power supply

hello builders
I'm hoping some one can answer these question;

1) Can the external 5v 500ma power supply be used whilst the brain v2 is connected to a computer or is it solely to be used when the brain is being used as a standalone device. I ask as i would like to reduce the power draw from my laptop.

2)Is the pin on the dc socket is  positive and the ring ground or is it  vice versa. 


thanks for your help.

gaf