Why do circuits need a loop? - Page 1
Forum section: electronics / beginners

author: Legion
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:12:41 am
content: I don't understand why circuits need a loop instead of just a voltage difference and source of electrons? I did a couple experiments, the results of which confirm that a loop is required for current to flow. But these experiments didn't really help me understand WHY the loop is necessary. Here's what I did.I took 2 brand new AA batteries. I connected the anode of one to one side of a resistor and the other side of the resistor to the cathode of the other battery like this:I checked the voltage drop across the resistor and got 0V. I then check the voltage between the anode of one battery and the cathode of the other and got 0V. I don't understand how this is possible. The anode of a AA has a 1.5V potential over it's cathode. So why wouldn't the anode of one fully charged AA have a 1.5V potential over the cathode of any other fully charged AA?If I take a AA on it's own there's clearly no connection between the anode and cathode internally, otherwise the battery would just short itself out. So if the electrons can't move from cathode to anode inside the battery, it stands to reason that electrons traveling through a closed circuit stop when they reach the anode of the battery. They don't continue on to the cathode and go around again.If you use the water analogy for electricity, a battery would be kinda like a water tower (cathode) that flows into a lake (anode), if we're talking electron flow. Why does the water tower care which lake it's water flows into as long as the lake is at a lower level (potential)?In other words, why won't the electrons flow from the cathode of the first battery (C1) through the resistor and into the anode of the other battery (A2) so long as C1 has electrons to spare and A2 has room for more electrons?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 22, 2014, 12:22:22 am
content: When you connect a meter across a component... you're making a loop, right?If you had an ideal electric field probe (which would be a single terminal device -- sort of...), you would measure the whole thing (batteries and resistor) as some potential, with (relative to the potential hovering around the middle) 1.5V more towards the end of one battery and 1.5V less at the other end.This is much easier demonstrated with some electrostatic tidbits, like charged balls of aluminum foil on a string, styrofoam beads, rubber balloons rubbed on hair, etc. Reason being, the voltages are much higher (several kilovolts), so it's easier to experience the electric force.No current flows, so no voltage drop appears on the resistor.There is a "charge drop", which the field illustrates, but this doesn't perform work: compare electric force to mechanical force -- you can compress a spring so that it's holding some force (which exerts an equal and opposite force on whatever is holding that force -- because it is in a loop as well!), but the spring, just sitting there, in and of itself, does not dissipate power. (Whereas, the resistor -- when it has current flowing through it -- gets hot and dissipates power.)So, think about current, and the requirement of current flowing through wires to... do stuff.Tim

author: free_electron
author_rank: Super Contributor
timestamp: February 22, 2014, 12:24:32 am
content: to push an electron in one terminal fo a battery you need to take one out the other terminal.the total amount of electrons is a constant. you just shift them.there is no such thing as a source of raw electrons. electrons are bound to atoms. there are a fixed number fo atoms in that battery.basic physics

author: Nerull
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:35:11 am
content: All points in a circuit are trying to reach equal electrical potential. In a battery, chemical reactions work to produce a potential difference. When you connect the ends of a battery together, the reactions in the battery release energy to continually 'fuel' this potential difference, keeping it from going to zero.When you connect the anode and cathode of two otherwise unconnected batteries together, the potential between them equalizes in a split second, and and there is nothing in the circuit to add further energy - the reactions in the cell only care about the potential between the anode and cathode of the battery - so no further potential develops.If you were to measure the extreme ends of your circuit, you would now find a voltage of 3V. The cathode of A1 and the anode of A2 have reached equal potential, but the 1.5V potentials across each battery remain. The potentials add to create double the potential from the anode of A1 to the cathode of A2.

author: Unknown
author_rank: Unknown
timestamp: February 22, 2014, 12:37:10 am
content: A simpler answer: batteries (power supplies, etc.) don't create a potential at each end - they create a potentialdifferencebetweenthe two ends. To measure or do work, you need to take advantage of the difference.In your water tower / lake example, it's gravity which is 'closing the circuit' and providing the difference - gravity operates everywhere on earth, so it doesn't particularly matter which lake the water flows in to. Remove gravity from the loop (e.g. in space) & the flow stops.

