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Unformatted text preview: et’s turn to the deﬁnition of electric potential:
Deﬁne a coordinate system.
Choose where the electron starts it motion
as x!=!0 and its direction of motion as +x. Electric Potential Electric Potential Answer Answer 0 = ∆K + ∆U
∆K = −∆U ∆U = q 2 ∆V ∆U = (−1.6 × 10−19 C)(+5000 V) = −8.0 × 10−16 J
So, the electron lost potential energy by moving
in this electric ﬁeld.
Where did this energy go?
It went into the kinetic energy of the system,
since the electric force is a conservative force.
We can then turn to conservation of energy to
solve for the ﬁnal velocity of the electron.
Did any energy leave the system?
No, it stays with electron. +x v= −2∆U
mv 2 = −∆U
2 −2(−8 × 10−16 J)
= 4.2 × 107 m/s
9.11 × 10 Wow! The electron got almost close to the
speed of light, with just a 5000!V difference. electron Volt It becomes very inconvenient to work with
Joules when you are dealing with electrons or
We then introduce a new unit, the electron Volt.
The electron Volt (eV) is deﬁned as the energy
that an electron gains when accelerated through
a potential difference of 1Volt.
An electron in a normal atom has about 10!eV
while gamma rays (light) may have millions of
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