Can you help me understand a resistor? - Page 1
Forum section: electronics / beginners

author: Fredtop
author_rank: Newbie
timestamp: August 06, 2016, 08:12:30 pm
content: Hi,I know very little about electronics (relatively speaking) and are struggling to visualize how a resistor works in my head.Do they create a smaller hole for the electricity to go through, or do they adsorb some of the electricity?Sorry, I'm really looking at this is a dumb way I know.I think the secondary part of the understanding, is if it works like my second example, does that get turned into heat? As in, its converted and wasted energy.If I put a resistor before a bulb, it will dim it. But is this because the resistor is stealing some of that power, or just not allowing as much to flow and get to the bulb. Is another way of explaining how I'm trying to see it.Many thanks for your patience and help.Fred

author: stoica adrian
author_rank: Contributor
timestamp: August 06, 2016, 08:24:59 pm
content: http://www.allaboutcircuits.com/education/

author: Fredtop
author_rank: Newbie
timestamp: August 06, 2016, 08:29:31 pm
content: Thanks for the link. Its good to understand it on paper for sure.But I need to visualize whats happening in my head. And that's something I'm struggling to get from reading about it or watching videos.I guess I'm imagining electricity as flowing water. Rightly or wrongly, that's how (at my basic level) I need to see it and see how a resister works in that way of thinking.Does it soak up the water and let it evaporate before it goes further, or does it just make a smaller hole, reducing how much can go through?Regards

author: helius
author_rank: Super Contributor
timestamp: August 06, 2016, 08:31:04 pm
content: Laypeople (Muggles?) are oft led astray by a preconception that "electricity" is a real thing, like bagels or buttercups. It isn't, the way engineers think about it. Electromagnetic phenomena are an inherent property of all matter, and they only discuss measurable aspects of that phenomena, like charge, current, or voltage.A resistor is a component that is designed to have a specific resistance. Resistance is also a property of all matter, specifically, it is the proportional relationship between the electric field strength (voltage) and the rate of movement of charge (current). These words sound cumbersome because they try to express a mathematical relationship, and all science is really impossible without using math more or less directly.Resistance is the amount of electric field (voltage) required to push electric charge through a material at a particular rate. Higher resistance means you need more voltage to push the same current; lower resistance means you can push the same current with less voltage, or higher current with the same voltage.In physical terms, a higher resistance presents more "obstruction" to moving charges, so as they move they crash into things more often. The more often charges crash into an obstruction, the more force (voltage) is required to keep them moving along.Edit: that video is pretty misleading. At 0:45 it compares electrical current to water (which is not a good analogy) and then proposes that a narrowing of a water tube causes a reduction in flow similar to a resistor, which is not true. The Bernoulli and Navier-Stokes equations govern water flow, and make the water speed up in this situation. Electrical current does not speed up in a resistor.

author: hamster_nz
author_rank: Super Contributor
timestamp: August 06, 2016, 08:31:57 pm
content: Think of it this way. In a good conductor electrons move unimpeded. In a resistor the bounce into the nonconductor bits, loosing energy as heat as the do so.

author: spacedementia87
author_rank: Contributor
timestamp: August 06, 2016, 08:37:52 pm
content: Electricity isn't really a thing it is just the name of a concept.Really it works because there can be differences in potential and there are things (electrons) that can flow from.an area of high potential to one of low potential.Water is often used as an analogy. Imagine a log flume. You pump the water up to a top of a high hill (a cell does this) this water now has a lot of potential. The water can now flow from that area of high potential to the bottom of the hill where there is low potential.If you make the tube the water is flowing through really narrow, it slows the flow of water down. Both before the narrow bit and after it.Circuits work in a similar way. A cell or battery gives electrons potential energy creating a **potential difference** between two point in a circuit. The electrons can then flow from the area of high potential to the area of low potential. Just give a nice big metal wire and the electrons can flow really fast. Lots of electrons passing a certain point every second is a high current.Put a resistor in and it is a material that electrons cannot flow though very easily. They have to slow down to move through the resistor. This causes a traffic jam behind. As such a resistor reduces the number of electrons per second moving around the circuit even though the potential difference (measured in volts) is the same, the current is lower.In the resistor the electrical energy is transferred into heat energy. Look up high wattage resistors. They are large devices with big metal heat sinks to dissipate the heat energy.Sent from my ONE A2003 using Tapatalk

author: Fredtop
author_rank: Newbie
timestamp: August 06, 2016, 08:48:49 pm
content: Thank you everybody. With a combination of the above posts, I now see it a lot better. Confirmation of the heat generation is very valuable.If you measure with and without a resistor, do you see the same voltage on a multi-meter?

