Electric potential homework problems

[gandharva_23]

mark the correct alternative :
The electric potential decreases from 120V to 80V as one moves on the X axis from x=-1cm to x=1cm . The electric field at the origin
a) must be equal to 20V/cm
b) may be equal to 20V/cm
c) may be greater than 20V/cm
d) may be greater than 20V/cm
i think all a , b and c should be correct but the answer given in the book is b and c . it will be b or c depending upon whether the V vs x graph is concave or convex . and b option will be correct for straight line graph .
what is incorrect in my approach ?

 

[nrqed]

mark the correct alternative :
The electric potential decreases from 120V to 80V as one moves on the X axis from x=-1cm to x=1cm . The electric field at the origin
a) must be equal to 20V/cm
b) may be equal to 20V/cm
c) may be greater than 20V/cm
d) may be greater than 20V/cm
i think all a , b and c should be correct but the answer given in the book is b and c . it will be b or c depending upon whether the V vs x graph is concave or convex . and b option will be correct for straight line graph .
what is incorrect in my approach ?

"a" can't be correct because of the "must be". If a was right, it would imply that this is the *only* possible answer (so it would not make sense to have a and another choice!).

 

[Physics Monkey]

Hi gandharva,

Simply knowing that the potential is decreasing from x = -1 to x = 1 is not nearly enough information to tell you what the electric field is at x = 0. All you know is that its positive since the potential is said to be decreasing. Try imagining or drawing situations where the slope of the potential graph is very small or very great at x = 0. See what possibilities you can come up with, and remember, the potential can do anything as long its decreasing the whole time.

Hope this helps.

P.S. As you have written it, the answer d) is the same as c). You might check to make sure you have posted all the information correctly.