Magnetism or Electricity

[Marblehead]

Big difference between "no more current flow" and "no more change in the current flow..."

And I will take your word for that as well. (No, I'm not going to test it in order to find out first hand.)

[ChiDragon]

Sounds valid to me. That is, except for one instant:

 

In the Army, in the old days, that is, my old days, in the 1950s and 1960s, we had a telephone that had a hand crank generator for giving a ring current. It was 90 volts pulsating DC current. And I promise you, no matter how long you held the wires you would get an electrical shock as long as someone was cranking the telephone.

 

Now, it may be true that this pulsating DC current was converted to AC. I never investigated that.

 

A pulsating DC is not a direct DC. It is considered to be AC because it was pulsating. A real DC is from a battery which a steady voltage(non-pulsating). It does go through an DC-to-AC converter to run a motor. Otherwise, there will be no magnetic field can be produced(without the change in current) to turn the motor.

[Apech]

 

A pulsating DC is not a direct DC. It is considered to be AC because it was pulsating. A real DC is from a battery which a steady voltage(non-pulsating). It does go through an DC-to-AC converter to run a motor. Otherwise, there will be no magnetic field can be produced(without the change in current) to turn the motor.

 

the commutator on the motor will do that

 

http://en.wikipedia.org/wiki/Commutator_(electric)

[Brian]

Not true, CD. Direct-current does NOT equate to constant-current (nor does it imply constant potential difference). Hook up a simple resistor to a battery and you have a DC source generating a unidirectional current (charges flowing in only one direction through the circuit) but the voltage starts changing right away, as does the current.

 

The only thing indicated by the AC vs. DC designation is whether the current vector reverses direction over time. This is not necessarily the same as a fluctuation in magnitude...

Edited September 15, 2013 by Brian

[ChiDragon]

the commutator on the motor will do that

 

http://en.wikipedia.org/wiki/Commutator_(electric)

 

Okay....

That is how a DC motor works. I guess I was talking about the external power source.

[Brian]

 

Okay....

That is how a DC motor works. I guess I was talking about the external power source.

 

What if the external power source is a potato? Or a photovoltaic cell? Or a neuron? Or a muscle?

[Apech]

What if the external power source is a potato? Or a photovoltaic cell? Or a neuron? Or a muscle?

 

 

Excellent thought we should design a vacuum cleaner that runs on potatoes.

[Brian]

http://www.science-projects-resources.com/how-to-make-a-potato-battery.html

 

http://en.wikipedia.org/wiki/Lemon_battery

 

http://www.miniscience.com/projects/airbattery/

 

Source of the potential difference doesn't matter very much except in terms of practicality...

 

[ChiDragon]

Not true, CD. Direct-current does NOT equate to constant-current (nor does it imply constant potential difference). Hook up a simple resistor to a battery and you have a DC source generating a unidirectional current (charges flowing in only one direction through the circuit) but the voltage starts changing right away, as does the current.

 

The only thing indicated by the AC vs. DC designation is whether the current vector reverses direction over time. This is not necessarily the same as a fluctuation in magnitude...

 

According to ohm's law:

If you have a good regulated DC power source with a variable resistor connected to it, then, only the current will be changed with the resistance but voltage will remain constant. Unfortunately, if you have power source like you have described, I think you should go get your money back.....

 

 

What if the external power source is a potato? Or a photovoltaic cell? Or a neuron? Or a muscle?

 

 

 

http://www.science-projects-resources.com/how-to-make-a-potato-battery.html

 

http://en.wikipedia.org/wiki/Lemon_battery

 

http://www.miniscience.com/projects/airbattery/

 

Source of the potential difference doesn't matter very much except in terms of practicality...

 

 

Yes, let's be more practical..........

Edited September 15, 2013 by ChiDragon

[Brian]

That's only if you have an idealized "constant-voltage source." They don't really exist but are a convenient concept when introducing someone to the principles of electricity.

 

 

 

 

And, for the record, Ohm's law says:

 

 

 

J=σE

 

or, in a simplified environment,

 

I=V/R.

 

In neither case is any term required or expected to be a constant.

 

Edited for clarity...

Edited September 15, 2013 by Brian

[ChiDragon]

Greeting, Brian....
What is your background.....???

[Apech]

That's only if you have an idealized "constant-voltage source." They don't really exist but are a convenient concept when introducing someone to the principles of electricity.

 

 

 

 

And, for the record, Ohm's law says:

 

 

 

J=σE

 

or, in a simplified environment,

 

I=V/R.

 

In neither case is any term required or expected to be a constant.

 

Edited for clarity...

 

 

isn't resistance a constant (more or less)???

[Brian]

Greeting, Brian....

What is your background.....???

 

I am a classically trained physicist with a concentration in electronic engineering, CD.

 

I recently had my TTB "name" changed from "A Seeker" to "Brian" so you may not have recognized me...

 

 

 

 

 

 

isn't resistance a constant (more or less)???

 

 

 

Only sometimes (and even then only in idealized "non-real-world" cases, when you look closely...)

 

EDIT: For instance, as soon as current begins flowing through a real resistor, it begins to heat up. That temperature change causes a change in resistance, which causes a change in the current, which changes the rate of change of the temperature of the resistor, etc., etc.