author: hamster_nz
author_rank: Super Contributor
timestamp: August 06, 2016, 08:55:40 pm
content: Quote from: Fredtop on August 06, 2016, 08:48:49 pmIf you remove the resistor, the measured voltage will always be higher, as power is no longer being lost in the circuit feeding the resistor (as long as the meter's impedance is higher that the one that has been removed for the test)

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2016, 08:56:17 pm
content: Yes. Try putting a 10k resistor across a battery and you should measure the same voltage, now put it in series with the battery and the meter and you should measure the same voltage.Note that this isn't perfect, since both the battery and the meter have some resistance. If the resistor is a very low value, the battery voltage will drop, when it's connected in parallel with it because a voltage is dropped across the battery's internal resistance. If the resistor is a very high value, then there will be a voltage drop across it when it's connected in series with the meter because the meter is not a perfect open circuit.

author: Brumby
author_rank: Unknown
timestamp: August 07, 2016, 03:06:15 am
content: Quote from: Fredtop on August 06, 2016, 08:48:49 pmIt depends.It depends on the resistor, how the resistor is connected, the multimeter and the circuit you are measuring.

author: bitwelder
author_rank: Super Contributor
timestamp: August 07, 2016, 05:38:11 am
content: Quote from: Fredtop on August 06, 2016, 08:29:31 pmOne common way to help understanding electricity is actually the 'hydraulic' or 'water-in-pipes' analogy:https://en.wikipedia.org/wiki/Hydraulic_analogy(if you google for it you will find plenty of results, some in textual, some in video form if you prefer that)While of course it won't explain everything (and it can also be a bit misleading when you move into more complicated issues), it can help visualize some basic facts about electricity and circuits.

author: edy
author_rank: Super Contributor
timestamp: August 07, 2016, 01:11:48 pm
content: A resistor makes it harder for electrons to flow through it, so they bump into atoms and their motion is converted into motion of atoms in the resistor, heating up the resistor (temperature is motion of atoms).I think of Ohm's law, V=IR. Apply same voltage through a resistor, say from a battery. If you increase R (resistance), you decrease I (current). So a resistor lowers the number of electrons passing through the circuit, given the same electromotive potential (voltage) that you are applying to it.Hope that makes sense.It's like pushing on water in a pipe with a certain pressure. If you have a clear pipe, lots of water flows per second past a point. Stick a bunch of stuff in the pipe so it clogs it up more and more, and with same pressure the water flow rate will be reduced.

author: timb
author_rank: Super Contributor
timestamp: August 07, 2016, 01:15:52 pm
content: Quote from: bitwelder on August 07, 2016, 05:38:11 amFrom that article:"Electricity (as well as heat) was originally understood to be a kind of fluid, and the names of certain electric quantities (such as current) are derived from hydraulic equivalents."But... But... That *is* true! What do batteries and electrolytic capacitors contain? Electrolyte! What is electrolyte? A fluid! (It's also what plants crave!)Ergo, electricity is a fluid!:smug:Seriously though, while the water analogy does start to fall apart once you get past the basics, it works well enough to help most laypersons understand the concept. I say most people, because some people couldn't find their ass with two hands and a flashlight.For example, I had a girlfriend who just couldn't understand even the water analogy. Why? She...didn't understand how water worked! Seriously! I asked her what she thought the pipes under the sink were for and she thought they some how produced the water. As in they just filtered dirty water that went down the drain. Apparently, when she was a kid in elementary school, they were learning about the space station (or the shuttle or something) and the teacher mentioned how waste water could be filtered and recycled. Nearest I can figure, she just assumed that's how kitchen sinks worked too...I understand a misconception like that when you're 5, but to not question it until you're 25!

author: Brumby
author_rank: Unknown
timestamp: August 07, 2016, 02:14:31 pm
content: I had to laugh .... but ^^^ that's ^^^ not a bad analogy.

author: metrologist
author_rank: Super Contributor
timestamp: August 07, 2016, 02:23:56 pm
content: How about this. Water used as a resistor. distilled water has a very high resistance. Add some electrolyte and the resistance goes down. Is that because it holds more free electrons that can then flow? Is that what makes resistor resistance values differ, how many free electrons are floating in the material that can pass through a cross sectional plane?

author: helius
author_rank: Super Contributor
timestamp: August 07, 2016, 07:35:03 pm
content: Quote from: metrologist on August 07, 2016, 02:23:56 pmResistivity.A channel of distilled water 2km across and 1mm long would have an extremely low resistance.QuoteNo. There are no free electrons at all, before or after.QuoteFor solid metals, the resistivity is related to the crystal structure of the material and its temperature.

