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I'm am electrical engineer and I thought the same as you. Had no clue it's a defined term

In general, Voltage and resistance determine amperage, that’s right.

Yes and no. These are AC/DC adaptors so they're rectifying the AC current (+/- AC to DC) and using transformers, resistors, etc to step down the voltage. Watts is V*A

On a power brick, the numbers of a matter of how many volts it pushes out and how fast it can push them out (amps).

The components inside your laptop need a relatively fixed amount of power at a variable amount of current. If it's working harder, it may pull power now quickly using more watts of power. The components within the laptop are designed with that amount of power in mind and the resistance is fixed within a certain tolerance.

The brick is similarly designed to provide a relatively constant amount of power at a maximal rate. You'll notice that a brick heats up with use. Pull power through it faster than capacity and it'll overheat and die.

If you compare to home electricity, think of it like this.

• 110V, 15A appliance in 220V, 15A circuit. That appliance is gonna melt something and catch fire. This is like a laptop with an over-voltage brick

• 220V, 15A appliance in a 110V, 15A circuit, it's probably not going to start or run properly. This is a laptop with an under-voltage brick

• 110V, 30A appliance on a 110V, 15A circuit. It may run for a bit if it doesn't immediately draw over the 15A, but when it does the breaker trips or a fuse pops (if they're working). If you have a bad breaker or fuse that doesn't trip in time, the wires in your wall will actually heat up and either burn out or set your house on fire

In the home circuit, that 110V is the constant voltage and 15A is a maximum. You can plug in a 1500W ([email protected]) microwave, or a 20W ([email protected]) wall wart. Just like the circuit from the wall to your panel, your brick has a fixed volts and max amps. Your laptop has a fixed volts and variable amps.