Question

Hi, I'm in the new 6.002 course that will begin on March 5. I've been reviewing the first two lectures, and I'm really lost. My extremely difficult question is with regards to HW question No. 1 [if anyone can help]. Are the figures that we are to use in order find the polarity of the voltages, current, etc. figures that we make up ourselves??

Answer 1

i second ask the same question

Answer 2

Yes; they must be. The simplest would be choose a convention so the answers are positive.

But know that the old homework you're looking at is not for the March 5 course; it's for an old course that was hand-graded by a TA who could examine your choice of current/voltage conventions. The course we're taking hasn't yet been posted yet, but won't have that sort of ambiguity when it is since it will be computer graded.

Answer 3

Well you're following some conventions that's all if you imply that all elements are getting the current into the + terminal then the power consumed (or disipated) would be positive at that element, at the same time the current flowing into the + terminal of a given source will be negative because you had already define the current in that branch for a given element remeber (positive though) it has a contrary sign to satisfy the conservation of energy , so in a given node (juction between two or more elements) the current going out are equivalent to the current going in, so alternating signs for a given current shared in two or more branches .

This also implies that a source has a negative power because it's not absorbing but instead delivering power to the elements in the circuit.