I'm on lesson 9 of these tutorial series with arduino.
About 22:00 minutes in, he starts explaining the formulas to determine current flow. But he did not explain whether or not the current flow is linear or if behaves parallel.
If V=I*R, then the more resistors we apply to a circuit the less flow (I) we will have. That makes sense. Does the wire leading up to the resister hold a max current until it hits the resistor(s) then drops down to the value I? He makes it sound like it's almost pushing through positive and negative sides of the circuit in his example. But maybe he is just trying to make clear the voltage must = 0 (is this "ground" or no?) for the circuit to be complete.
I am also puzzled on what the max current of 5v can be then without any resistors. If V=IR where (R=0), then that'd mean we have 0 for (I) as well. Which doesn't make any sense to me cause there has to be current in order to power a small LED or something on a circuit like that, right?
I didn't sit through the tutorial but perhaps I can help.
First: Not to confuse you but the tutorial deals with "ideal" components. Which if fine when working with normal values. But if we have a 5V source and connect 0 ohms then the current is infinite. Of course this is impossible because at these extreme conditions other factors come into play.
Second, The current through a simple electrical circuit (5V supply and 1 or more series resistors) is constant everywhere in the circuit. You can compare current to water flow in a hose and resistors as restrictions in the hose and nozzle. You can see the flow of water is the same everywhere in the hose.
Third:
In a simple circuit, current flow is linear with either change in voltage of supply or change in resistance.
I = E/R
Fourth:
I am also puzzled on what the max current of 5v can be then without any resistors. If V=IR where (R=0), then that'd mean we have 0 for (I) as well. Which doesn't make any sense to me cause there has to be current in order to power a small LED or something on a circuit like that, right?
No. You are not applying V=IR correctly. If R = 0 and I = 0, then the voltage must = 0 which is not correct. See "Third:"
aspen1135:
I'm on lesson 9 of these tutorial series with arduino.
About 22:00 minutes in, he starts explaining the formulas to determine current flow. But he did not explain whether or not the current flow is linear or if behaves parallel.
If V=I*R, then the more resistors we apply to a circuit the less flow (I) we will have. That makes sense. Does the wire leading up to the resister hold a max current until it hits the resistor(s) then drops down to the value I?
Outside the realms of static electricity and extremely high voltages, charge must balance out everywhere,
this means that the number of electrons in the wire matches the number of positive ions in the crystal
lattice of the metal itself. Add one electron in at one end and one must leave the other end to
maintain balance (assuming all at low voltage). This effect is due to the electric field and travels
at the speed of light. The electrons themselves travel much much slower, but they are controlled by
the electric field (effectively instantly).
So current does not travel along, get to an obstacle and then decide to do something in response - the entire
circuit is always aware of the total resistance (in effect), because this balance is always being maintained.
When a switch is opened, for instance, "news" of this new infinite resistance travels at the speed of light
and within nanoseconds or microseconds the entire circuit has zero current.
The full story is slightly more complicated, for instance charge can pile up in very small amounts, and this
causes measurable voltages due to the imbalance (these voltages then act to try to reduce
the imbalance). Capacitors are components that allow much more charge to pile up for less voltage,
and rely on the change being differential - charge on one plate is balanced by opposite charge on the other,
so that from outside no net piling up of charge is visible.
MarkT:
Outside the realms of static electricity and extremely high voltages, charge must balance out everywhere,
this means that the number of electrons in the wire matches the number of positive ions in the crystal
lattice of the metal itself. Add one electron in at one end and one must leave the other end to
maintain balance (assuming all at low voltage). This effect is due to the electric field and travels
at the speed of light. The electrons themselves travel much much slower, but they are controlled by
the electric field (effectively instantly).
When a switch is opened, for instance, "news" of this new infinite resistance travels at the speed of light
and within nanoseconds or microseconds the entire circuit has zero current.
OK. Circuit 'flow' makes sense now. What I am having troubles grasping at this point is the 'infinite' current when zero resistance is applied to only a 5v circuit. 5v itself is a limitation in size of power to some degree right? For the typical 'hose/water' metaphor, it'd be the total allowed girth of the hose. That must mean the total output power has to be limited in amps in some matter.
I've been dabbling in electric bicycles and want to use this as example. If Watts = Volts * Amps (of the controller), then if 5v were supplied and you took away the amp controller completely (0 resistance) then tried running it on just 5v, would the 'infinite' current even be able to power a 2000watt rated motor? I understand the application of resistors for adding practical variability. I just don't understand why you need 24,48,52v etc. instead of something like 5v to power a bike if you have infinite current and can just reduce the resistance.
The source of your 5 volt example also has to be considered because IT is part of your circuit. ALL voltage sources have internal resistance and this resistance is the limit to the infinite current your theory suggests.
aspen1135:
would the 'infinite' current even be able to power a 2000watt rated motor?
Good point! In an ideal world, it would be able to do that. There would be a current of 400A through the wires without any problems. But real-world copper wire has a resistance, too, so you will never have 0 Ohm connecting the battery and the motor. And real batteries have an "internal resistance" which does not allow them to provide "infinite" current. That is the reason why the battery wire in your car is thicker than your finger, an this is also the reason why electric bikes or power tools are equipped with 12V/24V/36V batteries: You can transfer the same electrical power with much less current.
It's true that if you need 2000 watts then you could in theory have 1V at 2000A or 100V at 20A. But for very high currents you need thick heavy unbendable cables and it's hard to control. 20A needs thinner cabling and there are far more components available to use for control.
It's not just a matter of theory, you have to think of practicality too.
aspen1135:
OK. Circuit 'flow' makes sense now. What I am having troubles grasping at this point is the 'infinite' current when zero resistance is applied to only a 5v circuit. 5v itself is a limitation in size of power to some degree right? For the typical 'hose/water' metaphor, it'd be the total allowed girth of the hose. That must mean the total output power has to be limited in amps in some matter.
This brings in the concept of a short circuit, so check out that link.
In practice, if you connect the two terminals of your power supply together, or via a very small resistance, then you no longer have your 5 V between them; it becomes zero or very low. The proportional relationship still holds.
The fact simply is that you will always have a limitation in the ability of the power supply to supply current because the power supply itself contains significant resistances. And this is part of the reason why we use regulators in power supplies, which work (generally by wasting power to some degree) to maintain the voltage at the specified level such as 5 V, despite different currents that may be drawn.
Yes. Like the vacuum clear commercial says: "It's a dirty world". An ideal 5V supply will be able to supply an infinite current, in to a zero ohm path.
But, reality is dirty with resistance. Resistance in the power source, resistance in the wires, and in the case of shorting a 5 V power source, this resistance will be distributed across the circuit. Some of it will be in the power source, some in the wires, and that resistance adds up to one total series path resistance, and that is what will limit the current.
In fact, it's the power sources internal resistance that limits how much current the power source can deliver. For instance, if a 5V power source has an internal resistance of 9Ω, and the wire being used to short out this power source has only 1Ω, then the total resistance is 10Ω, and the total current will be half an amp. Notice that if the shorting path is 0Ω, the current will be 0.56A, and that, my friend, is the limit of that power sources ability to deliver current.
I just don't understand why you need 24,48,52v etc. instead of something like 5v to power a bike if you have infinite current and can just reduce the resistance.
Aside from the point that you can’t get as much current as you want, when you are dealing with things like an inductive load it takes time to get the current to flow in the windings. This is because an inductor acts to reduce the rate of change of current.
A simple way round this is to provide a higher voltage to the inductor and this will get the current flowing faster. Thus developing more power in the inductor.
