I'd recommend kryptonite. The Evolution Mini is used by a number of my friends (messengers) and they use it many times in a day. Keeps things secure and it's small. If you want a bit more flexibility in what you can attach to then go for a larger sized one.

https://www.kryptonitelock.com/products/list.aspx?cid=1001&amp;scid=1000

Lock for extremely frequent use
Sparknotes' studyguide for the SAT II: Physics test says that for a point charge (1-dimensional, e.g. an electron), the formula for intensity of the generated electric field is given by $E=\frac{kq}{r^2}$. It also says that for a line charge (2-dimensional, e.g. a wire in a DC circuit), it's given by $E=\frac{kq}{r}$, and that for a plane charge (3-dimensional) it's given by $E=kq$. I have no example for a plane charge because I don't know what that would be, hence my question: How can a plane charge's electric field intensity NOT diminish over time? Shouldn't this violate Conservation of Energy?
