power tracks thickest ?

[fran1942]

Hello, a simple question here.
My tutor told me to make the negative power track on the PCB the thickest (widest).
I understand that the negative and positive power tracks should be the thickest tracks on the board. Can someone please tell me exactly why this is ?

Thanks kindly for any help.

 

[(*steve*)]

To reduce the impedance of the power supply.