W=qV or .5QV? Confused, please help!

[riseNshine]

Hi guys,

I'm doing TPR physics problems and having some difficulties understanding this concept:

1) How much work must be done to move a -2 coulomb charge from the positive plate to the negative plate of a capacitor charged to 10V.

Answer: they use W=Q*change in V, yielding the response 20V.

2) How much work is required to insert a dilelectric with the battery still connected. Battery is 12V. 2 uF parallel plate capacitors.

Answer: use W=change in PE=.5V^2(C'-C).

3) During charging, four microcoulombs of negative charge are transferred form one plate of an 8uF capacitor to the other plate. How much work was done by the electric field during this charging process?

Anwer: use W=change in PE


I'm confused on in what cases you would use W=change in PE versus W=q*V. From their explanations, it seemed you use W=change in PE when moving total charge from one plate to another, but then I don't understand why it was used in case 3.

Thanks all!

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[riseNshine]

anyone?
please?

 

[MedGrl@2022]

Hi guys,

I'm doing TPR physics problems and having some difficulties understanding this concept:

1) How much work must be done to move a -2 coulomb charge from the positive plate to the negative plate of a capacitor charged to 10V.

Answer: they use W=Q*change in V, yielding the response 20V.

2) How much work is required to insert a dilelectric with the battery still connected. Battery is 12V. 2 uF parallel plate capacitors.

Answer: use W=change in PE=.5V^2(C'-C).

3) During charging, four microcoulombs of negative charge are transferred form one plate of an 8uF capacitor to the other plate. How much work was done by the electric field during this charging process?

Anwer: use W=change in PE


I'm confused on in what cases you would use W=change in PE versus W=q*V. From their explanations, it seemed you use W=change in PE when moving total charge from one plate to another, but then I don't understand why it was used in case 3.

Thanks all!

Hi,

I was so confused by this too! I think you just have to know that for the potential energy of capacitors U=1/2QV... for point charges, and electric fields U=QV but for capacitors it is U=1/2QV. My examkracker book tried to explain it to me and I read somewhere about 1/2V being the average V but I really didn't understand. I hope someone else can enlighten us.

I hope that I helped. 🙂

Best,

Verónica