Do I understand the terminology of voltage/current? - Page 1
Forum section: electronics / beginners

author: Richardcavell
author_rank: Contributor
timestamp: April 22, 2024, 02:14:53 pm
content: Hi.I want to make sure that I understand this circuit. Please tell me if anything I'm saying here is wrong.There is a battery, which is drawn as 3 electrochemical cells in series. The voltage of the battery is the sum of the voltages produced by each cell. The internal resistance of the battery is the sum of the internal resistances of each cell. The current will be the same as that produced by the weakest cell.By convention, current is said to flow from positive to negative. So this is clockwise for this circuit.There is a voltage drop from A to C. In fact, there is a voltage drop from A to B, and also from B to C. The voltage drop from A to B plus the voltage drop from B to C equals the voltage drop from A to C. In fact, the resistances add that way as well. The current at point A is the same as the current at point C.The wire that point A is located on has a resistance so low that for most purposes we can say that it has no resistance. Therefore, the voltage at all points on that wire is the same. Same with the bottom wire.We can say that the bottom wire is "ground". That refers to zero voltage. The top wire is some positive value above ground.I could equally say that the top wire is ground, and that the bottom part of the circuit has a negative voltage with respect to it.It is my choice which wire I refer to as ground. In fact, I could refer to point B as ground if it is convenient for me.Is it correct to say that the voltage is converted to heat? Or is it something like, the energy expended by the current flowing through the resistors is equal to the integral of power with respect to time. The power is a function of voltage, current and resistance. That energy is given off as heat.Richard

author: wasedadoc
author_rank: Super Contributor
timestamp: April 22, 2024, 02:22:47 pm
content: Voltage, current and resistance are variables but only any two of them are independent. Once you set any two of them the third is automatically determined. (Ohm's Law).

author: rstofer
author_rank: Super Contributor
timestamp: April 22, 2024, 03:16:18 pm
content: A good understanding of Kirchhof's Laws is very helpful:https://courses.lumenlearning.com/suny-physics/chapter/21-3-kirchhoffs-rules/Current can't pile up at a node and voltage can't spill out on the ground.

author: tggzzz
author_rank: Super Contributor
timestamp: April 22, 2024, 04:38:27 pm
content: Quote from: Richardcavell on April 22, 2024, 02:14:53 pmThat's all correct so far, and you can go further...The concept of "ground" is essentially a fiction, a convenient "story" that has some usefullness in some circumstances - but which can be misleading in others. Rather than "ground", it is better to think in terms of yourchoosingto define one node as being at the reference voltage (of 0V), and thereafter measuring all voltages with respect to that reference node.The components you designate as R1 and R2 can be generalised to non-linear components such as a diode. All your statements will remain valid,except"In fact, the resistances add that way as well."As rstofer noted, this is formalised by Kirchoff's current law (KCL) and Kirchoff's voltage law (KVL)KCL states that the sum of all currents into a node must be zero.KVL states that the sum of all voltages around a loop must be zero.You already realise those w.r.t. your circuit, but they are also true when there are multiple nodes and multiple loops in a circuit. Once you get a "feel" for what that means, you will have a powerful conceptual tool for analysing more complex circuits.QuoteNot quite.The power dissipated in each component is the voltage across that component multiplied by the current through that component (i.e. P=VI). The energy dissipated is indeed the integral of power over time. That is true for linear components (e.g. resistors) and non-linear components (e.g. diodes).

author: m k
author_rank: Super Contributor
timestamp: April 23, 2024, 09:03:26 am
content: Quote from: Richardcavell on April 22, 2024, 02:14:53 pmEnergy can do work, for that it needs time.More work during that time needs more power.So energy and power are sort of the same.Electricity can't be forced, it is taken.Resistance is taking energy of one form and converting it to the other.Two fluids can't mix if there is a gap.Electricity can't flow if circuit is not closed.Flowing electricity is balancing potential differences.Resistance is resisting electricity flow by creating friction, and heat.But it's not forcing, only balancing.After balance there are no more free circuit participants to balance more.Electricity is on the high ground, it has a potential.When it drops down its shape decides the rest of its effect.Medium through what it is dropping decides that shape.Since electricity is taken, it is also given, if available.If current is not available voltage must drop, to maintain balance.Maybe it's demanded.

author: jwet
author_rank: Frequent Contributor
timestamp: April 24, 2024, 03:13:30 pm
content: I think that is a very cogent description.One quibble that might shows some misunderstanding is saying the that the "current at point A..."- voltage applies to a point but current applies to a path. You might say that "the current along the top conductor..." or the "current through point A...". Its a small point and anyone would know what you meant as long as you understand.

author: IanB
author_rank: Super Contributor
timestamp: April 24, 2024, 03:21:55 pm
content: Quote from: Richardcavell on April 22, 2024, 02:14:53 pmEverything you said was spot on, except for this last part.A correct statement is that electrical power is consumed by the resistors and converted to heat. The instantaneous power (in watts) consumed by a resistor is equal to the voltage difference across the resistor times the current flowing.In physics, energy is conserved. So looking at the system as a whole, chemical energy stored in the cells is converted to electrical energy flowing in the circuit, which is converted to heat in the resistors (and also in the internal resistance of the cells).Lastly, energy is a total quantity (measured in joules), and power is a rate of change of energy or flow of energy (measured in joules per second, equal to watts).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 25, 2024, 11:48:47 am
content: This is also not entirely true:Quote from: Richardcavell on April 22, 2024, 02:14:53 pmInside the battery, current flows from the negative terminal to the positive terminal. Current also does not go from one place to the other, current always runs in loops. (Or else you build up static charges with very high voltages really quickly).And there is of course also:https://xkcd.com/567/

author: tggzzz
author_rank: Super Contributor
timestamp: April 25, 2024, 12:04:00 pm
content: Quote from: Doctorandus_P on April 25, 2024, 11:48:47 amWhile true, such thoughts can be confusing in the longer term.It is simpler and more consistent to think (and formulate KCL equations) in terms of the current's direction in/out of a component's more positive terminal.From that fundamental statement valid by definition for all components, it is possible to derive your statement about a particular component, i.e. a battery.

author: radiolistener
author_rank: Super Contributor
timestamp: April 25, 2024, 12:42:14 pm
content: Quote from: Richardcavell on April 22, 2024, 02:14:53 pmNo. Power is not voltage.Electric power = voltage * currentThe heat power = ΔT * h * AwhereΔT is temperature difference (analog of voltage in electric power)h is heat transfer coefficientA is surface areah * A is heat transfer capacity (analog of current flow in electric power)So, in this case electric power (voltage * current) is converted to heat power (ΔT * h * A).

author: Terry Bites
author_rank: Super Contributor
timestamp: April 25, 2024, 03:49:13 pm
content: The cell voltage is a measure of its potential energy. Energy is transported via the current ino the circuit resistances.