A320.9 ng sample of an unknown radioactive substance was placed in storage and its mass measured periodically. after 47 days the amount of radioactive substance had decreased to 40.11 ng. how many half‑lives of the unknown radioactive substance have occurred?

Explain why electric current cannot exist if a current doesn't have a voltage source.

Four point charges each having charge q are located at the corners of a square having sides of length a. find symbolic expressions for the following. (a) the total electric potential at the center of the square due to the four charges (use any variable or symbol stated above along with the following as necessary: ke.) vtotal= (b) the work required to bring a fifth charge p from infinity to the center of the square (use any variable or symbol stated above along with the following as necessary: w=

Asign is hung between two cables as illustrated below. if the sign weighs 350 n, what is the tension (in n) in each cable?