Maybe you’ll believe this guy.
Who is this hack?
Oh!
I mean… Oh
Damn it, OK already.
Ring - I sure believe that guy and I was not trying to offend you. But you have to understand that Coriolis forces are not Radial forces that may increase the diameter of a rotating wheel. The only radial force that does that is Centrifugal Force.
andy_fl: You may just have to accept that the universe is a stranger place than you thought.
Eyer8 referred you to Maxwell’s equations, and it’s there that you’ll find the answer to your OP.
Trouble is, Maxwell’s equations border on incomprehensibility. They were the cause of the whole frame-of-reference problem, and the first time around, you didn’t just have to be an Einstein to work it out, you had to be the Einstein.
Sigh. Well let’s see if quoting another expert works.
Matti Meron isn’t Einstein but he is a Ph.D. physicist at the University of Chicago.
Also, I suggest you go back to the Einstein quote and see if you can’t find where he talks about the Centrifugal force. You must have have missed it.
If this isn’t enough for you then I give up.
Note that “The Centrifugal Force” is something very different than radial forces.
For example, if you’re standing on a rotating merry-go-round, you’ll feel as if you’re being flung radially outwards by a mysterious force. In fact it’s just inertia, and your body is simply trying to move tangentially in a straight line while the rest of the merry-go-round keeps moving in a curved path.
But radial forces in rotating objects are different: they’re REAL. They cause rotating objects to stretch. They cause people to stick to the inner surface of the space station in 2001. Fling a rotating dough ball into the air and it stretches into a pizza. Or tie a string between two balls and throw it like a bolo. The balls orbit each other and the string stretches. The string is experiencing a very real, non-illusory force because the balls are moving in curved paths and each ball accelerates inwards. It’s just an F=MA force.
My original point was that, if you spin a wheel, you cause motion which causes acceleration which causes a force by F=MA. If you hadn’t spun the wheel, that force wouldn’t have appeared. Creating acceleration creates force.
PS
If a bank of car batteries is like a perfect Voltage Source (twelve volts at almost any value of current,) then a VandeGraaff electrostatic machine is like a perfect current source (ten microamps at almost any value of voltage.) Hook various resistors to a VDG machine and you’ll find that the current stays the same, but the voltage across the resistor is proportional to the resistance. Current sources work backwards when compared to voltage sources: to turn a VDG machine off, just short it out with zero ohms. That gives you ten microamps at zero volts (zero watts output.)
(Expanding my earlier post)
The OP assumes that voltage and current are different (although as you concede, related) things.
Maxwell unified the theories of electricity and magnetism to produce a theory of electromagnetism.
In the electromagnetism theory, voltage and current are not different things, just aspects of the same thing.
Electromagnetism simply can’t be explained in Newtonian terms, e.g. you can’t use F=ma analogies except in limited circumstances.
It was Maxwell’s theory that called Newtonian physics in to question in the first place. If Maxwell’s theory was correct, then all sorts of strange things could occur, like time dilation. Einstein’s special theory holds that these strange things really do occur.
Using pre-Maxwellian terms to describe electricity is useful in lots of circumstances, just as Newtonian mechanics is still useful.
But, in certain circumstances, like the causality question in the OP, you have to recognise that you’re only using an approximation, and the approximation will break down if pushed.
Similarly with the centrifugal force thing. You just can’t explain non-Newtonian mechanics in Newtonian terms.
a lot of this is related to potential and kinetic relationships
the voltage awaits
the resistance exists
the current results
centripetal forces are the result of angular velocity, which can be variable.
centrifugal forces are the opposite and equal reactions to the centripetal.
Thanks all for your posts.
>> is there such a thing as a "constant current source?
A car’s alternator and any other electromagnetic generators are current generators, not voltage generators. By adjusting the field current you vary the output current. A control feedback circuit adjusts the field current (and indirectly the output current) to maintain a certain voltage. But the generator is generating current, not voltage. So almost the totality of the power we use is generated by current sources, not by voltage sources.
OTOH chemical batteries are sources of voltage but they provide an insignificant amount of the power we consume.
>> is there such a thing as a "constant current source?
Oh sure.
- A transistor is often modeled as a constant current source.
- Constant current lab supplies can be readily purchased.
- A linear power supply operating in “current limit” acts as a constant current source.
- A high resistance in series with a voltage source can approximate a constant current source.
- A photovoltaic cell is usually modeled as a constant current source.
Let’s look physically, at what’s going on. For a resistor, the OP description is essentially correct. A voltage is set up across the resistor, which means there is a non-zero electric field, causing current to flow. Even using Crafter_Man’s current source, the current deposits charge at the ends of the resistor to cause the voltage.
For the inductor, the current supports a magnetic field. Think of an inductor as a small slinky, with the magnetic field running along the slinky axis. Changing the current changes the magnetic field, and this time-varying magnetic field creates an electric field which wraps around the slinky.
For both the resistor and inductor, then, current flow is in response to an electric field. It’s just that in one case, the electric field is caused by charge, and in the other, it’s caused by a changing magnetic field, which is in turn caused by (changing) current.
For the capacitor, current doesn’t really flow through the capacitor. The simplest capacitor is two parallel sheets of metal. Current flows in one end onto one sheet, and other current flows out the other end, coming from the other sheet. The charges on the plates cause a voltage between them. The charge is the integral of the current. So in this case, it’s more physical to say that the charge (the integral of the current) causes the voltage. Taking the derivative of both sides of V = integral(I)/C gives you dV/dt = I/C.
It should also be noted that a commercially-made constant current source is really a voltage source that automatically adjusts the output voltage to maintain the desired constant current. A closed loop controller (consisting of a series transistor, reference, comparator, etc.) is used to maintain the constant current.