One of my daughters mentioned on Shabbos that she learned in school that if one does melacha on Shabbos one cannot have benefit from that work, e.g. (her example) if you turn on a light, you would not be allowed to sit and read by that light. But is this correct? (I admit: I am naturally suspicious of anything my kids learn in school : ) The gemara's discussion of this halacha uses the example of violating the issur of bishul. The Chayei Adam has a safeik whether the rule would be the same for a melacha where the object has not changed - e.g. if one violated the melacha of carrying in a reshus harabim, would one be prohibited from having benefit from the object? When one turns on a light, the wiring of one's home has not changed - it has simply started carrying a current. On the other hand, the bulb that was dark has now become illuminated. Is this comparable to ma'avir or bishul??
Another interesting safeik is whether this halacha applies to issurei derabbanan as well as issurei d'oraysa. One could debate whether tuning on a light in indeed an issur d'oraysa (too lengthy a discussion for now).
I don't know how much of these details my daughter absorbed in our few minute discussion, but then again, I wonder if her teacher thought through them when presenting the case (yes, I should give her the benefit of the doubt).