Used to watch these videos when I was a high school dropout and had a dream to study physics. Since I watched all of your videos, I have gotten my GED, done 2 years at a community college, and now am a Physics Major at University of Maryland working in a high powered laser optics lab. These videos started it all. I would watch them while at work and take notes on little sticky notes, which are still on the wall in my room. Thank you DrPhysicsA, seriously. You changed the entire trajectory of my life.
I had small particles and figments of knowledge from my teachers, but after watching this and taking notes, they've all combined to make a large mass of consolidated information. Thank you, my good Sir.
Hello. Can I say that I have really enjoyed watching this video and other videos of yours. I have been able to gain lots of information, in terms of my studies at both GCSE and A Level Physics, which the A Level Physics I'm doing distance learning. I'm also doing A Level Maths and A Level Further Maths, distance learning also. If it wasn't for people like you, who give up their free time to produce videos on such a good scale, I would be in a worse place. Can I say thank you very much for your help and support, it's appreciated very much. Regards Dominic G
V and r (internal) are constant. R (load) is variable. I = V/(R+r). Power thro Load = I^2 R = V^2 R/ (R+r)^2 = V^2/ R + r^2/R + 2r. Now for the power to be a maximum, the denominator must be a minimum. Diff w.r.t R and set equal to zero. You get 0 = 1 - r^2/R^2. So r^2 = R^2. So r=R.
Yes indeed. I can't now remember why I plotted it that way round. I suspect I was simply trying to show that there is a direct proportionality between current and voltage. But as you say, the slope of the graph which I drew is 1/R not R itself.
Thanks for kind comments. My videos are an amalgamation of AQA, OCR, Edexcel and Cambridge A level syllabus for both AS and A level (with the exception that I don't tend to do the biophysics parts). The videos in the A level revision playlist "Atomic Structure - A Level Physics" contains atomic energy levels and photon emission when electrons fall down to lower energy levels.
at 16:20 shouldn't it be -I3R3 + I2R2? since if we're going with the current direction, there must be a voltage drop and if we're going against the current direction then it will be a voltage gain?
Well there is a quantum mechanical underpinning to all this, but for purposes of A level all I meant was that as the current passes thro the filament of the lamp it heats the filament (and thus produces the light). As the filament heats up, its resistance increases so the current falls. R increases because the electrons in the wire get more excited and therefore there are more collisions and interactions between electrons which impedes current flow.
Unless I've missed a point, the answer is that you need about 0.7v across the LED before any current will flow in the circuit. So for the first 0 - 0.7v I would expect no current. Thereafter, assuming there is a resistance in the circuit I would expect current to grow proportionately with voltage.
You're a life saver. Your videos saved me like 50-60 pages of reading. Your explanations are thorough yet simple to follow and idk you just made my life THAT much easier.
EMF is voltage. So it is electric potential (V) - potential energy per unit charge. Electric Potential Energy (Ep) is work done moving a charge (q2) from infinity to a point r away from q1. The Electric Field (E) is the Force per unit charge.
Thank you so much. I have always had a problem with grasping the abstract essences of electricity. Your profound video has helped healed my wound and made me get it. Thanking you a thousand times would still not repay what you have done to the world.
Yes indeed. You will find a set of vids on QM in my Quantum Mechanics playlist (see main page of my UA-cam channel) and you will find additional Waves vids in the playlist on A Level Physics.
It is "Atomic Physics 3: Semiconductors, Diodes and Transistors". But you might want to watch videos 1 and 2 first to set it all in context. You can find these on my home UA-cam page (drphysicsa) in the playlist Atomic Physics. Thanks for kind comments.
Thanks so much, with only a couple of weeks till my exam I wasn't sure if I could pass them, after watching your videos everything seems much clearer :)
Took physics in university a decade ago. Our professor stinks at really teaching us anything. Didn't understand a damn thing and its a miracle I managed to pass the courses. Anyway, I'm currently settled in my career unrelated to physics...but out of sheer intellectual curiosity decided to re-learn physics. I find your youtube channel highly valuable --"subbed." I am sure my life/career choices may have ended up differently had I actually understood this stuff a decade ago.
i am so confused. Why do you sometimes use the direction of conventional current and sometimes you use the "actual"direction.?? What should we assume in the exam?? Thank You!!!
