amine2,
The relationship between voltage, current, and resistance is described by [u]Ohm's Law[/u]. Ohm's Law is the 1st thing you learn when you take an electronics class.
Resistance means the resistance to the flow of electrical current. With high resistance, less current flows. With low resistance, more current flows.
In general, the voltage is fixed (or controlled) and the current depends on the resistance.* They don't tell you the resistance for a chip, but the datasheet will give you the current. (With a chip, the actual current usually also depends on what's connected to it.)
At your wall socket there is either 120V or 240V and that's constant. The amount of current depends on what you plug in. If you plug-in too many things, too much current flows and you blow a breaker (and the voltage drops to zero).
You can use the water flow in a pipe analogy (NOT a perfect analogy**)...
- Current is water flow
- Voltage is water pressure. With higher pressure, more water flows.
- High resistance is a skinny pipe and very little water flows. Low resistance is a fat pipe and water flows easily.
For example if you connect an LED (with an appropriate series resistor) to a "little" 12V power supply, you might get 20 milliamps (0.02 Amps). You can connect that same LED & resistor to a 12V car battery that's capable of 500 Amps and you'll get the same 20mA current.
Or, here's an "opposite" example... You can take a couple of regular 9V batteries and put them in series to get 18V. That's more voltage than a car battery, but if you try to start your car with that setup the batteries are only capable of around 1Amp (that's a guess) for a very short period of time before they die... I don't know what the resistance of a car starter, but it's much-much less than 1 Ohm. Ohm's Law is a law of nature and it's always true. So, when you try to apply 18V across your car's starter with 2 small 9V batteries, the voltage drops to almost zero and the starter doesn't move.
* Or impedance. Resistance and impedance are both measured in Ohms.
* The biggest problems with the water analogy are - If you cut a pipe, the resistance goes to (nearly) zero and water flows-out freely all over the place. If you cut a wire, the resistance goes to infinity and no current flows.
And with water, there's nothing wrong with low-resistance... Nothing bad happens with no water-resistance. If there's no electrical resistance, usually too much current flows, bad things happen and things burn-up.
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