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I have some slight confusion about voltage drop across electronic components.

If the current before and after the resistor is the same, why is there a voltage drop across the input and output terminals of the resistor?

Voltage drop is the joules of energy lost when moving 1 coulomb of charge across 1 ohm. So its essentially a force which acts on electrons at a point.

If the current is the same before and after the resistor, why is there a difference of electromotive force?

The only other thought i have may be that the drop is compared to when the resistor is not there

Can anyone clarify these concepts please?>

enter image description here

Malemna
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    What does OHm’s Law tell you? – Tony Stewart EE75 May 25 '21 at 13:04
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    Does this answer your question? [What exactly is voltage?](https://electronics.stackexchange.com/questions/50976/what-exactly-is-voltage) – Mitu Raj May 25 '21 at 13:05
  • Voltage is a measure of energy per unit charge [J/C]. When a unit of charge passes through a resistor and heats it up, it has lost some of its energy. – vir May 25 '21 at 13:13
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    Maybe the answers to this related question will help: https://electronics.stackexchange.com/questions/566529/why-does-voltage-drop-in-a-series-circuit-but-current-stays-the-same – ErikR May 25 '21 at 13:16
  • V= I * R........ – Tony Stewart EE75 May 25 '21 at 13:33
  • If a unit of charge passes through a resistor, it has lost some of its energy? To Heat right? But why is the current the same after the resistor if it has lost energy? – Malemna May 25 '21 at 13:43
  • And Ohms law jsut says that the voltage across a resistor is proportional to the current, assuming the internal structure does not change. It tells you that the energy required to move a unit of charge across the resistor is proportional to the current through that resistor. – Malemna May 25 '21 at 13:48
  • @TonyStewartEE75, OP is asking 'why is Ohm's Law' so Ohm's Law itself tells you nothing. It's an equation, not an explanation of rationale. – TonyM May 25 '21 at 15:03
  • One current does not describe the whole circuit but Ohm's law tells you there is always a voltage across any resistance with current flowing thru it. Are you looking for some other reason? It's just that simple. – Tony Stewart EE75 May 25 '21 at 15:43
  • @TonyStewartEE75, can't explain it to you further. You've had the reasons but you're not budging from one view. So there's no prospect of a discussion and I lost interest a while back. – TonyM May 25 '21 at 15:52
  • I wasn't discussing with you, and just stating the obvious – Tony Stewart EE75 May 25 '21 at 16:43

1 Answers1

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Electric Potential Difference OR Voltage:

enter image description here

• Consider the task of moving a positive test charge within a uniform electric field from location A to location B as shown in the diagram.

• In moving the charge against the electric field from location A to location B, work will have to be done on the charge by an external force.

• The work done on the charge changes its potential energy to a higher value; and the amount of work that is done is equal to the change in the potential energy.

• As a result of this change in potential energy, there is also a difference in electric potential between locations A and B.

• This difference in electric potential is represented by the symbol ΔV and is formally referred to as the electric potential difference.

• By definition, the electric potential difference is the difference in electric potential (V) between the final and the initial location when work is done upon a charge to change its potential energy.

• Because electric potential difference is expressed in units of volts, it is sometimes referred to as the voltage.

Current:

• Current has to do with the number of coulombs of charges that pass a point in the circuit per unit of time (not with how much of potential energy that charge carries).

Answer:

As the charge moves through the circuit it loses its electric potential hence voltage drops. The charge is simply the medium which moves the energy from location to location.

While the energy possessed by the charge may be used up (or say that the electric energy is transformed to other forms of energy), the charge carriers themselves do not disintegrate, disappear or otherwise become removed from the circuit.

That's why voltage drops but current is same (charge carriers that pass a point does not change only their energy becomes used up)