The problem arises because electricity was discovered before anyone knew anything about electrons. It was assumed that electricity travelled from positive to negative and that is what we still call the conventional current. But we now know that the electricity is the flow of electrons which actually travel from negative to positive because they are negatively charged and are therefore attracted to the positive terminal. We retain the idea of a conventional current travelling from plus and minus even though nothing actually flows in that direction.
DrPhysicsA "historical reasons" i thought the the whole selling point of science was that it was willing to change ideas based on evidence. Physics has failed. Archimedes rolls in his grave.
At 3:56, the point I'm trying to convey is that N if the total number of electrons flowing per second, Q is the total charge flowing per second and that that must therefore represent the current I.
N is number of charge carriers passing a given point per second. Q is the total charge passing a given point per second. But charge flowing per second is current. I=Q/t
I think it was Benjamin Franklin who started the idea of electricity flowing from + to -. You will recall that electricity was used long before it could be explained quantum mechanically and before electrons had been discovered. It was therefore just unfortunate that current flow was determined to be the opposite direction to the particles which actually flow - ie the electrons.
To help better my understanding of how electricity works, I envisioned myself being an electron whizzing around an electron which constantly reversed EPE as I deliver energy to different electrical components and release energy as light quanta.
Electricity which flows through miles of power cables suspended from pylons is still subject to a potential difference, otherwise no electricity would flow. Of course, for the most part the electricity is alternating current. Because the power cables are themselves part of the enormous circuit that is why it is essential to transmit at high voltage so as to minimise power losses through the cable.
3:40 It is a bit confusing that 'v' denotes distance in 'Q=Avnq/sec' and velocity in 'I=Avnq'. 'Q' usually denotes charge (not charge per second) and 'v' usually denotes velocity, not distance. Hence, perhaps the first equation should rather be 'Q=Avnq·sec', or am I wrong?
At 5:20 the straight line's gradient represents 1/resistance.... If voltage is on the y-axis and current on x-axis, then the gradient represent Resistance
0:36 The direction of current in the source and in the circuit is different! From "minus" to "plus" in the source and from "plus" to "minus" in the circuit. It's because the electrons flow in the circuit is not free, but driven by the source. The circuit also must be closed otherwise there'll be no current. It's written in books about historical convention, but it's purely practical as you can see. Means that everytime you see a drawing of a current in a book it is a part of the circuit not the source, if nothing else is specified. Many people are confused about it. I wonder if it was so hard to put in books this simple explanation instead of "historical convention" just to make it clear from the bigining?
1:15 - isn't that equation change in current over change in time, as opposed to the amount? I may not fully understand the mathematical meaning of triangles, otherwise- it just seems rather confusing.
About the positive-to-negative flow of current and the negative-to-positive notation, is that universal? For instance, I live in the U.S., so are our engineers and electricians using that reverse notation in their daily life? Or are there situations, besides personal preference, where it may actually be taught and trained in the way that makes sense? It just seems like something that could cause unnecessary problems.
Brilliant video! Very helpful, thank you! Is it right that in a DC circuit the charge flows because it is subject to an electric field created by the battery? How does charge then flow through 300 miles of pylon, where is the electric field originating from in this case?
How a charge actually flows in a circuit? Does the battery opens space to the previous electron so that the nucleus next to it pulls it and so on? I know charges repel or attract because of the photon exchange between them which sort of tells to its electron mate to repel each other but how does that even work?
Great video!! There was only one think I was unclear about though, which was is the EMF the same as the E? As I understand, the EMF is the total amount of energy given by the cell, as is the E - is this just different notation?
Taylor Shill potential difference across a parallel circuit is the same across each branch in every scenario. This means that, the same electromotive force is doing work on these charges, and if there are resistors, the current will be distributed through each equal to the ratio of the resistances. That is to say, the same force is pushing the electrons, so less go through high resistances and more pass through the lower resistances. If you have a parallel circuit with a battery with internal resistance, you can first find the total external resistance using the reciprocal addition of resistances rule, and then treat that as a single resistance in series with the source of emf. The difference is, in a parallel circuit the voltage is the same across the whole and in a series circuit it’s the current that’s always the same. This means that, in a series circuit the voltage has to be greater at higher value resistances in order to push electrons through and maintain the constant current. In this way, whilst in parallel the ratio of currents are equal to that of the resistances, in a series circuits it’s the ratio of voltages that’s in the ratio of the resistances.
