1. The problem statement, all variables and given/known data My Physics course does not give an in-depth overview of electrostatics at all (it is rather shallow, focusing on electronics and circuit analysis). However, I find that I really need to understand it fully to form the basis for magnetism and generation. So... I have a few questions that I wanted to ask: 1. Electric potential and charge signs - I know that the electric potential (energy) is set to be zero at infinity, i.e. PE = 0 for r = ∞. For a stationary positive charge and a positive test charge, I know that, as distance decreases, potential energy increases... But what about: PE of negative charge relative to positive stationary charge? I think it is zero at ∞ but gets negative as distance decreases, as W = -ΔPE so as distance decreases, ΔPE is negative, and W is positive, so speed increases (correct!) PE of negative charge relative to a stationary negative charge should be same scenario as positive/positive? PE of positive charge relative to negative stationary charge should be the same as the neg/pos scenario My books (and Khan Academy) only look at the positive/positive case. So what about electric potential? I know it depends only on the stationary charge (i.e. the central charge of interest). So what about the signs of potential for a given charge? The potential relative to a positive charge should be positive, and should be negative for a negative charge. This would agree with what I'm figuring from PE (if we put signs into the equation V=kq/r 2. Signs of potential, and ‘potential across …’ – what does it mean to say ‘there is a potential difference across the cell wall of 1.7 v’ for example? What is the meaning of sign here? Should I take an increase in positive charge concentration as a positive p.d., and a decrease in positive charge concentration as a negative p.d.? Is there such a convention? For example: “Another interesting biological example dealing with electric potential is found in the cell’s plasma membrane. The membrane sets a cell off from its surroundings and also allows ions to selectively pass in and out of the cell. There is a potential difference across the membrane of about –70 mV” What is the meaning of the negative sign of 70 mV? I understand the potential difference of 70 mV as meaning that charge on either side of the membrane will have a difference in potential relative to the other side of magnitude 70mV, but whether it is higher or lower than the other side, how do I know? So if it was magnitude, no problem, I understand what it means. But -70mV? What would that mean? Which side is at higher potential? The cell or the surroundings? Would the negative be meaningless unless extra info on which side was positive/negative was known? (Just a bit confusing) 3. What do batteries actually do? I know they create a potential difference, hence a potential gradient ⇒ electric field. So this accelerates charges through the circuit and produces energy. Makes sense. But what about capacitors? The battery transfers charge from one plate to another. Now, do the terminals of the battery actually have net charge? I’m pretty sure they don’t, I can’t see any electrostatic effects happening from the negative end of a battery. So why can a battery produce charge separation on a capacitor, when it doesn’t do it at its terminals? Or is the average battery far too weak for this kind of thing? 4. This question is not on electrostatics, but it is an application of magnetism – the force between two infinite current-carrying wires. (See attached image) So what happened here is – the force due to wire 1 on wire 2 is calculated, and apparently that’s it – the force between the two wires is equal to that…. But shouldn’t the force be multiplied by 2 because of the force due to wire 2 on wire 1? Could I have an explanation as to why it is not necessary? I don’t get this… L Help please! 2. Relevant equations Voltage : V = kQ/r 3. The attempt at a solution ^^ See above I would be very grateful if anyone could help me debunk these confusing points!