author: spacedementia87
author_rank: Contributor
timestamp: August 07, 2016, 11:09:49 pm
content: It was asked if the "voltage" changes when you change the resistance. This question is hard to answer because of the term voltage here.Voltage is like metreage. What we measure is distance, we measure it in metres.Volts can be used to measure potential difference and electromotive force (emf).The emf of a cell is what we measure when no load is attached.As soon as we draw a current the potential difference between the terminals drops. The better the cell the less this drop will be.Quote from: metrologist on August 07, 2016, 02:23:56 pmNo, solutions conduct electricity because of free moving ions. Not electrons.Sent from my ONE A2003 using Tapatalk

author: Brumby
author_rank: Unknown
timestamp: August 08, 2016, 01:35:58 am
content: There's a lot of stuff being presented here that I see will only confuse someone who is struggling with afundamentalunderstanding.IMO there are two things that have been said which best help this understanding.....An electrical desciption:Quote from: edy on August 07, 2016, 01:11:48 pmand a water analogy:QuoteI particularly like this water analogy - and I would like to repeat it with some annotations that bring it a bit closer to the electrical concept.(One suggestion though, when you first read this think of the word 'resistance' as a mechanical attribute where water is trying to flow. When that is clear to you, read it again and then think about the electrical side of the analogy.)Quote

author: metrologist
author_rank: Super Contributor
timestamp: August 08, 2016, 02:10:20 am
content: Quote from: helius on August 07, 2016, 07:35:03 pmWell, that's odd. I recall doing electrolysis using a 12V battery charger. When I put the electrodes in pure water, there was no measurable current , until I added some type of electrolyte. Then I was able to get over 10 amps of current flow through the mixture.In your example, are the probes 1mm apart or 2 km? What is anextremelylow resistance? So many things are intuitive.QuoteI was considering it within the realm of common conductors and resistors, and sort of like the Drude Model:https://en.wikipedia.org/wiki/Drude_model

author: hamster_nz
author_rank: Super Contributor
timestamp: August 08, 2016, 02:27:06 am
content: Quote from: metrologist on August 08, 2016, 02:10:20 amI guess in effect what was being said was a big enough stack of 1 Mega Ohm resistors could have low resistance, as long as you connect enough of them in a parallel....

author: metrologist
author_rank: Super Contributor
timestamp: August 08, 2016, 03:14:49 pm
content: Yes. I meant un-intuitive. Never would have expected glass to have a low resistance either, unless that was a typo in the article I was reading.http://hypertextbook.com/facts/2006/SamTetruashvili.shtml

author: mikerj
author_rank: Super Contributor
timestamp: August 08, 2016, 04:18:31 pm
content: Quote from: helius on August 06, 2016, 08:31:04 pmA fluid will speed up through a restriction, but the flow rate (current) for a given pressure drop (potential difference) will reduce, exactly like a resistor so it's not misleading in that sense.

author: Gyro
author_rank: Super Contributor
timestamp: August 08, 2016, 05:58:04 pm
content: I can't wait for you guys to start helping the OP understand jfets and mosfets!Some interesting analogies though.

author: Brumby
author_rank: Unknown
timestamp: August 09, 2016, 04:21:07 am
content: Quote from: helius on August 06, 2016, 08:31:04 pmThis "reduction in flow...which is not true" presumes you talk about speed - but speed is not relevant in the analogy. Nowhere in any basic discussion of resistor operation is speed a factor. If, however, flow refers to something else ...Quote from: mikerj on August 08, 2016, 04:18:31 pmAny analogy will not be completely accurate - but that is not its purpose. Attacking the analogy is ONLY valid if it addresses afundamentalandrelevantproblem with the analogy itself. Leave Bernoulli out of this.The OP needs a basic mental picture to help understand the fundamentals of the concept and the water analogy WORKS EXTREMELY BLOODY WELL - especially for this topic.All these other tangents are just going to confuse the heck out of the poor person.

author: hamster_nz
author_rank: Super Contributor
timestamp: August 09, 2016, 06:20:40 am
content: Quote from: helius on August 06, 2016, 08:31:04 pmA couple of trivial points...What you are saying seems to imply that when the value of a resistor attached across an ideal current source, the amount of current flowing the resistor should change with the resistance. Of course this is false... Likewise, if the water pressure is kept constant, a smaller pipe *will* cause a reduction in flow.And those equations do not "govern" anything (although they are called the 'governing equations') the model it, and only as long as you do not take things to the extreme where they break dow.One of the Navier-Stokes eqns is that net fluid flow across any volume is zero, which is incorrect if a vacuum is created (e.g. cavitation). Or super-sonics, or shock discontinuities or ...And maybe electrons do speed up in a resistor. With less charge carriers available, they must be travelling faster to deliver the same amount of current???(And do charge carriers act as a fluid or a gas? Can you push more charge/electrons into a conductor if you push them hard enough?)