First slide

Electric field

Question

A block of mass m containing a net negative charge -q is placed on a frictionless horizontal table and is connected to a wall through an unstretched spring of spring constant k. If the horizontal electric field E parallel to the spring is switched on, then the maximum compression of the spring is

Moderate

A

$\sqrt{q E / k}$
B

$2 q E / k$
C

$q E / k$
D

$z e r o$

Solution

Force on the block: F = qE toward left.
Let spring be compressed maximum by x. Then
$F x = \frac{1}{2} k x^{2} or q E x = \frac{1}{2} k x^{2} or x = \frac{2 q E}{k}$

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