This is clear if you do the maths and if you look at the formulas, but it's not as clear in an "intuitive" sort of way: how can more resistors resist... less?
The water-flow analogy IS intuitive.* Two thin-restrictive pipes, or two partially-on valves give you two water-flow paths, and more total water-current flows.
You can probably derive all of this from Ohm's Law, but it's [u]Kirchhoff's Laws[/u] that describe how voltages & currents add-up or divide in series and parallel circuits.
Now... how do you normally end up using resistors while making up circuits? What's the most commonly used patterns?
The most common use of resistors in digital circuits is as [u]pull-up resistors[/u]. And frequently, the pull-up resistor is part of a [u]wired-or connection[/u].
In analog circuits, resistors are often used as [u]voltage dividers[/u] to provide a (low current) reference or bias voltage.
Resistors are also used simply to limit/control current in non-linear circuits** with LEDs or with the bass-emitter junction of a transistor.
The one I can see from several projects is:
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You have a 5V power source
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You connect things in parallel to that power source
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You make sure whatever you connect gets the right amount of amps within its tolerance using a resistor
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You make sure you don't connect "too much", as you will eventually run our of amps the battery can actually supply
Yes... If you have a power supply rated at 1 Amp and you connect a lamp (or something) that takes 1 Amp, that's fine. If you connect a 2nd lamp, the 2nd one will also try get 1 Amp and something unpredictable can happen... The voltage may drop, the power supply may burn-up etc.
The same thing will happen if you plug-in two hair dryers and a toaster to the same power outlet. (They are connected in parallel). You'll draw excessive current and blow a circuit breaker. On the other hand, nothing bad will happen if you plug-in 2 or 3 100W lamps, and a maybe a TV.
- You make sure whatever you connect gets the right amount of amps within its tolerance using a resistor
NO!!! You'll get a voltage drop across the resistor (Ohm's Law) so you won't have 5V for whatever you're powering. You are making a voltage divider, but the resistance of the load is unknown, and usually not constant. For example, we could measure the current drawn by the Arduino and calculate it's resistance... But if you turn-on an LED (powered by the Arduino) the current will increase, which means the resistance has changed.
- The water flow analogy is imperfect, especially since an open water pipe allows unrestricted water flow and water spills-out everywhere, whereas an open electrical connection is infinite resistance and no current flows. Also, nothing bad happens with zero water-flow resistance, but bad things can happen with no electrical resistance (a short circuit).
** An LED (like all diodes) is non-linear. That is, the resistance changes (drops) as voltage increases. At the diode's breakdown voltage ("operating voltage" for an LED) resistance suddenly drops to almost zero... A very tiny increase in voltage results in a huge increase in current. If there is no resistor to limit the current, the LED (and sometimes whatever is driving the LED) will burn-up.