I use a 5V 2A charger for my Raspberry Pi. If I use a short (50 cm long) wire everything works fine.
When I use a long (280 cm) wire, the lights from the board are turned off (thus the cable is not interrupted) but the Ethernet socket does not work and the lights from it are off.
Why does that happen? How can I solve this?
This is a power supply problem, as cables get longer, they can lose some efficiency. On top of that, longer cables are usually thinner as well. This compacts the problem even further.
If there are no lights on the board (especially the PWR led) when the board is plugged in, then the Pi is not receiving enough power.
You may need to stick with the shorter cable, or try to find a thicker long cable.
In short: short and thick is best.
Powering the Pi with different cables is talked about some here.
Also: A powered USB hub is always a good idea.
The quality (subjective), gauge, material and length of the wire determines the voltage drop from source to the target.
as a rule of thumb, worst the quality (in modern terms, cheaper the wire, cheaper the metal), higher the loss.
To get more details about the resistance in copper wire, this a good read.
In a nutshell, longer the wire, higher the resistance in the wire. Higher the resistance, greater the voltage drop the wire induces.
Further reading: Ohms Law
