21 Replies Latest reply on May 14, 2015 8:36 PM by nick123


      I missed the fundamentals of electronics like How current, resistance and voltage work. I Never really understood current, resistance and voltage and even semiconductors. Could anyone explain?

        • Re: Electron

          The following is an analogy and not meant to be exact definition:

          Voltage is like the push of electrons  comparable to water pressure.

          Current is the amount of electrons or the flow of water.

          Resistance is the stoppage of electrons like a dam. Current will flow more to the path of least resistance.


          Ohms Law



          Voltage = I(current) Resistance  or

          I = V/R or

          R = V/I



            • Re: Electron

              Thanks, But one more question, Why does current flow to path of least resistance?

                • Re: Electron

                  If I have two resistors in parallel, more current will flow on lower resistance. Remember Ohms law predicts this.

                  • Re: Electron

                    Hi Nick,

                    This is a very good question. "Why does current flow to the least resistance?" The question itself makes us think of resistance and current backwards. Current itself  does not make the choice. To illustrate what I mean I am also going to use an analogy. Let's imagine that we have a light source, like a 100 watt bulb. For this illustration the brightness of the bulb is analogous to the voltage in an electric circuit.  We are going to look at the bulb to see how bright it is but we are also going to place different materials between the bulb and our eyes. For example, if we put a piece of clear glass in front of our eyes we would see a lot of the light from the bulb. While the glass would still block a little bit of the light most of it would get through to our eyes. The glass has a low resistance to the passage of light. Now if we put a different piece of glass that is dark or cloudy between our eyes and the light we would still see some light from the bulb but not as much. This glass has a higher resistance to the passage of light than the clear glass. Finally let's put a piece of cardboard between our eyes and the bulb. This time we can't see the light from the bulb because the cardboard is not a conductor of light but rather an insulator. The light from the bulb in our analogy is the voltage, the material blocking the light is the resistance, and the amount of light that gets through is the current. Now you can see that the material that is blocking is really what determines how much light gets through. In other words the resistance of a circuit determines how much current flows. For the purpose of this thought experiment we have left the brightness of the bulb constant. You can imagine that if we increase the brightness ( voltage ) of the bulb and keep the same piece of darkened glass (resistance) in place that we will see the light get brighter (Increased current). The relationship between the brightness of the bulb (Voltage), the darkened glass (Resistance), and the brightness that we see through the cloudy glass (Current) is mathematically described by Ohm's Law. Ohm's Law says that the Voltage is equal to the Resistance times the Current. V = I * R. Hopefully this little visual thought experiment will help to clarify how Voltage, Resistance, and Current work together in an electrical circuit.


                  • Re: Electron



                    The picture you used shows conventional current flow (positive to negative) which, while useful in some circuit design situations is not correct (sorry Ben).

                    Electron current flows from negative to positive.





                    Current (in this case electrons moving through a conductor) just like anything else that flows always takes the easiest (least resistant) path to get

                    where it is going. The resistance to flow itself provides the guiding force for this. In the case of water in a pipe the resistance is provided by the friction

                    between the water molecules and other water molecules as well as the water and the walls of the pipe. In the case of electrons the reasons for the

                    resistance is a little more complicated but in its simplest form  the electrons do not flow directly from one place to another but instead bounce around

                    off other electrons and atoms to get where they are going. As they bounce around they lose energy and this energy is released as heat. An interesting

                    side note and one of the complications is that normally the hotter a conductor is the more resistance to current flow it has. The formula for finding how

                    much power is used (in watts) is:


                    P (power) = I2 (current squared) * R (resistance).


                    It can also be calculated as:


                    P (power) = I (current) * V (voltage).


                    Ohm's law and it's variants (such as the power formulas) provides the mathematical tools to explain this and to design circuits needed to control  it.


                    One other quick note, the above discussion is about DC (Direct Current). There is also AC (Alternating Current) but that is an explanation needing

                    another (several) days.

                      • Re: Electron


                             Which is GND, positive or negative. My picture does not show electron flow! But is useful  for understanding GND which I gave for explanation. Hence:

                        The following is an analogy and not meant to be exact definition:

                        Thank you


                          • Re: Electron

                            The arrows with the I (current) directly above and below them in the picture show the direction of current flow.


                            Like numbers in math positive and negative voltage are relative terms. Ground is more negative than any positive voltage and more positive than any negative voltage. 3 volts is more negative than 5 volts,

                            -3 volts is more positive than -5 volts. A long time ago (1977) I maintained a Fairchild Sentry integrated circuit tester that connected -11 volts (it was called TCOM for tester common) to the ground pins on

                            the ICs under test. As long as it supplied -6 volts (5 volts more positive than -11 volts) to the 5 volt pins of the ICs it worked fine. Occasionally the TCOM buffer board which supplied the -11 volts to the

                            test head would die which usually ended up with the IC being tested also very dead.


