# If resistors do not decrease flow in a circuit then why are they able to make things like an LED short when otherwise it would without the resistor

So I just spent the last 8hrs trying to comprehend how resistors function in a circuit and still cannot find a source of explaining it to me. And please not another water analogy. I already feel hydrated and I only drank one cup today.

It is said that every component in a circuit "absorbs voltage" like a 9V battery going in a 2V LED making the voltage coming out the LED 7V. I thought that once electrons are through a circuit they must go somewhere and that nothing can take away power (energy cannot be destroyed) like the case of a resistor where it's just slowing down the electrons. So where did that voltage go to?

In the case of the LED/resistor relation, if the resistor is not 'reducing' the amount of electrons then how is it preventing an LED from smoking if it's just slowing down the electrons? Which still makes the current the same through out the circuit.

I'm tired after an uneventful day so the wording is really butchered. But hopefully my questions go through.

I think my understanding of the fundamentals is jaggedy. I also feel like I have to be really good in algebra to be able to grasp these aspects.

• Simple linear algebra covers most EE equations. $1/2piRC$, etc – Sparky256 Feb 11 '18 at 6:53
• "good in algebra" and "simple linear algebra" are rather relative terms depending on the perspective of the person. – Ariser Feb 11 '18 at 9:55

See my answer to Is voltage the speed of electrons?

No analogies? Use the real physical definitions. In this case from wikipedia:

The voltage between two points is equal to the work done per unit of charge against a static electric field to move a test charge between two points

Voltage is a field. Fields have the ability to "do work" - to transfer energy from the field to something else. An electric field exerts a force on an electron. That electron moves a distance while that force is applied: force x distance = work, for all kinds of forces.

Resistors also exert a force on electrons, but in this case it's like friction: it acts to slow them down. This force removes energy from the electrons. You're correct that it does not destroy the energy: it ends up as heat, like most "waste energy" in the universe.

The other key thing is that in a conventional circuit current loop the rate of flow of electrons must be the same everywhere - they're "incompressible", because they repel each other strongly. It's more like a bicycle chain. If you apply a resistance (brake) to the chain at any point, it slows the whole thing down. You don't get the chain going fast on one side and slow on the other. Similarly, you don't get electrons going "fast" before a resistor and "slow" after it - the rate of flow of electrons is the same at the battery, at the resistor, and at the LED.

• It's starting to come together. The bicycle analogy makes sense. So electrons in a circuit are like linked? I always thought of electrons being 'individuals' in that if one is entering a resistor and slowing it down it has no effect on the others that are moving through the circuit on the other side. But Im confused on one thing. Why when people talk about powering LEDs they say they want XmA to run through it but when adding a resistor the mA stay the same? I thought we just established that resistors will slow down the current by transferring the energy from voltage into heat AKA lower mA – Django Feb 12 '18 at 2:22
• LEDs are (normally) current driven and if you have a voltage source and don't want to build a current-control, you can drive leds with a resistor in series. By doing that, you create a voltage divider. This causing a very basic closed-control loop where the current is regulated when the led changes it's voltage, which for example may be caused by temperature change. – Finkman Feb 12 '18 at 13:09
• So how would I know how much mAmp an LED needs? If I have no info on an LED? – Django Feb 13 '18 at 2:23
• @Django Well, that's a problem - for small LEDs the answer is usually 20ma. You might be able to make measurements of temperature and colour to work out whether you're overdriving an LED, but then you have no guarantee of its lifetime. This is why you should make sure you have datasheets or at least key ratings for components. – pjc50 Feb 13 '18 at 9:52

It's not slowing the electrons (charges, actually) down (since they're going into the resistor at the same rate that they're coming out), it's absorbing energy from them and turning it into heat. Volts is joules per coulomb, and since charge is a physical (or physical-equivalent, if you want to get technical) amount, the only way to reduce voltage is to remove energy from the charge.

While you start your question with the function of resistors, you are interested in a generic idea of how to learn the principles of circuit analyis in general.

The most common flaw of analogies is, that they aren't consistent at some point or the persons using the analogy fail to match every thing and aspect in the analogy and the real problem. The most dominant failure of the water analogy is, that the fluid resembling the electrons is not compressible. Electrons behave like an ideal gas in most media (vacuum and conductors), so if there's an analogy which fits best, it's a more or less pressurised gas flowing between parts of the circuits with differing pressure. But in general, forget phrases like "absorbs voltage". Because this wording does not fit the concept of voltage. It wouldn't fit to say, a small tube "absorbs the pressure" when using the water or gas model, either, would it?

But to get back to the basic things that happen in a circuit. If there's a current flowing in a circuit this must have a cause. In concentrated circuit elements (this is what we assume in basic circuit analysis, you don't have to mind about that now), the cause for a current to flow is, as already mentioned by @pjc50 an electrical field. The electric field represents the voltage between the two ends of a device with a defined geometric size regardless if it's a resistor, a diode or a spark gap.

What happens inside a resistor? Electrons have a mean free travelling distance, during which they are accelerated by the electrical field inside the resistor. After being accelerated over a distance and taking up kinetic energy, which may be more or less than this named distance, they accidentally and eventually hit some atoms (i.e. the orbitals of the non-free electrons), get deflected and loose their kinetic energy. The energy is now transferred to the atoms of the resistor resulting in vibrations (which is basically thermal energy). The resistor heats up.

When the electron leaves the resistor, it statistically has the same speed as it had, when it entered the resistor, because inside the resistor the acceleration due to the electric field and the deceleration due to collisions balance themselves! This is a very important property of ohmic resistance.

So, the whole topic is about balance, then. That's where algebra comes in. The speed of electrons traversing something like a resistor and the number of them doing that simultaneously at any cross section of the device are related to the voltage and the physical properties of the material. There are a lot of parameters and equations but they are fairly simple for an ohmic resistor. You should read about them and strive to solve them once for a resistor, then I think some basic understanding may arise about the concept of resistance.

After that it will be easy to understand the other circuit elements and finally getting an idea, what a circuit is after all.

Current is rate of flow of charges, or here, rate of flow of electrons. LED doesn't gets smoked because current through it gets decreased, which results into less heat.