Capacitors and related Voltages.

[Badger33]

Homework Statement

1) A capacitor plate holds a given charge Q. Why is the magnitude of the plate voltage small when a grounded plate is near, but large (for the same Q) when it is alone?

2) If the charge Q on a capacitor is doubled, what is the change in the voltage one measures on the capacitor? (Remember that Q is the magnitude of the charge on each plate, both positive and negative.)

3) How could you double the positive charge on one capacitor plate without changing the negative charge on the other plate?

2. The attempt at a solution

1) The magnitude of the plate voltage is smaller when a grounded plate is near, because when a plate is grounded the ground is an infinite receiver to store electrons and is also an infinite supply of electrons. When the plates are brought into proximity of each other the ground offsets the charges, thus effectively lowering the magnitude of the voltage.

2) the voltage doubles. Look at the equations V=Q/C and also V=(Qd)/(εA).

3) You would have to change the area of the plate.

Solution Rational
For question 1 I am 98% sure I am correct. 2) Think I am 100% correct. 3) This is where me and my roommate are in differencing opinions, I think that you will have to reduce the area by half and he thinks that you will have to double the area. Who is correct on number 3 and is my rational on the previous 2 questions sound?

Thanks for all the help in advance!

 

[jbsteele]

Answer: For question 3, you would need to double the area of the plate in order to double the positive charge on one capacitor plate without changing the negative charge on the other plate. Your rational for questions 1 and 2 is correct.