                            For ground I copied this from a Wikipedia entry:


                            In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.


                            There is nothing absolute about what voltage potential ground is at which is why when you connect signals from two circuits that use different power supplies together you must also have a ground connection

                            between them to make sure both power supplies are at the same ground potential. All the voltages from both of the power supplies will then, as the description says be referenced to the same ground.


                            Home power AC circuits use a direct connection to Earth for ground. The white (common) and green (or bare) wires in a home AC wiring are connected together at the circuit breaker box and these are
                            also connected to a wire or pipe that goes into the ground (as in dirt) from the circuit box. Again the voltage potential of that ground is not absolute and will vary from house to house or even at different

                            places around the same house.

                          • Re: Electron

                            Clem's picture and explanation are in fact, correct.


                            It is ALWAYS best to describe current flow from positive (Vcc source) to negative (ground drain). This is particularly true for all gpio situations with the Raspberry Pi, the Arduino, the Intel Edison, the pyboard, etc.  It is completely irrelevant which direction the little chunk of matter ( the electron ) in the wire is 'actually' moving...  what matters is how we keep current flow organized mathematically and metaphysically from design schematic to breadboard to pcb...  and it is ALWAYS best to picture current flow from positive to negative... this is especially true of pictorial representations of current flow through a transistor or diode, in which the arrows on the conventional schematics ALWAYS point in the direction of the positive current (holes) in the circuit through the diode or transitor.  It is NOT helpful to confuse the topic by pointing out that the electron actually moves in the opposite direction (especially to a newbie)!




                              • Re: Electron

                                Hi Marcus,


                                "It is completely irrelevant which direction the little chunk of matter ( the electron ) in the wire is 'actually' moving..."


                                But that is current flow ! How can you know electronics without knowing the basic concept of what current actually is and how and why it moves? When discussing things like electromagnetism

                                knowing the actual direction of current flow is not irrelevant.



                                "this is especially true of pictorial representations of current flow through a transistor or diode, in which the arrows on the conventional schematics ALWAYS point in the direction of the positive current (holes) in the circuit through the diode or transistor.  It is NOT helpful to confuse the topic by pointing out that the electron actually moves in the opposite direction (especially to a newbie)!"


                                But we are not talking about transistor or diode circuits, we are talking about basic DC circuits. Since you brought it up perhaps it would help if you explained what "holes" are in a way that

                                would not confuse a newbie. Of course to do this properly would require at least a preliminary discussion of semiconductors, valence bands, band gaps, and other things that make semiconductors

                                useful (doping, p/n junctions, recombination etc.). After that you could discuss transistor theory and explain why using conventional current flow when designing and displaying these circuits in

                                schematics is useful. That is a very big plate of confusion for any newbie. I'm just trying to point out that in this case your idea of less newbie confusing requires a lot of knowledge to be that way.



                                I've been over this a couple of times in these forums but once again electronics is a complicated and confusing subject. If a newbie wants to go beyond fiddling about, which some of them

                                (Hi Clem) clearly do, a good grasp of of the underlying physics, fundamental AC and DC electronic circuits, and more than just basic math are required. If a newbie really wants to understand

                                these things there will be some confusion getting there (YMWV). And while is not impossible to do that in a forum like this it does require a dedicated long term effort by a forum member to

                                cover all the basics and good feedback to help eliminate confusion as well as a newbie that is dedicated to learning it. Other alternatives are of course available such as books (I always had

                                trouble with books because you can not ask them to clarifying things that you do not understand from what you read) or Community College (sometimes the professors are not very good at

                                clarifying either, but other students may be).

                                  • Re: Electron

                                    I am willing to step aside, if you are willing to tutor the people on electronics here. Why not start a blog that starts from the basics and continues on  to say a DC to DC buck converter. Or maybe to electronics using  TTL logic like NAND and NOR gates building a simple accumulator.



                                    • Re: Electron

                                      In my book:


                                      Current flows from positive to negative - it's  a completely arbitrary thing measured in arbitrary units (amps).


                                      Electrons flow the other way, we don't usually talk about electron flow when analyzing circuits but we do need to think about it when describing how devices work.


                                      It would have been nice if the early pioneers had guessed right but they didn't so we are stuck with the problem.