At 16:58 you're stressing that you use a minus sign because you're going opposite the current. Why is it then, that you're not using plus where you're going in the same direction as the current?
What is the proof that you will always get the necessary number of equations and unknowns to calculate the currents in each branch by writing kirchoff's laws for the circuit? ( I'm basically asking why is kirchoff's laws a foolproof method to find the variables in a circuit..)
I am not talking about the validity of the laws themselves. I am asking why is it that you always have the correct number of equations and unknowns to calculate the currents in every branch of the circuit? Is the method of writing kirchoff's rules a foolproof way to calculate the currents in a circuit?? If so, Is there a proof to support that statement??
Anand Krishnan As I said, the principle is that energy is conserved and that this means that if you go round a circuit and come back to where you started you must be at the same potential energy. Whether or not you have enough information to calculate what the currents are in any particular loop will depend on how much information you have been given.
Great stuff - one thing to correct you on, however. In the derivation of I = nAve, you say that n is the 'number of electrons per m^3'. Actually, it's the number of 'free electrons per m^3'; most of the electrons can't carry a current, as they're bound into the shells of the atoms, and it is only those which have been 'delocalised' that can carry a current. A small point, but one that I have seen marks for in mark schemes (WJEC in particular, who love it).
I got a little confused in the beginning when you talked about v being both a velocity and a distance. Wouldn't it be more accurate to talk about v as the velocity and vdt as the distance? In which case dN = nAvdt. Oh, and great series of physics videos. I got here via your mechanics videos. What you have made is very much appreciated :)
Hi just another question, if you're performing an experiment about the relationship between the current and the potential difference in LED's, what kind of line on a graph would be expect to get?
double circuit confuses, why it is zeo when you go baxk to where you started>>? I think until the loop is closed it should be continuing without break and voltage cannot be 0 because current continues to flow until it won' slow it down to the stop. I might be wrong but yet I don't understand why you substract the current from total energy??
Dear SIr, Thank you so much for the wondeful lesson on logic gates. i am starting to like the subject i was afraid of!! sir could you tell me where can i find your lecture on how a transistor works?
I don't understand the cancellation at the end. How do the E's in the numerators all cancel in the parallel circuit example and why do the I's cancel in the series circuit example. Other than that I found this very useful indeed!!!
do be careful not to confuse E for energy with epsilon for EMF. Also rho for resistivity should be lower case with a long tail and power should be a capital P.
Usually we plot a graph of resistance against 1/current. But if we invert the axis to 1/current on y axis against resistance. How do we display this in the form of y=mx+c?
***** Well we have the equation V=e-Ir so we rearrange this equation to make it appropriate for y=mx+c V=-rI+e so if you graph this you can look at the y-intercept for the e.m.f and take the absolute value of -r (|-r|) for the internal resistance.
e= IR +Ir divide by current e/I=R+r divide by emf 1/I=R/e+r/e Gradient is 1/e Y intercept is r/e Im assuming you mean the equation of the line on the graph of 1/current against resistance
considering Kirchhoff s 2nd law , what if there is a circuit with no appliance(no resistance) and only a cell connected , so as current flows through cell it would gain energy and this energy will add up when it completes one complete circuit and passes through cell for the second time, so current would keep on gaining voltage ? how is Kirchhoff law applied here
![Lp. Сердце Вселенной #60 РОЖДЕНИЕ ЛОЛОЛОШКИ [Финал] • Майнкрафт](http://i.ytimg.com/vi/YoR0pAV9FVQ/mqdefault.jpg)
Used to watch these videos when I was a high school dropout and had a dream to study physics. Since I watched all of your videos, I have gotten my GED, done 2 years at a community college, and now am a Physics Major at University of Maryland working in a high powered laser optics lab. These videos started it all. I would watch them while at work and take notes on little sticky notes, which are still on the wall in my room. Thank you DrPhysicsA, seriously. You changed the entire trajectory of my life.
You are a credit to the internet.
My procrastination has lead to last minute panic revision.... once again.
I am glad there is someone in the same boat as me!
Oh I'm 100% sure its not just us two either I can name at least 50 from my college alone :P
Lol, I'm watching this video a few hours before the exam. I am the one who's ACTUALLY doing last minute revision.
loool Aditya Chaturvedi ikr me too
Same here. But i have 10 hours i also need to sleep at some point...
I had small particles and figments of knowledge from my teachers, but after watching this and taking notes, they've all combined to make a large mass of consolidated information. Thank you, my good Sir.
