Question about electricity. Experts please.

I have a compressor that is rated at 18 amps. When I plug it into a socket that is on a 15 amp circuit with no extension cord it works fine. Borderline tripping the breaker I know but it's ok. When I use a 50' extension cord it will trip the breaker. Now as far as I know using a long extension cord does not cause more current draw and only adds resistance. So why does it work without an extension cord and doesn't with an extension cord? Thanks.

If you add resistance, the voltage is lower at the motor. With lower voltage, the motor needs more current to deliver the same power to the load. The motor will take whatever current it needs to deliver the power that it takes to run at the speed that it is designed for.

As you say - the chord adds resistance. You're over peak amp to begin with.

One idea is to use a bigger, thicker extension cord - thus less resistance.