Question

A long straight charged wire has the electric field measured as $5N/C$ and pointing down at a distance of $10cm$ above it.
What would the electric field measure at $10cm$ below it?

• A. $10N/C$ toward the bottom of the screen.
• B. $5N/C$ toward the bottom of the screen.
• C. $10N/C$ toward the top of the screen.
• D. $5N/C$ toward the top of the screen.

Answer 1

Answer: (D)

$5N/C$ toward the top of the screen.

The electric field $E$ due to a straight charged wire at a distance is given by ,

$E=\frac{1}{2\pi {\epsilon }_0}.\lambda /r$

where $\lambda =$ linear charge density of wire ,

by symmetry the electric field due to a straight wire is same on any side of the wire , it depends on distance , which is same in the given diagram below the wire as above the wire therefore magnitude of field will be $5N/C$ below the wire .

Now as the electric field is directed radially inward to wire it means wire has negative charge , as the field lines goes toward the negative charge therefore electric field will be toward the top of the screen .