author: VK5RC
author_rank: Unknown
timestamp: February 22, 2014, 12:40:10 am
content: From my simple understanding the electrons do flow through the battery (a pump in the water analogy) UNTIL the movement of charge (electrons) has created sufficient voltage (the batteries 'Voltage') to oppose the flow, this initial "flow" is very small and happens very quickly upon making the battery.The ratio of voltage change to charge (inversed) is a measure of capacitance, and all electrical circuit components esp batteries have some capacitance.A pump full of water but with closed inlet and outlet valves will move a little bit of water until the pressure (voltage) opposes the further flow. For more flow to occur a source of water (electrons) is needed.In Alternating Current (alternating + and - voltage applied) this small flow both 'in and out' can 'add up' giving the reality of AC current "flowing" across a capacitor.Dave has a good video about this in the last few months.It is really important to get your head around this, esp the differences between voltage and current, in AC they are often 90 degrees out of phase.It clicked for me some years ago when I read the early chapters of the Amateur Radio text ARRL handbook which had a good introduction to AC, capacitance and Inductance.

author: Maxlor
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:41:19 am
content: Quote from: free_electron on February 22, 2014, 12:24:32 amHardly.As for the OP's question: if you must employ the water analogy, your setup is more akin to water in zero gravity. Nothing will happen. Straining that image a bit more, you might see your batteries as pumps, with one end closed. Nothing will happen. As soon as you connect the two ends that were disconnected until now though, you'll actually get some current.However, I don't think the water analogy is all that helpful except for the simplest of cases. What I find more helpful is remembering that there is no absolute voltage, it's always relative to something (most often called ground, but it doesn't have to be, or even have to have the function of ground.) In your circuit, the plus poles of your batteries might be 1.5V higher than the minus poles, but there's nothing in the circuit that says that one battery's plus pole is equal to the other's plus pole. The opposite in fact, one battery's plus pole is equal to the other's minus pole. So basically, a battery's plus pole is 1.5V relative to it's minus pole only. Until you tie the minus pole to some other part of the circuit, and you've only attached the plus pole, that plus pole will conveniently float at the the same voltage of whatever(*) it's attached to (measured relative to what said whatever(*) is measured against), which means, nothing happens.

author: mazurov
author_rank: Frequent Contributor
timestamp: February 22, 2014, 01:14:48 am
content: You don't need a (conducting) loop for AC. Or any electron flow which doesn't need to remain constant forever. Do some experiments with a capacitor - AA battery, 10K resistor, 1000uF cap and a DMM is all you need.

author: c4757p
author_rank: Super Contributor
timestamp: February 22, 2014, 02:11:04 am
content: Quote from: mazurov on February 22, 2014, 01:14:48 am"Does current flowthrougha capacitor?"(Please don't answer that.)

author: codeboy2k
author_rank: Super Contributor
timestamp: February 22, 2014, 02:35:19 am
content: Quote from: c4757p on February 22, 2014, 02:11:04 am

author: edy
author_rank: Super Contributor
timestamp: February 22, 2014, 03:04:12 am
content: In a battery, electrons from one end need to combine with atoms on the other end because there is a chemical reaction that has to be balanced. Inside the battery, a chemical has to give up the electron and pass to another. You can't do that across two batteries because the chemicals in 2 separate batteries are not in contact. So electrons have no reason to jump out of the end of one battery... and the other battery won't take them. The chemicals must be mixed in intimate contact and we take advantage of the electronic exchange reaction going on between atoms by just redirecting the electrons through some wires and making it do some useful work for us.In other words, the potential energy or "push" on the electron is due to the fact that 2 chemicals are in special contact with each other and want to exchange electrons as they recombine chemically. It is this exchange within the same battery that moves electrons from one end to another through an external circuit.The confusion here is thinking that a battery is somehow made of one side with too few electrons and the other side with too many. That is not the case. The charges in a battery are balanced and it is not "full of electrons bursting to pop out" like a can of gas, with the other end empty of electrons.Remember that while electrons move one way, chemical ions have to move the other way in the battery (taking a shorter route) and so the electrons HAVE TO find their way back to the same battery to complete the ion-electron exchange. So if you hook up your 2 batteries in a loop, they will allow the other end of each battery to complete the electron exchange.