Hello.
Can I say that I have really enjoyed watching this video and other videos of yours. I have been able to gain lots of information, in terms of my studies at both GCSE and A Level Physics, which the A Level Physics I'm doing distance learning. I'm also doing A Level Maths and A Level Further Maths, distance learning also. If it wasn't for people like you, who give up their free time to produce videos on such a good scale, I would be in a worse place. Can I say thank you very much for your help and support, it's appreciated very much.
Regards
Dominic G
U completed education? Now u do research?
@@mathematicsfanatic832 he's a crack dealer
@@homunculus3646 Guys keep it civil please
your mum works at the crack den. She's a lieutenant
V and r (internal) are constant. R (load) is variable. I = V/(R+r). Power thro Load = I^2 R = V^2 R/ (R+r)^2 = V^2/ R + r^2/R + 2r. Now for the power to be a maximum, the denominator must be a minimum. Diff w.r.t R and set equal to zero. You get 0 = 1 - r^2/R^2. So r^2 = R^2. So r=R.
Yes indeed. I can't now remember why I plotted it that way round. I suspect I was simply trying to show that there is a direct proportionality between current and voltage. But as you say, the slope of the graph which I drew is 1/R not R itself.
One day challenge - learn 2 years of A level physics in 1 DAY! *LET'S GO* -im gonna fail-
Silent Pixel did u pass?
Dinesh Raj I'd say I failed ;-;
i'd be the next one! 1 day before the exam!!!!!!!!!
1 day challenge is back!
24:18 challenge is backkkkk
Outstanding. Every physics class I took (3) presented 1/R = 1/r1+1/r2+1/r3 as axiomatic. I've never seen this (simple) derivation before. Thank you.
Thanks for kind comments. My videos are an amalgamation of AQA, OCR, Edexcel and Cambridge A level syllabus for both AS and A level (with the exception that I don't tend to do the biophysics parts). The videos in the A level revision playlist "Atomic Structure - A Level Physics" contains atomic energy levels and photon emission when electrons fall down to lower energy levels.
at 16:20 shouldn't it be -I3R3 + I2R2? since if we're going with the current direction, there must be a voltage drop and if we're going against the current direction then it will be a voltage gain?
Mahmoud Matar yep
Ya something seemed off to me. So straight to the comments I went lol.
Me toooo
One of the best videos explaining this concept.
Well there is a quantum mechanical underpinning to all this, but for purposes of A level all I meant was that as the current passes thro the filament of the lamp it heats the filament (and thus produces the light). As the filament heats up, its resistance increases so the current falls. R increases because the electrons in the wire get more excited and therefore there are more collisions and interactions between electrons which impedes current flow.
Unless I've missed a point, the answer is that you need about 0.7v across the LED before any current will flow in the circuit. So for the first 0 - 0.7v I would expect no current. Thereafter, assuming there is a resistance in the circuit I would expect current to grow proportionately with voltage.
You're a life saver. Your videos saved me like 50-60 pages of reading. Your explanations are thorough yet simple to follow and idk you just made my life THAT much easier.
How long is your paper?
SmoothPinnacle Its A3 size
Didn't know Bruce Forsyth could do Physics... ;-)
I haven't got great physics teachers and electricity if not taught right is hard to understand, and this is really helpful! Thanks a lot!
EMF is voltage. So it is electric potential (V) - potential energy per unit charge. Electric Potential Energy (Ep) is work done moving a charge (q2) from infinity to a point r away from q1. The Electric Field (E) is the Force per unit charge.
5 years after graduating college, i now know what I want(ed) to major in: physics.
Thank you so much. I have always had a problem with grasping the abstract essences of electricity. Your profound video has helped healed my wound and made me get it. Thanking you a thousand times would still not repay what you have done to the world.
Yes indeed. You will find a set of vids on QM in my Quantum Mechanics playlist (see main page of my UA-cam channel) and you will find additional Waves vids in the playlist on A Level Physics.
It is "Atomic Physics 3: Semiconductors, Diodes and Transistors". But you might want to watch videos 1 and 2 first to set it all in context. You can find these on my home UA-cam page (drphysicsa) in the playlist Atomic Physics. Thanks for kind comments.
