electric field?

Question

complete data:
two large horizontal plates are separated by 4mm .the lower plate is at a potential of -6v.what potential should be applied to upper plate to create an electric field strength of 4000 v/m upward in space between the plates
relavant equation:
E=v/d
attempt at question:
4000=v/0.004
v=16v
but the answer of this question is -22v
how it will be solved

Answer 1

You are doin it right. But the answer you got tells you the potential difference between the plates. Remember it is the potential difference between the plate that causes the electric field. Now you will ask then why not the answer is 10V because it will give the same potential difference of 16V (which you got). Read the question carfully what it says, electric field in upward direction and as you know electric field lines flow from higher to lower potential (just as the water flows from a higher height to a lower height) that means potential at the upper plate should be lower because then only the electric field can move upwards. So the only answer left is -22V.

Answer 2

Arrrgghh. Nightmare.

Field flows from +ve to -ve, right?

Wrong. It flows from more +ve to more -ve.

The diff?

You need the bottom to be more +ve than the top, [because they said the field goes from bottom to to]
but the bottom is actually -6V

So the top must be -16V more -ve than that.

-6 -16 = -22

You were right that there is a 16V difference. This question is worded to separate the A students from the C students. { half the B students will make a lucky guess}

Hope the explanation is simple enough to understand and make sure you don't so it again

Answer 3

the potential difference is 16V and not the absolute potential.
=>the other potential = -6-16 = -22V