author: mtdoc
author_rank: Super Contributor
timestamp: February 22, 2014, 03:40:48 am
content: Batteries (or any voltage or current source) have potential energy between terminals. Without a path for current to complete the circuit, there is no path to release that energy and do work (or produce heat). Just as the potential energy in a boulder at the top of a hill needs a path down the hill, an electrical circuit needs a path from higher potential to lower potential.

author: edy
author_rank: Super Contributor
timestamp: February 22, 2014, 03:57:21 am
content: QuoteThere is no connection of anode and cathode internally, but there is an electrolyte which is in contact with both in between, containing chemicals which want to react with the anode and cathode. Except these chemicals can't react because electrons need to be taken from one end and supplied to the other to complete the chemical reaction. Only when you complete the circuit loop does it allow the reaction to continue. An electrolyte in your battery is internally reacting with the anode and cathode and changing its chemical composition which is why eventually your battery runs out when all the chemicals have reacted and changed form. But your battery is a complete system made up of 2 half-cells which are internally joined.... You cannot connect a half-cell from one battery (like the minus pole) to the half-cell of a completely different battery (like the plus pole) since there is no electrolyte exchange bewteen the 2 batteries in that configuration and therefore no potential difference.In other words, while electrons can flow through a wire from one battery into another, an equal and opposite FLOW OF IONS in your electrolyte cannot flow through the wire from one battery to another. Within a battery, what you don't see is that IONS are moving in the electrolyte in an opposite direction to the electrons which are flowing externally in a loop around your circuit. Electrons go one way but directed through wires outside the battery, while electrolyte ions go the other way within the battery. That is what drives the electromotive force. Two batteries connected may allow electron flow only between half-cells on each end, but since electrolyte are not physically connected across the 2 batteries then the ions have no path to travel in the opposite direction.

author: Rigby
author_rank: Super Contributor
timestamp: February 22, 2014, 04:17:26 am
content: I imagine it as a rigid water hose that doesn't leak at open ends. one can not put more water in without removing the same amount, and one cannot remove water without adding the same amount. it must be part of a circuit in order to conduct the water around.I like edy's explanation better, though.

author: Skimask
author_rank: Super Contributor
timestamp: February 22, 2014, 04:32:42 am
content: Magic Fairy Dust...That's my answer...I'm sticking to it...

author: noah_fakelastnamelike_bob
author_rank: Regular Contributor
timestamp: February 22, 2014, 04:35:48 am
content: i agree with skimaskbut it is pixidust not magic fairy dust whatever that is

author: Skimask
author_rank: Super Contributor
timestamp: February 22, 2014, 04:37:36 am
content: Oh no...it IS magic fairy dust in my little world.

author: Joule Thief
author_rank: Regular Contributor
timestamp: February 22, 2014, 05:08:02 am
content: Maybe this helps. Imagine the outer (unconnected) battery terminals to be closed instead of open pipes.

author: VK5RC
author_rank: Unknown
timestamp: February 22, 2014, 11:51:49 am
content: Its THE MAGIC SMOKE going around in circles not Fairy Dust, everybody knows that.

author: free_electron
author_rank: Super Contributor
timestamp: February 22, 2014, 12:28:48 pm
content: Quote from: c4757p on February 22, 2014, 02:11:04 am[dr. Evil mode] number two, please deploy the Alan Parsons Project and aim at c4757p [/dr. Evil mode]

