- #1
kliang1234
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Information given:
A parallel-plate capacitor has capacitance C = 12.5pF when the volume between the plates is filled with air. The plates are circular, with radius 3.00cm. The capacitor is connected to a battery and a charge of magnitude 25.0pC goes onto each plate. With the capacitor still connected to the battery, a slab of dielectric is inserted between the plates, completely filling the space between the plates. After the dielectric has been inserted the charge on each plate has magnitude 45.0pC.
Question:
1) What is the electric field at a point midway between the plates before the dielectric has been inserted?
2)What is the electric field at a point midway between the plates after the dielectric has been inserted?
Apparently, the correct answer for both questions is 999V/m.
The answer for the first part is calculated with E=Q/Ae
The justification for #2 is that because the plates are still connected to the battery, potential does not change, therefore by E=V/d, the electric field does not change after the dielectric is inserted.
This is where I'm confused. I understand their justification that if potential doesn't change, electric field doesn't change.
However, the given information specifically states that the charge increases from 25pC to 45pC. From what i know, such an increase in charge should increases electric field.
Lets assume that i attempted problem 2 with the same approach i took problem 1.
E=Q/Ae. The increase in Q increases electric field if that equation was used.
Can anyone explain this to me. I'm very confused.
Thanks
A parallel-plate capacitor has capacitance C = 12.5pF when the volume between the plates is filled with air. The plates are circular, with radius 3.00cm. The capacitor is connected to a battery and a charge of magnitude 25.0pC goes onto each plate. With the capacitor still connected to the battery, a slab of dielectric is inserted between the plates, completely filling the space between the plates. After the dielectric has been inserted the charge on each plate has magnitude 45.0pC.
Question:
1) What is the electric field at a point midway between the plates before the dielectric has been inserted?
2)What is the electric field at a point midway between the plates after the dielectric has been inserted?
Apparently, the correct answer for both questions is 999V/m.
The answer for the first part is calculated with E=Q/Ae
The justification for #2 is that because the plates are still connected to the battery, potential does not change, therefore by E=V/d, the electric field does not change after the dielectric is inserted.
This is where I'm confused. I understand their justification that if potential doesn't change, electric field doesn't change.
However, the given information specifically states that the charge increases from 25pC to 45pC. From what i know, such an increase in charge should increases electric field.
Lets assume that i attempted problem 2 with the same approach i took problem 1.
E=Q/Ae. The increase in Q increases electric field if that equation was used.
Can anyone explain this to me. I'm very confused.
Thanks