Question

I am really confused about Physics 2 in relation to the topic of Electric Work, Electric Potential Energy and Voltage.
I understand that delta(U) = -W or W = -delta(U).
However, I do not understand certain areas of this electric work.
My book says that a external force does NEGATIVE WORK when it it moves two particles of the same charge AWAY from each other. But looking at delta(U) = -W does not make sense.
The book also states that W from a to b = Ua - Ub.
Ugh! I have been reading different books, Khan Academy, MIT, Yale... I don't understand this work thing at all.

Answer 1

This is the most confusing paragraph ever. One the first view they say the work of pushing two like charges away is positive, then on the second view they say now it's negative. :(

Answer 2

I understand the electric field would do positive work on a +Q moving away from another +q, because +Q would be moving in the same direction of the electric force, away from +q. The electric field would do negative work if +Q is to be brought towards +q correct?

Answer 3

But, then how do they state later on that negative work is done by an external force when moving two particles away from each other.

Answer 4

whats ur doubt in U=-w?

Answer 5

i mean start with something caz u have written so many things at a time...

Answer 6

Honestly, I don't even know why W = -delta(U). At this point I'm so confused, everything feels crossed.

Answer 7

take the case of earth and a object of mass m. For now, forget the charge, electric field.

Answer 8

it's fine with u?

Answer 9

yes

Answer 10

suppose the ball is at A initially...and it is being taken to point B

Answer 11

now work done by gravity= -mgh

Answer 12

right?

Answer 13

correct, and the work that I do to move the ball is mgh (positive and opposite to gravity)

Answer 14

change in potential energy= Ub- Ua = work done by the external agent= -( work done by the gravity)

Answer 15

yes

Answer 16

it's fine?

Answer 17

ok yes that I understand

Answer 18

External agent is taking the ball slowly such that there is no change in kinetic energy of the ball

Answer 19

my book that mention that. about applying the external force slowly, that is to keep the Potential Energy U unaffected correct?

Answer 20

When we define the potential energy...we apply the external force such that there should not be any change in the kinetic energy..in what context ur book says this?

Answer 21

that context that you mention. you made that part clear, thank you. please proceed.

Answer 22

as we saw in the last example...the ball has been taken from A to B...there must be some external agent...such that there is no change in kinetic energy of the ball.
so work done by external agent= change in potential energy+ change in kinetic energy(=0)
work done by external agent = change in potential energy

Answer 23

Correct.

Answer 24

now we come to electric charge...

Answer 25

as u have only written...
delta(U) = -W or
W = -delta(U).
here w= work done by the field(gravity or electric field)
now it does make sense to u?

Answer 26

ahhh ok

Answer 27

when they mention that on Physics books, W = -delta(U) they refer to the Work done by the Field not the external agent.

Answer 28

yes...work done by the field not the external agent

Answer 29

Because positive work by ME moving the ball up results on positive work, and a positive change in P.E, which is the energy spent moving the object up. Work by gravity would be the opposite.

Answer 30

yes...perfect

Answer 31

now we come to.....However, I do not understand certain areas of this electric work.
My book says that a external force does NEGATIVE WORK when it it moves two particles of the same charge AWAY from each other. But looking at delta(U) = -W does not make sense.

Answer 32

there are two charges and moving away from each other..then external force will does the negative work because force and displacement are in opposite directions...right?

Answer 33

but, would not I make the force in the direction of the particle away from the other one ?

Answer 34

yes that is the picture on my mind

Answer 35

that is the electric force...but suppose if we(external agent) move the charge particle slowly then we'll do the work against the electric field...so work done is negative

Answer 36

right?

Answer 37

both charges are of the same type...

Answer 38

hmmm that is what i dont understand give me one sec I draw my question

Answer 39

ok

Answer 40

drawing almost done, do you know how to do colors?