Who The Feck is Ohm?

[Chienmortbb]

3 hours ago, Bill Fitzmaurice said:

And that's why Mesa Dual Rectifier amps give you the choice. 😉

Are those the ones that make Amp Techs have nightmares?

[bremen]

43 minutes ago, SimonK said:

Here's another helpful picture (from https://en.wikipedia.org/wiki/Ohm's_law😞

 

 

I have this tattooed onto my belly.

[agedhorse]

54 minutes ago, Chienmortbb said:

Are those the ones that make Amp Techs have nightmares?

Certainly those who are under-qualified tend to struggle, but qualified pro techs generally don't have many issues. We have factory techs that can  turn around 4 or 5 amps a day (depending on the problems of course), but they are excellent and good troubleshooters too. 

 

It's no different than for any product, the better qualified the tech is, the easier it is to fix. When I work on class D bass amps, I typically spend less that 1 hour on each amp. I find them easier and quicker to repair than my older class AB/G/H designs.

[Chienmortbb]

23 minutes ago, agedhorse said:

Certainly those who are under-qualified tend to struggle, but qualified pro techs generally don't have many issues. We have factory techs that can  turn around 4 or 5 amps a day (depending on the problems of course), but they are excellent and good troubleshooters too. 

 

It's no different than for any product, the better qualified the tech is, the easier it is to fix. When I work on class D bass amps, I typically spend less that 1 hour on each amp. I find them easier and quicker to repair than my older class AB/G/H designs.

I must admit that I found designing much easier than repairing, so a good tech is to be admired.

[TimR]

I'm sure someone has done this in the past and it was asked to be a sticky, but for some reason the Mods didn't agree.

 

Or may be a different forum...

[TimR]

On 14/01/2025 at 15:36, SimonK said:

 

Strictly speaking this is wrong.

 

The volt is defined as the difference between 2 points when a current of one amp flows through a resistance of 1 ohm (although it's expressed as power dissipation 1Watt). 

 

ie the current flow causes the volt difference. Which is a very difficult thing to get your head around and why the water flow/pressure analogy is more correct. 

 

Think of it as lots of blue men on the left piling up trying to get through the tube at the same time, with fewer blue men running off to the right as they pop out of the restriction. While the blue man stuck in the tube gets more hot and flustered and red in the face. 

 

 

Anyway. As you were...

Edited Wednesday at 22:04 by TimR

[agedhorse]

3 hours ago, TimR said:

 

Strictly speaking this is wrong.

 

The volt is defined as the difference between 2 points when a current of one amp flows through a resistance of 1 ohm (although it's expressed as power dissipation 1Watt). 

 

ie the current flow causes the volt difference. Which is a very difficult thing to get your head around and why the water flow/pressure analogy is more correct. 

 

Think of it as lots of blue men on the left piling up trying to get through the tube at the same time, with fewer blue men running off to the right as they pop out of the restriction. While the blue man stuck in the tube gets more hot and flustered and red in the face. 

 

 

Anyway. As you were...

Actually, something is required for the current to flow, and that is the volt. Without a volt, and without a resistance, no current would flow.

 

It's like the "which came first, the chicken or the egg?" argument.

[TimR]

7 hours ago, agedhorse said:

Actually, something is required for the current to flow, and that is the volt. Without a volt, and without a resistance, no current would flow.

 

It's like the "which came first, the chicken or the egg?" argument.

 

There are many ways of inducing current without voltage. Current comes before voltage as the first current was measured in a moving wire in a magnetic field. Your bass pickups induce a current with no voltage. 

[SimonK]

11 hours ago, TimR said:

 

Strictly speaking this is wrong.

 

The volt is defined as the difference between 2 points when a current of one amp flows through a resistance of 1 ohm (although it's expressed as power dissipation 1Watt). 

 

ie the current flow causes the volt difference. Which is a very difficult thing to get your head around and why the water flow/pressure analogy is more correct. 

 

Think of it as lots of blue men on the left piling up trying to get through the tube at the same time, with fewer blue men running off to the right as they pop out of the restriction. While the blue man stuck in the tube gets more hot and flustered and red in the face. 

 

 

Anyway. As you were...

 

Yes this is gets us into a fascinating discussion in the philosophy of science/physics where we have to acknowledge that our descriptions of the natural world are often analogies that may only work up to a certain point. The classic example is wave and particle duality in physics wherein one analogy works well in some contexts, and the other in other contexts, but neither in all contexts. So with things like that picture (volts/amps/ohms) you have to consider whether it does the job for the context - personally I find it helpful just to remind myself whats going on when I'm thinking about matching cabs with amps etc., but I do appreciate that there is a whole world that sits underneath the picture where the only way to really get a grasp of what's happening is by diving into mathematics.

 

Of course another philosophical problem is why maths works so well as it is also an analogy of the physical world, but an unreasonably good one...

[Chienmortbb]

1 hour ago, TimR said:

There are many ways of inducing current without voltage. Current comes before voltage as the first current was measured in a moving wire in a magnetic field. Your bass pickups induce a current with no voltage. 

The problem with Ohms law (it is not really a problem) is that without any of the three attributes, the others exist only in name. Yes, you can have a battery with a Pd of 12V but without a path for the current to flow through, the 12 volts has potential, but it's "all dressed up, no place to go".

 

Similarly, you can induce current in a coil, but the only if it is connected to a circuit.

 

I introduced a new term above, Pd, the potential difference. Volts are the units that measure potential difference, in the same way that amperes are the units we give to current. Confusingly, current is designated I, as using C would confuse things as capacitance is C in the electrical world. One of the things I often see is people talking about the Amperage. No such term, it is current measured in amperes. It is common and quite correct to abbreviate that to amps.

