velocity) of zero, so that the current is constant.

The progression is therefore electrical potential energy converts to kinetic energy which converts to heat. The net result: Conversion of electrical potential energy to heat.

Hope this helps.

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  • $\begingroup$ Now I understand. Thank you! $\endgroup$ Commented May 18, 2020 at 11:40
  • $\begingroup$ @Tony2015 So the answer is acceptable? $\endgroup$
    – Bob D
    Commented May 19, 2020 at 12:07
  • $\begingroup$ Yes thanks now it's very clear. I just have a doubt about the potential energy that the electrons lose when they collide with the atoms of the material. My question is, why is it that they lose potential energy and not kinetic energy? Because when they collide my common sense assumes that they lose speed, hence kinetic energy, so I don't understand why they would lose potential energy (because to lose potential energy they would have to move closer to the positive terminal, and that's done by the movement of the electrons, not by the collisions right? I can't see the relation. $\endgroup$ Commented May 23, 2020 at 14:59
  • $\begingroup$ @Tony2015 Potential energy IS converted to kinetic energy. But then kinetic energy is continuously converted to heat so that the overall change in kinetic energy is zero (constant velocity) See update to my answer showing the progression. $\endgroup$
    – Bob D
    Commented May 23, 2020 at 15:36

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