I Reff = I R1 + I R2 + I R3
I Reff = I (R1 + R2 + R3)
Divide both sides by I
Reff = R1 + R2 + R3
Similar approach with the parallel circuit
Thanks so much, with only a couple of weeks till my exam I wasn't sure if I could pass them, after watching your videos everything seems much clearer :)
Took physics in university a decade ago. Our professor stinks at really teaching us anything. Didn't understand a damn thing and its a miracle I managed to pass the courses. Anyway, I'm currently settled in my career unrelated to physics...but out of sheer intellectual curiosity decided to re-learn physics. I find your youtube channel highly valuable --"subbed." I am sure my life/career choices may have ended up differently had I actually understood this stuff a decade ago.
Thanks.. You just gave me a reason why i need to learn this.. Haha
I can only learn stuff from highly formalized concise lectures otherwise it doesn't stick. Don't ask why. The good Doc's a Godsend. ;)
i am so confused. Why do you sometimes use the direction of conventional current and sometimes you use the "actual"direction.?? What should we assume in the exam?? Thank You!!!
The problem arises because electricity was discovered before anyone knew anything about electrons. It was assumed that electricity travelled from positive to negative and that is what we still call the conventional current. But we now know that the electricity is the flow of electrons which actually travel from negative to positive because they are negatively charged and are therefore attracted to the positive terminal. We retain the idea of a conventional current travelling from plus and minus even though nothing actually flows in that direction.
so in the exam...
DrPhysicsA "historical reasons" i thought the the whole selling point of science was that it was willing to change ideas based on evidence. Physics has failed. Archimedes rolls in his grave.
Brill lecture doc. You need to set up a patreon account
honestly we must do smthing to convince him to start again, when I'm bored I always watch his videos
Thanks. Glad it was of some help. Good luck in any exams you may be taking.
At 3:56, the point I'm trying to convey is that N if the total number of electrons flowing per second, Q is the total charge flowing per second and that that must therefore represent the current I.
N is number of charge carriers passing a given point per second. Q is the total charge passing a given point per second. But charge flowing per second is current. I=Q/t
I think it was Benjamin Franklin who started the idea of electricity flowing from + to -. You will recall that electricity was used long before it could be explained quantum mechanically and before electrons had been discovered. It was therefore just unfortunate that current flow was determined to be the opposite direction to the particles which actually flow - ie the electrons.
Because you put it in a circuit to measure current. If it had a sizable resistance it would reduce the current in the circuit.
To help better my understanding of how electricity works, I envisioned myself being an electron whizzing around an electron which constantly reversed EPE as I deliver energy to different electrical components and release energy as light quanta.
I've spent weeks Trying to grasp electric theory.. This is a handy lecture
Electricity which flows through miles of power cables suspended from pylons is still subject to a potential difference, otherwise no electricity would flow. Of course, for the most part the electricity is alternating current. Because the power cables are themselves part of the enormous circuit that is why it is essential to transmit at high voltage so as to minimise power losses through the cable.
Its a quantum mechanical effect. See my 3 videos on Atomic Physics 1, 2 and 3.
At 1:49, there's a symbol that looks like a sideways ribbon that you said is current. What is that symbol called?
3:40 It is a bit confusing that 'v' denotes distance in 'Q=Avnq/sec' and velocity in 'I=Avnq'. 'Q' usually denotes charge (not charge per second) and 'v' usually denotes velocity, not distance. Hence, perhaps the first equation should rather be 'Q=Avnq·sec', or am I wrong?
wrong.....
adil m...?
Laureano Luna Understand the confusion. I was simply saying that in one second the velocity in metres per second represents the distance.
0:25, are the electrons which are flowing originally from the copper atoms in the wire?
Because I have assumed that I2 goes down the central limb. I2 will of course have elements of I1 in it.
At 5:20 the straight line's gradient represents 1/resistance....
If voltage is on the y-axis and current on x-axis, then the gradient represent Resistance
that right. I thought I had put an annotation to that effect.
R eff , a gcse concept explained so much more elegantly than my GCSE teachers!
0:36 The direction of current in the source and in the circuit is different! From "minus" to "plus" in the source and from "plus" to "minus" in the circuit. It's because the electrons flow in the circuit is not free, but driven by the source. The circuit also must be closed otherwise there'll be no current. It's written in books about historical convention, but it's purely practical as you can see. Means that everytime you see a drawing of a current in a book it is a part of the circuit not the source, if nothing else is specified. Many people are confused about it. I wonder if it was so hard to put in books this simple explanation instead of "historical convention" just to make it clear from the bigining?
thank you so much! my teacher didnt explain how to find I2 and I3 and i struggled a lot with it, but im so glad i found your video! :)
sir, i like ur ., videos.
this brings students for more information about a level ..
thank you sir
Where did I do that?
