work equation re: electricity and electrostatics

[salsasunrise123]

Hello, I am reviewing electricity and electrostatics and am confused about the equation for work. According to PR, the equation is W-PE, but according to Nova physics the equation is W=qV. I am confused as to which formula to use between they both don't give you same answer in problems such as this:

Parallel plate capacitor with 10 V battery. +3 charge is on negative plate and you need to move it to positive plate. calculate work.

PR method: w=-pe -> -(3*10) = -30

Nova method: w=qv -> 3*10= 30

Any help would be greatly appreciated. Thx

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[salsasunrise123]

bump

 

[gwjib04]

Before using equations just think about it:
Does a + charge favor moving to the + plate? No, because like charges repel. Therefore, the work done on the object is being added to the PE of the charge. The ΔPE must be + then because we associate a +ΔPE with gain and a - ΔPE with loss.

Your confusion probably lies in how the equations are defined. In general we say ΔPE = -W because when you drop books (most common example) their PE decreases due to the gravitational force converting their PE to KE as they travel some distance to the ground where their PE_final=0. So the gravity has done +work on the books, the books have lost PE (final minus initial where final is 0 and initial was mgd).

So when you're dealing with a charge in an electric field, its the same scenario but the force is the electrostatic force. Best to pay more attention to the fact that we're taking about CHANGE IN PE here because charges often don't end up at 0 PE. That's especially true here because we're actually lifting the books up (moving a + charge to a + plate).

So the equation for the work done ON a charge in an electric field is given by
W=F_externald not W=qEd because the field won't move it to the + plate
In this case the Work done ON the object is negative because you've taken the charge it to a more repulsive area. The PE of the charge is + though: PE_final-PE_initial where final PE is greater than initial