Images used and explanations from physicsclassroom.com website

  • Exactly how is the energy possessed by the charged used up? It collides with atoms with high kinetic energy. This kinetic energy is produced by an electromotive force which is produced by a higher concentration of charged particles at the anode terminals a battery supply. Still, if the current before and after the resistor remains unchanged, how can electric potential energy be lost? The potential energy is what causes a current or movement fo charged particles – Malemna May 25 '21 at 14:03
  • My only understanding of how this can be true is that the concentration of charged particles at the anode decreases the longer you have connected circuit. When the electrons charged particles are accelerated with an electric force as a result of the higher concentration of charged particles, the electrons then collide with atoms turning kinetic energy into heat energy by excitation and de-excitation of valence electrons. As they collide there is possibly a similar effect to back EMF, so the reduction of concentration at the anode can possibly keep the current constant at the output of resistor – Malemna May 25 '21 at 14:18
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    Do not get confused between charges and energy possessed by these charges. Even if the energy possessed by the charge is used up by the load (resistance or collisions with other atoms in the wire) still the charge carriers itself are present. Suppose if you have an object at a height h on table (It has potential energy due to virtue of height and no kinetic energy as its at rest). When you drop it to ground, potential energy changes to kinetic energy and as it drops to ground, it transfers it energy to the ground and its energy gets used up. –  May 25 '21 at 14:20
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    [Continuation] Only its energy is getting used up, Object itself is still present on the ground. You can apply this analogy to charges too. Only energy possessed by the charges is getting used up due to collisions (hence voltage drops). But charges itself are present (current is numbers of coulombs of charges that pass a point in the circuit). Only energy possessed by the charges is getting used up. –  May 25 '21 at 14:21
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    [Continuation] Inside the battery, charge carriers gain energy (through electrodes and electrolyte action). When battery is connected to a circuit, this energy gained in battery is lost (transformed to other forms of energy heat, light etc) by the charges as they travel through wires, resistors (bulbs)etc. Only energy is used up (transformed to other forms] Charges itself are present in the circuit. –  May 25 '21 at 14:28
  • I think you are confused with the term 'energy'. A charged posses energy due to its position which is then called electric potential energy, because it has the 'potential' to perform work. It performs work on the atoms in the resistor by transfer of kinetic energy, when they collide they are slowed down. The fact that the current remains constant says there is no real drop in potential energy relative to the terminals of the resistor but possibly relative to the anode. – Malemna May 25 '21 at 14:32
  • You'll have to explain exactly why does the electric potential energy caused by concentration of charges differ across the terminals of the resistor when the current through the terminals remain constant. – Malemna May 25 '21 at 14:38
  • @Malemna Do you understand that an object is not the same thing as its energy? Do you understand that charge can move with no change in its energy? Do you know the law of conservation of charge? What happens if the rate of electrons into and out of a resistor are not equal, where do the extra electrons all go? – Matt May 25 '21 at 14:46
  • Energy is a property of objects. Do you understand that charge can move with no change in its energy? You'll have to explain this one. As for the conservation of charge, yes into and out the resistor the number of charges in the same. You've confused yourself, there is a difference between the number of charged particles into and out of the resistor and the 'rate' in which they move into and out of the resistor – Malemna May 25 '21 at 15:05
  • You may have 1000 particles moving into the resistor, those same 1000 particles will move out of the resistor but the rate at which they move is not necessarily the same. – Malemna May 25 '21 at 15:06
  • In a simple resistive circuit, we do observe steady state almost immediately after connecting a circuit. Current (charge/time) rate at which charge flows is everywhere same in the circuit. Here in this paper they have described circuit in terms of charge and field. may be you can have a look and see if this can clear your confusion http://ithaki.eng.auth.gr/elab/microscopic-view-of-elecrrical-circuits.pdf –  May 25 '21 at 16:01
  • @Malemna It is in fact both rate and count of electrons in and out that must be equal. You failed to answer my question, if the rates are not equal where do all the electrons go? Follow up question: if the rates are not equal but the count in and out are equal, how did you accomplish that? Note: everything in your question so far has been about DC analysis, so I dont expect an answer along the lines of "the rate isnt constant". If my highway has a tunnel, and I told you that 10 cars/min enter but only 1 car/min comes out. What would you say about this situation? – Matt May 25 '21 at 16:02
  • @Malemna Im not sure how to explain my comment about charge moving without an energy change, but here are some examples: shoot some electrons through a vacuum or use a superconductor. (I will admit this doesnt happen in circuit analysis, but I brought it up in case the overall idea helps clear something up) A change in energy doesnt imply movement of charge either. – Matt May 25 '21 at 16:06
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    @Malemna - Hi, Polite technical criticism of an answer (or question) is allowed, but please be careful not to cross the line into an "ad hominem" attack of a *person*. Therefore one of your comments has been deleted. I recommend you re-read the [Code of Conduct](/help/conduct). Thanks for your help with this. – SamGibson May 26 '21 at 12:19