# Conceptual Question 27.13

#### phyBnewb

1. The problem statement, all variables and given/known data
A parallel-plate capacitor consists of two square plates, size L x L, separated by distance d. The plates are given charge +-Q . Each part changes only one quantity; the other quantities have their initial values.

Part A

What is the ratio E(final)/E(initial) of the final to initial electric field strengths if Q is doubled?

2. Relevant equations

I know that:E = kQ/r^2 ,where k= 8.99*10^9

3. The attempt at a solution

I am not sure where to start here for this question.
Since it is a ratio of E final / E initial would Q cause E to be something like this?
E = k(2Q)/r^2 => 1/2E for E final?

#### phyBnewb

I tried 1/2 but it was incorrect. Help please!

#### nasu

You are using the formula for a point charge in the case of a parallel plate capacitor.
The electric field inside the capacitor is constant and is given by E=V/d where V is teh voltage and d is the distance between the plates.

#### phyBnewb

i figured it out. it wasnt 1/2, for Efinal/Einitial = 2

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