 

The real world is a complicated place!

Edited September 15, 2013 by Brian

[Apech]

I am a classically trained physicist with a concentration in electronic engineering, CD.

 

I recently had my TTB "name" changed from "A Seeker" to "Brian" so you may not have recognized me...

 

 

Only sometimes (and even then only in idealized "non-real-world" cases, when you look closely...)

 

I get it Ohm's law is a idealised specific rule ... yes I seem to remember that (it's a long time since I did any physics).

[Brian]

Exactly, Apech. You are spot-on.

[ChiDragon]

That's only if you have an idealized "constant-voltage source." They don't really exist but are a convenient concept when introducing someone to the principles of electricity.

 

And, for the record, Ohm's law says:

 

J=σE

 

or, in a simplified environment,

 

I=V/R.

 

In neither case is any term required or expected to be a constant.

 

Edited for clarity...

 

isn't resistance a constant (more or less)???

 

All three factors can be variables.

 

I=V/R is a constant voltage source. The voltage must be constant, in order, for the variable magnitude of the current to be controlled by the resistance.

 

V= IR is a constant current source. The current must be constant, in order, for the variable magnitude of the voltage to be controlled by the resistance.

 

 

 

 

PS......

@ Brian ----- By the way you talk, I had detected that you are a physicist by intuition. This is why I've called myself as a grokking Taoist.....

 

Edited September 15, 2013 by ChiDragon

[Brian]

In anything beyond trivial examples, none of these terms will be constant, CD. We often "pretend" that we hold one as fixed and vary a second so that we can see the effect on the third but that is just a simplification for the purpose of explaining to a beginner or getting a handle on the concepts for a circuit.

 

In reality, you have to think about the partial derivatives of each component with respect to all the others (and you have to remember that a real resistor is also a capacitor and an inductor at the same time...)

Edited September 15, 2013 by Brian

[Apech]

In anything beyond trivial examples, none of these terms will be constant, CD. We often "pretend" that we hold one as fixed and vary a second so that we can see the effect on the third but that is just a simplification for the purpose of explaining to a beginner or getting a handle on the concepts for a circuit.

 

In reality, you have to think about the partial derivatives of each component with respect to all the others (and you have to remember that a real resistor is also a capacitor and an inductor at the same time...)

 

You are a Physics God Brian.

[Brian]

Not at all! Just a real physicist (I actually prefer the old term "natural philosopher.")

 

I suspect there are quite a few readers with backgrounds in science or engineering who have been nodding throughout this thread but haven't felt a need to interject...

 

My family learned long ago to be careful about asking questions like "why is the sky blue?" or "how does a radio work?" or "what is fire?" or "why do stars and diamonds 'twinkle?'" unless they really wanted an answer!

Edited September 15, 2013 by Brian

[Dagon]

In anything beyond trivial examples, none of these terms will be constant, CD. We often "pretend" that we hold one as fixed and vary a second so that we can see the effect on the third but that is just a simplification for the purpose of explaining to a beginner or getting a handle on the concepts for a circuit.

 

In reality, you have to think about the partial derivatives of each component with respect to all the others (and you have to remember that a real resistor is also a capacitor and an inductor at the same time...)

 

Maybe he means the energy composed in a circuit that will only operate when a continuous source of power is supplied to it, unlike a spark which is a sudden discharge?

 

Thanks for furthering our understanding!

 

The power supply is going to be steady rather than constant at +5/-5%, otherwise the output through resistance would vary too much, and we could fry components?

Edited September 15, 2013 by Dagon

[Brian]

Maybe he means the energy composed in a circuit that will only operate when a continuous source of power is supplied to it, unlike a spark which is a sudden discharge?

 

Thanks for furthering our understanding!

 

I'd like to think that CD & I have only language issues...

[Dagon]

Neurons are kinda weird, I heard that they fire randomly, unlike electricity going through a circuit, which can be predicted. Is that true?

Like trying to predict which neuron is going to fire next, is like predicting where lightning will strike.

The randomness was determined by the Poisson distribution. (IIRC)

Edited September 15, 2013 by Dagon

[Brian]

Electrons are only "predictable" in the common sense of the word when they treated en masse -- as in an electrical circuit where we are dealing with millions or billions of them. Viewed individually, they cannot really be described in classical terms and can't even be realistically thought of as "particles" (heck, trying to talk about the existence of an electron becomes problematic.)

 

Quantum electrodynamics is beyond the scope of this thread, though.

 

The kung fu student says "teach me dim mak" and the instructor replies "let's learn how to stand first..." It's kinda like that.

[Dagon]

Then why aren't they differentiated, when en masse verse not, why should we call the phenomena electrical in both instances? Especially considering they act completely different.

Edited September 15, 2013 by Dagon

[ChiDragon]

Neurons are kinda weird, I heard that they fire randomly, unlike electricity going through a circuit, which can be predicted. Is that true?

 

Like trying to predict which neuron is going to fire next, is like predicting where lightning will strike.

 

The randomness was determined by the Poisson distribution. (IIRC)

 

Neurons fire only when the brain sends a signal to do a particular thing. For example, when you want to move a finger or contract a muscle, signals will be sent to those areas to fire up the neurons.

Edited September 15, 2013 by ChiDragon