P 1.5
The charge on an electron is –1.6e19 coulombs.
A current of 1 ampere flows in a wire
carried by electrons.
How many electrons pass through a cross section of the wire each
second?
Hint the definition of 1 amp of current is 1 coulomb of charge passing a point
on the wire in 1 second.
Answer:
1 amp =
1 C/s = N electrons/s x 1.6e19 coulombs/electron
N = 6.25e18 electrons per second
P 1.6
The ends of a length of wire are labeled
a
and
b
.
If the current in the wire is
i
ab
= –5A,
are the electrons moving toward
a
or
b
?
How much charge Q
(not the # of electrons)
passes through a cross section of the wire in 3 seconds?
Answer:
i
ab
means the current is from
a
to
b
but the –5A means the current is from
b
to
a
, but electrons flow in reverse, so the current is reversed again so that the electron flow
is from
a
to
b
.
The electrons are flowing toward
b
.
For a constant current (DC), the
charge Q flowing past a point in 3 seconds is
Q = 5 amps x 3 seconds = 15 coulombs.
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 Fall '08
 Staff
 Energy, Volt, Electrons, Electric charge

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