Hey guys im making brownies but i don't have a whisk. Can i substitute with a electric mixer on low switching on and off occasionally? Would that lead to overbeating the mixture
Brownies don't need whisking. Whisks are handy tools that make it easier to blend the ingredients in together but a fork to beat the eggs and a spoon to mix everything will do just fine. You won't get any real benefit from using a mixer, you'll just make more dishes dirty.
A mixer with whisk attachment is generally a good substitute for a whisk. Both are used to integrate air into a mixture or combine the ingredients. If the recipe calls for a whisk and I have non at hand, I simply use a mixer. Overbeating shouldn't be much of a problem, if you stop as soon as everything is combined/airy enough. In case I don't have a mixer, too, or it is just a very small amount, for which a mixer would be overkill, I simply use a fork. It is basically a very rudimentary whisk. It takes longer and is definitely more tedious, however, i even made meringue with a fork once (here it helps to add a little bit of salt and have the egg whites at room temp).
That said: For brownies a good spatula or spoon should work quite fine. No mixer needed.