                                      If you want to be really clear you need to talk about "conventional current" and "electron flow" but it's a bit pedantic sounding - one of them has to get the everyday name and pos to neg got there first


                                      I'm a full time electronic engineer and have been for more than 40 years - and I've never had a need to use anything other than the conventional model to design and solve circuits - although occasionally I've needed to think about electrons to understand why a device does what it does.


                                      I like the water model for starting off - you can think of the electricity as water flowing though a pipe and conveniently leave the fact that the electrons are going the other way until later.



                              • Re: Electron

                                Hi Clem, Gary, and Marcus,

                                I am feeling left out here. Can't anyone find something wrong with my crazy light analogy? The fact is that the questions are easy to ask and the answers are often difficult to formulate especially for guys like yourselves who know the subject to the very base of its foundation. Where do we begin our necessary simplification and where do we stop. I personally found all your explanations understandable and better than mine. I am sure Nick appreciates the time, thought and effort that goes into trying to explain the intricacies of electronics. If he has an abiding interest he will have to get a book and start reading. then we will be able to address more focused questions which he will encounter as he learns.


                                • Re: Electron
                                  John Beetem

                                  There's a very good description of basic electronics in "Max" Maxfield's Bebop the the Boolean Boogie: http://www.amazon.com/Bebop-Boolean-Boogie-Third-Edition/dp/1856175073


                                  Max uses water flow to represent current and water pressure to represent voltage, which works quite well as analogies.


                                  I was able to pick up a slightly-used 2nd edition for "a fistful of dollars" (shipping "for a few dollars more"), but Amazon currently only seems to have the first and third editions.

                                  • Re: Electron



                                    Might I suggest a book?


                                    Getting Started in Electronics: Forrest M. Mims III: 9780945053286: Amazon.com: Books


                                    Forest Mimms uses simple and easy to understand images and words to describe current flow.


                                    I highly recommend it!  Check it out!

                                      • Re: Electron

                                        Thanks, but another question. How does resistance work?

                                          • Re: Electron
                                            Jan Cumps

                                            Resistance uses material that has limited conductivity. That way it makes it difficult for current to flow.

                                            Copper has high conductivity. Current flows freely through copper.

                                            Carbon has much less conductivity. That's a characteristic of that material. It is much more difficult for current to run through carbon. This 'more difficult' thingy is called resistance.


                                            copper, with its good characteristic to let current flow through it, has very low resistance (almost 0 Ohm). It's a conductor. We use copper for wires.


                                            carbon, with its characteristic to restrict current flow, has a high resistance. It's a resistor. We use carbon to build resistors.


                                            The longer, narrower and thinner the piece of carbon is, the higher the resistance will be.


                                            You can use that analogy with water again. When you pour water from a bucket to the ground, it flows freely down (the air gap between bucket and ground is conductor). All water will go down at the same time.


                                            When you empty that same bucket through a pipe with a narrow diameter, much lass water can pour down at once. The pipe acts as a resistor.

                                              • Re: Electron
                                                John Beetem

                                                Jan Cumps wrote:


                                                copper, with its good characteristic to let current flow through it, has very low resistance (almost 0 Ohm). It's a conductor. We use copper for wires.

                                                The resistance of a copper wire depends on its cross-sectional area.  Thin wire such as AWG 28 is about 65 milliohms per foot at room temperature, which doesn't sound like much but if your wire is 10 feet long that's 650 mOhm.  If you put 1A through that 650 mOhm you'll get a 0.65 V voltage drop which changes your 5.0V power supply to 4.35V at your Raspberry Pi, causing it to malfunction.  So beware of Micro USB cables with thin wires, and keep the cable as short as possible.


                                                As the current passes through your cable it will warm up, which increases its resistance: AWG 28 is 72 mOhm/foot at 50 C (122 F).


                                                If you want really low resistance, go with 0000 welding cable.  It's 50 microOhm per foot at room temperature, or 20,000 feet (about 4 miles) per Ohm.  That weighs about 13,000 pounds.


                                                Here's my explanation "to the masses" about resistance in metal.  At the atomic level, metal is a lattice of atoms bonded to their nearest neighbors.  There are lots of extra electrons, so if you put a voltage across a block of metal the electrons can easily move towards the positive voltage, giving you an positive current in the opposite direction.  Heat causes the lattice to vibrate, which makes it easier for electrons to collide with the atoms, slowing down their progress.  The collisions cause additional vibration which makes the metal warmer, causing more collisions.


                                                This is a simplified, rather fanciful explanation.  If you want the gory mathematical details, take a senior/grad level solid-state physics course :-)

                                                • Re: Electron

                                                  Thanks, Great Explanation.