Which A level syllabus are you studying?
such a helpful and well explained vid thank you
1:15 - isn't that equation change in current over change in time, as opposed to the amount? I may not fully understand the mathematical meaning of triangles, otherwise- it just seems rather confusing.
The current will be the change in charge divided by the change in time.
thank you for clearing that up.
About the positive-to-negative flow of current and the negative-to-positive notation, is that universal? For instance, I live in the U.S., so are our engineers and electricians using that reverse notation in their daily life? Or are there situations, besides personal preference, where it may actually be taught and trained in the way that makes sense? It just seems like something that could cause unnecessary problems.
Sir? Did you know that you rock? THANK YOU for rocking!
These are a very valuable set of videos you've produced, great revision tool.
I'm thanking you now, but hopefully we'll all come back here tomorrow and thank you more because the exam went well, thanks to you.
I finally understood where the 1/R+... and R+... formulas originate from, thank you!
Not that I am aware. And I was never much good at chemistry.
You have a voice of Gordon Ramsey when he cooks, except that you, you deliver knowledge about physics :D
+Deeya Bheeroo Gordon Who?
Brilliant video! Very helpful, thank you! Is it right that in a DC circuit the charge flows because it is subject to an electric field created by the battery? How does charge then flow through 300 miles of pylon, where is the electric field originating from in this case?
How a charge actually flows in a circuit? Does the battery opens space to the previous electron so that the nucleus next to it pulls it and so on? I know charges repel or attract because of the photon exchange between them which sort of tells to its electron mate to repel each other but how does that even work?
Great video!! There was only one think I was unclear about though, which was is the EMF the same as the E? As I understand, the EMF is the total amount of energy given by the cell, as is the E - is this just different notation?
Exam morning watching these videos,
That feeling of knowing everything i need
I have a question, what happens to the potential difference in a parallel circuit containing branches with components of different resistances?
Taylor Shill potential difference across a parallel circuit is the same across each branch in every scenario. This means that, the same electromotive force is doing work on these charges, and if there are resistors, the current will be distributed through each equal to the ratio of the resistances. That is to say, the same force is pushing the electrons, so less go through high resistances and more pass through the lower resistances. If you have a parallel circuit with a battery with internal resistance, you can first find the total external resistance using the reciprocal addition of resistances rule, and then treat that as a single resistance in series with the source of emf. The difference is, in a parallel circuit the voltage is the same across the whole and in a series circuit it’s the current that’s always the same. This means that, in a series circuit the voltage has to be greater at higher value resistances in order to push electrons through and maintain the constant current. In this way, whilst in parallel the ratio of currents are equal to that of the resistances, in a series circuits it’s the ratio of voltages that’s in the ratio of the resistances.
samuel McDonagh I thought the current was the same in any point in series circuit due to Kirchhoff’s 1st Law?
Your Videos are so frickin useful! thanks so much for helping us all through history A-level!
At 16:58 you're stressing that you use a minus sign because you're going opposite the current. Why is it then, that you're not using plus where you're going in the same direction as the current?
I am simply saying at this point that on the basis of the directions I have assumed, I3 is travelling in the opposite direction to I2.
I love those graphs that explain the mathematical process!
Yes if you are making the point that the charge is negative.
Thanks. I'm glad the first part helps. I hope that in due course the second part will make sense too.
13:50 won't the current be the other way around??
Great video really, But I wonder where is the part of the potential devider and potentiometer?, Thanks.
Very helpfull...but why did you stop uploading lectures???
thanks for the amazing videos,Dr Bob eagle.
Thank you so much DrPhysicsA I was gonna fail before now and now I see hope. Was wondering if you had any videos on waves and quantium physics.
What is the proof that you will always get the necessary number of equations and unknowns to calculate the currents in each branch by writing kirchoff's laws for the circuit? ( I'm basically asking why is kirchoff's laws a foolproof method to find the variables in a circuit..)
Essentially it comes down to energy conservation. If kirchoff's rules did not apply that would imply that energy had been gained or lost.
