If the electrical potential energy of two point charges is U when they are a dis

Question

If the electrical potential energy of two point charges is U when they are a distance d apart, their potential energy when they are twice as far apart will be U/4. U/2. U 2U. 4U. An electron is released between the plates of a charged parallel-plate capacitor very close to the right-hand plate. Just as it reaches the left-hand plate, its speed is v. If the distance between the plates were halved without changing the electric potential difference between them, then the speed of the electron when it reached the left-hand plate would be 2v. ( 2)v. v. v/ 2. v/2. A cylindrical metal rod has a resistance R. If both its length and its diameter are doubled, its new resistance will be: 8R. 4R 2R R/2. R/4.

Explanation / Answer

(3) The potential energy is defined as
U = Kq1q2/r
therefore on doubling the distance , the potential energy get U/2.
(4)
The electric field between the plates is doubled which results in the acceleration of the electron being doubled. However, the distance travel by the electron is halved so these changes exactly cancel each other so that the final velocity stays the same.

(5) R = pL/A
when length ad diameter get double
L' =2L
A' = 4A
New resistance R' = R/2

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