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64. (a)When the electron is released, its energy is
K
+
U
= 3.0 eV
−
6.0 eV (the latter
value is inferred from the graph along with the fact that
U = qV
and
q =
−
e
).
Because of
the minus sign (of the charge) it is convenient to imagine the graph multiplied by a minus
sign so that it represents potential energy in eV.
Thus, the 2 V value shown at
x
= 0
would become –2 eV, and the 6 V value at
x =
4.5 cm becomes –6 eV, and so on.
The
total energy (
−
3.0 eV) is constant and can then be represented on our (imagined) graph as
a horizontal line at
−
3.0 V.
This intersects the potential energy plot at a point we
recognize as the turning point.
Interpolating in the region between 1.0 cm and 4.0 cm, we
find the turning point is at
x =
1.75 cm
1.8 cm.
≈
(b) There is no turning point towards the right, so the speed there is nonzero.
Noting that
the kinetic energy at
x
= 7.0 cm is
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 Spring '10
 Rebello,NobelS
 Charge, Energy

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