I am not talking about the validity of the laws themselves. I am asking why is it that you always have the correct number of equations and unknowns to calculate the currents in every branch of the circuit? Is the method of writing kirchoff's rules a foolproof way to calculate the currents in a circuit?? If so, Is there a proof to support that statement??
Anand Krishnan As I said, the principle is that energy is conserved and that this means that if you go round a circuit and come back to where you started you must be at the same potential energy. Whether or not you have enough information to calculate what the currents are in any particular loop will depend on how much information you have been given.
I wish I could repay you for all the help!
Great stuff - one thing to correct you on, however. In the derivation of I = nAve, you say that n is the 'number of electrons per m^3'. Actually, it's the number of 'free electrons per m^3'; most of the electrons can't carry a current, as they're bound into the shells of the atoms, and it is only those which have been 'delocalised' that can carry a current. A small point, but one that I have seen marks for in mark schemes (WJEC in particular, who love it).
This is such a useful resource, thank you! You explain it so much better than my teacher did :)
Great video! Just to ask, is there a Chemistry equivalent to this channel?
I got a little confused in the beginning when you talked about v being both a velocity and a distance. Wouldn't it be more accurate to talk about v as the velocity and vdt as the distance?
In which case dN = nAvdt.
Oh, and great series of physics videos. I got here via your mechanics videos. What you have made is very much appreciated :)
Thanks. All I mean is that if the speed is v meters per second then in one second the distance travelled is v meters.
With your videos I understand it instantly, making my revision fun and productive. Thank you so much for posting these revision videos :) James Yr.12
Hope it goes well. All good wishes.
Hi just another question, if you're performing an experiment about the relationship between the current and the potential difference in LED's, what kind of line on a graph would be expect to get?
Thanks a lot for your videos, this are extremely helpful and I am truly grateful
Does Kirchhoff's Law apply if the resistor have different values? Does I3 + R3 - I2 + R2 = 0? If R is different values.
Not done that yet. I'll put it on the list.
double circuit confuses, why it is zeo when you go baxk to where you started>>? I think until the loop is closed it should be continuing without break and voltage cannot be 0 because current continues to flow until it won' slow it down to the stop. I might be wrong but yet I don't understand why you substract the current from total energy??
Dear SIr,
Thank you so much for the wondeful lesson on logic gates. i am starting to like the subject i was afraid of!! sir could you tell me where can i find your lecture on how a transistor works?
How did we discover the charge of the electron? Not the sign, the magnitude? How do we know that?
I don't understand the cancellation at the end. How do the E's in the numerators all cancel in the parallel circuit example and why do the I's cancel in the series circuit example.
Other than that I found this very useful indeed!!!
Millikan's oil drop expt. See my vid on Charge and Mass of an Electron - A Level Physics
do be careful not to confuse E for energy with epsilon for EMF.
Also rho for resistivity should be lower case with a long tail and power should be a capital P.
In the exam (A level), should I use the + to - (conventional current) OR - to + ?
You use conventional current for the exam
Lauren Cutler Thanks!
How can I send my questions Sir ?
You are really doing great work, in very simple manner .
would the current not grow with a constantly increasing slope as the resistance of a diode decreases with temperature and therefore current
Why is the current shown as goin in the direction of electron movement?
How do i put this emf in form of y=mx+c?
Why do you want to do so?
Usually we plot a graph of resistance against 1/current. But if we invert the axis to 1/current on y axis against resistance. How do we display this in the form of y=mx+c?
***** Well we have the equation V=e-Ir so we rearrange this equation to make it appropriate for y=mx+c
V=-rI+e so if you graph this you can look at the y-intercept for the e.m.f and take the absolute value of -r (|-r|) for the internal resistance.
e= IR +Ir
divide by current e/I=R+r
divide by emf 1/I=R/e+r/e
Gradient is 1/e
Y intercept is r/e
Im assuming you mean the equation of the line on the graph of 1/current against resistance
***** Is PhysicsNet.co.uk your site?????
You are excellent teacher wow thnaks for a level videos!
considering Kirchhoff s 2nd law , what if there is a circuit with no appliance(no resistance) and only a cell connected , so as current flows through cell it would gain energy and this energy will add up when it completes one complete circuit and passes through cell for the second time, so current would keep on gaining voltage ? how is Kirchhoff law applied here
Thanks. Good luck in the exam.
I think you saved mynlife. I now have a bit of hope for tomorrow's test on electricity. :D Keep doing what you're doing!