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If the current in a series circuit remains the same, then what is the use of a resistor? I read somewhere that as electrons flow through a circuit, they are accelerated towards the positive terminal, but in collision with other electrons and fixed positive ions, they lose their kinetic energy and so their speed remains the same as they exit the resistors.

This gave me an answer as to why the current in a circuit remains the same, but made me think that if the current is the same, what is the use of the resistors? Do resistors of greater resistances cause the electrons to collide more among themselves and lose more kinetic energy, causing the wire to heat up? If this is the case, then shouldn't the speed of the electrons decrease with higher value of resistances?

(While answering the questions, please bear in mind that I am a tenth grade student who has just been introduced to current, emf, etc. please correct me if I have made any mistake in the body of my question)

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  • $\begingroup$ The question on the title may be too broad to answer; depending on what you classify as a "use", there are potentially thousands of possible answers, all correct but none complete. $\endgroup$ – probably_someone Nov 15 at 12:21
  • $\begingroup$ @SolomonSlow If you have relevant information, you should post an answer. Comments are for asking for clarifications or suggesting edits to improve the question. $\endgroup$ – Aaron Stevens Nov 15 at 15:55
  • $\begingroup$ What do you mean by "the current is the same"? Are you thinking that all series circuits have the same current no matter which resistors are in the circuit? $\endgroup$ – Aaron Stevens Nov 15 at 15:58
  • $\begingroup$ @Aaron Stevens yeah... Please correct me if I'm wrong $\endgroup$ – Nisha Prakash Nov 15 at 16:59
  • $\begingroup$ @NishaPrakash When we say that the current is the same in series circuits what we mean by that is that it is the same at every point along the circuit (since there is only one path for the current to travel along). i.e. each resistor and the power source all have the same current flowing through them for that circuit. This does not mean that all series circuits have the same current or that the current through a given series circuit does not depend on the resistors/power supply in that circuit. $\endgroup$ – Aaron Stevens Nov 15 at 17:48
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Well, the thing is the concept of flow of current is very easy to misunderstand. As you are the tenth grader, you must be struggling by the definition of current as rate of flow of charge i.e. $$ I = \frac{q}{t}$$. The concept of flow is used because the fluid mechanics was the immediate precursor of Electricity .
Now, coming to your logic of why the same current flows through all the resistors when they are in series, I don’t think (personally) that that’s the correct logic. Same current flows through them is an experimental fact, and it’s explanation can be illustrated in the light of fluid dynamics. Here (https://www.quora.com/Why-does-the-current-always-remain-the-same-in-a-series-circuit-When-the-charges-encounter-a-resistor-wouldnt-the-speed-of-flow-decrease-If-the-speed-decreases-shouldnt-the-rate-of-flow-decrease-at-the-same-time) I have given a link to a very nice explanation of the concept which you have mentioned.
Now, your main question

What is the use of resistors connected in series?

Suppose you have connected a very high resistance resistor (around 10,000 ohms) directly into your home plug which creates a voltage of $220 V$ , then the current flowing through our resistor is $$ I = \frac{V}{R}$$ $$ I = 0.022 A$$ and now you come with some coiled tungsten contained in a glass and have two terminals ( I mean a bulb) and connected it in series with our high resistance resistor, then $$ Power = I^2 R $$ $$ Power = 4.84\times 10^{-4} ~R$$ Well you see, just by increasing the resistance you can get as much glow as you want ( Power) and resistance can be increased just by taking a longer tungsten wire ($ R = (length /Area) \times \rho) .
And again, it’s natural fact that same current flows through the resistors when they are in series and it’s use is something we have to find, it’s all on us. It’s not something invented for a purpose but was found to be true and hence use depends on how we use it.

Hope you have found it helpful.

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  • $\begingroup$ Thanks for the answer! I had a question regarding the answer in the link provided. The answer mentioned that the greater the resistance, the lesser the rate of flow of charge.. but isn't that nothing but current? Current isn't the amount of electrons flowing through all together, but the number of electrons which pass per second through a cross section right?! $\endgroup$ – Nisha Prakash Nov 15 at 17:02
  • $\begingroup$ @NishaPrakash, Re, "the greater the resistance, the lesser the rate of flow of charge." That might be true in some particular circuit, (e.g., in a circuit that is supplied by a constant voltage source) but that is not generally true. The one thing that you can always count on a resistor to do is, you can count on it to obey Ohm's Law. $\endgroup$ – Solomon Slow Nov 15 at 18:01
  • $\begingroup$ @NishaPrakash Let’s say you have reservoir full of charge $Q$ then rate of decrease of charge in the reservoir i.e. $ \frac{-dQ}{dt}$ is the current, do you agree? And the charges which flow from reservoir will pass through obstacles, so the obstacle gonna hinder movement of charges across it, hence lease will be the current. $\endgroup$ – Knight Nov 15 at 18:02

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