 

 

[Crusoe]

15 hours ago, bremen said:

I have this tattooed onto my belly.

Just be warned that you might get taken for a BMW driver

[Bill Fitzmaurice]

3 hours ago, TimR said:

 

There are many ways of inducing current without voltage. Current comes before voltage as the first current was measured in a moving wire in a magnetic field. Your bass pickups induce a current with no voltage. 

If there's no voltage there's no signal, nor current. The strings vibrating in the magnetic field of the pickups induce both current and voltage. You can't have the one without the other except with a theoretical superconductor.

[Lozz196]

1 hour ago, Crusoe said:

Just be warned that you might get taken for a BMW driver

No, they have “Remember, don’t indicate” tattooed on theirs, in reverse so they can read it in the mirror when shaving.

[Chienmortbb]

I have just updated the third part, AC Power, of this at the start. I did most of my theory 50 year ago, so forgive me if I have made an error.

[agedhorse]

Voltage and current have to coexist (in resistive circuits) until you add reactive elements... then it's possible to have current with no voltage due to the phase shift between the voltage and the current (for AC) because power is stored as voltage or current in the capacitor or inductor. This is an entirely different topic however.

[TimR]

Weird. I thought that moving magnets exerted a force on the electrons, causing them to move. That's the EMF. There's no voltage moving the electrons. The voltage is caused by the  electrons moving from one place to another. 

 

That's why voltage is defined in terms of current, not the other way round. A higher voltage is just where you have a higher density electro magnetic, electro chemical, theromionic field, or capacitive storage.

Edited 19 hours ago by TimR

[Downunderwonder]

On 16/01/2025 at 21:50, TimR said:

 

There are many ways of inducing current without voltage. Current comes before voltage as the first current was measured in a moving wire in a magnetic field. Your bass pickups induce a current with no voltage. 

Chicken, prepare to meet egg, on your face.

[SimonK]

1 hour ago, TimR said:

Weird. I thought that moving magnets exerted a force on the electrons, causing them to move. That's the EMF. There's no voltage moving the electrons. The voltage is caused by the  electrons moving from one place to another. 

 

That's why voltage is defined in terms of current, not the other way round. A higher voltage is just where you have a higher density electro magnetic, electro chemical, theromionic field, or capacitive storage.

 

I broadly agree with this - as Biochemist/Biophysicist speaking (at least originally) - there are plenty of biochemical systems that rely on the flow of protons rather than electrons, with the resulting electrochemical gradient measured in volts, so I would say volts are often used to describe the state of a system.

[TimR]

1 hour ago, Downunderwonder said:

Chicken, prepare to meet egg, on your face.

The SI unit for voltage is the volt (V), which is defined as one kilogram meter squared per second cubed per ampere kgm^{2}s^{-3}A^{-1}

[SimonK]

16 minutes ago, TimR said:

The SI unit for voltage is the volt (V), which is defined as one kilogram meter squared per second cubed per ampere kgm^{2}s^{-3}A^{-1}

 

It can also be defined as Joule per Coulomb, so the amount of energy needed to move a charge through a component (hence also being a descriptor).

[Chienmortbb]

16 hours ago, agedhorse said:

Voltage and current have to coexist (in resistive circuits) until you add reactive elements... then it's possible to have current with no voltage due to the phase shift between the voltage and the current (for AC) because power is stored as voltage or current in the capacitor or inductor. This is an entirely different topic, however.

Volts and current do have to coexist, in a resistive circuit, I think that going into phase shift, reactance etc. is a step too far for here. In fact, things have probably gone too deep for most already.

 

[agedhorse]

1 hour ago, Chienmortbb said:

Volts and current do have to coexist, in a resistive circuit, I think that going into phase shift, reactance etc. is a step too far for here. In fact, things have probably gone too deep for most already.

 

Agreed.

[agedhorse]

13 hours ago, TimR said:

Weird. I thought that moving magnets exerted a force on the electrons, causing them to move. That's the EMF. There's no voltage moving the electrons. The voltage is caused by the  electrons moving from one place to another. 

 

That's why voltage is defined in terms of current, not the other way round. A higher voltage is just where you have a higher density electro magnetic, electro chemical, theromionic field, or capacitive storage.

There are only 2 ways that voltage or current can exist without the other, these are both defined as limit or boundary functions... meaning that they are more theoretical than practical (though you can get close under ideal conditions).

 

Voltage can exist with no current flow IF the load resistance is infinite (an open circuit with no parasitic shunt leakage) because I = V/R, and as R approaches infinity, I approaches zero no matter what the value of V.

 

Current can exist when the load resistance approaches zero (a short circuit with no parasitic series resistance) because V = I x R, and as R approaches 0, the voltage will approach zero  no matter what the value of I.

 

There is no way to split apart the voltage and current relationship in resistive circuits except when R is either zero or infinity. While the voltage is generated by electrons moving from one place to another, an outside force or element must exist for there to do so. The outside force generates a voltage with no net current if the load is open, a current with no net voltage if the load is shorted and somewhere in between if the load is resistive. 

[agedhorse]

12 hours ago, SimonK said:

 

I broadly agree with this - as Biochemist/Biophysicist speaking (at least originally) - there are plenty of biochemical systems that rely on the flow of protons rather than electrons, with the resulting electrochemical gradient measured in volts, so I would say volts are often used to describe the state of a system.

It doesn't matter if it's protons or electrons, an outside energy force (mechanical, chemical, thermal) is required for there to be motion.