author: Psi
author_rank: Super Contributor
timestamp: February 22, 2014, 12:49:08 pm
content: Quote from: Legion on February 22, 2014, 12:12:41 amThink of it this way.You cannot have a voltage difference without two voltagesrelativeto each other.If you take two batteries and a resistor, like you have in your diagram, the voltage on each side of the resistor isn't relative to anything because the other ends of the batteries are floating.Here's something else to consider that may help you.The positive terminal of a 1.5V battery is only at 1.5V if the negative terminal is at 0V.So,If the negative terminal is connected to 5V then the positive terminal is now 6.5VIf the negative terminal is not connected to ANYTHING then it's not at 0V or 5V or any other voltage. Only when you connect it to something do you have a voltage there.(I think i'm making sense but its 2am here atm so, yeah)

author: Legion
author_rank: Frequent Contributor
timestamp: February 22, 2014, 02:38:45 pm
content: Thanks everyone. That helps. I found the comments about gravity and viewing batteries as pumps instead of electron sources to be especially helpful. Sometimes I feel like I'm just memorizing rules and equations without having a proper model of what's going on.

author: IanB
author_rank: Super Contributor
timestamp: February 22, 2014, 06:05:44 pm
content: Quote from: Legion on February 22, 2014, 12:12:41 amMaybe there actually was a potential difference between the anode of one battery and the cathode of another? But maybe you did not have anything sensitive enough to measure it?Consider that when you connected the meter probes between the two points the meter made connection joining the two points together (the meter does not have infinite resistance). So the meter immediately caused them to have the same potential and then naturally you will see zero.A potential cannot exist indefinitely all by itself. It requires some kind of work or energy to sustain it. Otherwise when you connect the meter probes the energy will be discharged and the voltage will drain away.QuoteBut why do you say "clearly"? Clearly thereisa connection internally otherwise the battery could not work.Why doesn't the battery short itself out? Actually, it does, a little. This effect is called self-discharge. If you leave a battery long enough it will become discharged all by itself.How then does a battery work? There is a chemical pump inside the battery that moves electrons from the positive electrode to the negative electrode. This pump has a certain force, and it causes all the electrons to pile up on the negative electrode until the voltage there exactly balances the chemical force. Once that happens everything stops until you connect an external circuit to the battery and provide a path for the electrons to follow.QuoteThey do, they do. However, there is only space for a tiny number of electrons to do that before the potentials in the system "push back" and stop any more electrons flowing. The "room for more electrons" is incredibly tiny. Much smaller than you can measure with a simple meter. If you want more electrons to flow you have to give them an "out" and close the circuit.

author: Zero999
author_rank: Super Contributor
timestamp: February 22, 2014, 07:02:39 pm
content: Quote from: IanB on February 22, 2014, 06:05:44 pmYes, in addition, if you connect the anode to a large piece of metal, then quickly connect the cathode to the same large piece of metal, a small current will flow for a short period of time.

author: Shock
author_rank: Super Contributor
timestamp: February 22, 2014, 07:31:29 pm
content: I found these videos do a real good job of explaining the basics.

author: Legion
author_rank: Frequent Contributor
timestamp: February 22, 2014, 08:45:46 pm
content: Quote from: Shock on February 22, 2014, 07:31:29 pmI love these old military videos. I watched one on AC motors and it explained it so well.

author: edy
author_rank: Super Contributor
timestamp: February 24, 2014, 12:56:07 am
content: I guess another way to think about it is this.... It is like musical chairs.Electrons don't simply move "in bulk" from one place to another. If they did, they would create a huge charge difference and what you'd be talking about is static electricity. Static can be pretty impressive but not enough energy there to power much, maybe for a split second. And the electrons would soon find the forces so great that they will want to move backwards to equalize things.Instead, it is like the game musical chairs. A bunch of chairs are in the center and people continuously move from one chair to another, in a loop. Think of the chairs as the metal in your wire and people are the electrons moving. Music is the electromotive force causing the people to move. Stop the music and everyone sits in the chairs and the circuit is off.So in a circuit, you NEED A LOOP because electrons are just jumping from one atom to the next in the metal conduction path. As electrons move to the next atom, they will leave a space in the previous atom that needs to be filled by an electron that also jumps, and so on. Around and around it goes. Without a loop, the electrons in the wire will have to be pushed all the way over to one side, making the entire charged positive on one end and negative on the other. The electrons wouldn't be able to move anymore and there would be nothing happening after only a split second.