Think of it like this:
Volts is the measurement across a battery. The battery provides current, measured in Amperes (Amps, or milliAmps for low power devices) to a load. (sometimes presented as water pressure, and flow in gallons per minute or similar).
How many amps will flow depends on the voltage and the load.
An ideal 1V battery will provide 1A of current into a 1 ohm load. That is Ohms Law; Voltage (V) = Current (I) x Resistance (R). V = IR and this can rearranged, as V/R = I, or V/I = R.
Say an ideal 9V battery was connected to a 5 ohm resistor. The current will be 9V/5ohm = 1.8A
Similarly you can divide Volts by desired current to determine a current limit resistor.
Say you wanted to limit the same battery to just 0.5V; 9V/0.5A = 18 ohm resistor.
We use this to determine current limit resistors for LEDs.
Say you had an Arduino, and you wanted to limit the current from a pin to 8mA so it wasn’t so bright.
The Voltage from the pin, across the LED, across the resistor, to Gnd will be 5V.
An LED has a current where it turns on, say 2.2V for a Red LED. This is Vf.
The 5V source will be split across the LED and the resistor. If the LED needs 2.2V, that means there will be 2.8V across the resistor. The current thru the LED and the resistor will be the same - there is only path for the current, so could it be otherwise.
So, resistor value needed = 2.8V/8ma (0.008A) = 350 ohm.
You may see this as (Vs - Vf/current = resistor. (5V - 2.2V)8mA = 350 ohm.
Or say you had a resistor and wanted to know how much current it would allow to flow:
(5V - 2.2)/200 ohm = 14mA (0.014A).
Capacity is how many mA can be produced by a battery over a time period.
An ideal 9V battery with 400mAH capacity could produce 0.4A for one hour into a load (like a 22.5 ohm resistor).
However, small battery internal chemistry limits the creation of electrons * to a smaller amount; if more are asked to be created than the battery can supply, the voltage drops. So a small square 9V battery will maintain 9V for smaller currents, like 10-20mA, and trying to draw more will not work - the battery may overheat.
A car battery on the other hand has different battery capacity and can create many more electrons, often expressed as Cold Cranking Capacity - this can be many hundreds of amps short term to turn the engine over while you start the car, or stretched out over an hour or two while you sit in the car and listen to music with engine off.
Watts are just a measure of the Voltage and current being used. Power = Voltage x Current, or P = IV.
You know V = IR, so can you do some substitutions to find other things. V=IR, V/I = R
P = IV, and V=IR, so P = I x IR, or I^2R. How many watts will a 350 resistor dissipate with LED above?
P = .008A x .008A x 350 ohm = 22.4mW, so part rated for 1/10W would be okay to use.
P = IV, and I = V/R, so P = V x V/R or V^2/R. 2.8V x 2.8V/350 = 22.4mA, so there a couple ways to determine power dissipation.
8 batteries connected in series add up voltage-wise. 8 x 1.5V (AA battery) = 12V.
AA batteries have a capacity of 2.2AH (2200mAH), so the current available will be limited to that, but at a higher voltage, say for driving a small motor.
If many, many, are connected in series, you can have 36 for 48 volts for powering an electric bike.
Take that string of 32 batteries (48V), and connect many strings in parallel, and the current available adds up.
10 strings at 2.2A = 22A for one hour.
Put a lot more in series, and many strings in parallel, and you can supply more volts and more amps → more power.
That’s what electrics cars do - big battery packs of small batteries in strings, with many parallel strings.
- electrons are not really produced - they are charged up by voltage, and their charge jumps from electron to electron, and this is what gets referred to as current. The higher the voltage, and the more electrons present, determine how much current can flow into a load. Resistors keep current from moving too much. Capacitors store up charge, and keep direct current from flowing thru - but will let variations in current flow. Inductors do the opposite - they let steady current thru, but prevent variations from flowing thru.
As an example, capacitors and inductors are used to make filters to use with speakers. Low pass filters let the low variations into the woofers for low notes, while high pass filters keep the low notes out of the tweeters for nice crisp high notes.