PHY138Y – Electricity and Magnetism – Quarterly Test Solutions
February 8, 2005
Multiple Choice Questions
1. Two point charges,
Q
and
q
, are a distance
r
apart. The electric field is zero at a point
P
between the charges, on the line
segment connecting them. Then
(A)
Q
and
q
must
have the same magnitude and sign;
(B)
Q
and
q
must
have the same sign, but
may
have different magnitudes;
(C)
Q
and
q
must
have equal magnitudes and opposite signs;
(D)
Q
and
q
must
have opposite signs, and
may
have different magnitudes;
(E) the point
P
must
be midway between
Q
and
q
.
Remember the electric field points away from a positive charge and towards a negative charge.
If the charges have
opposite sign (choices
C
and
D
here) then some electric field lines will point directly from the positive charge to the
negative charge in the region in between. If
Q
and
q
have the same sign and magnitude then there will be a zero electric
field half way between the charges (choices
A
and
E
) so these are possible solutions – but because there are two of them
you should have realised you need to go deeper. In fact the
need not
have the same magnitude, nor does the point where
E
=
0 be half way in between. As long as the charges have the same sign then there will be a point with
E
=
0 somewhere
along the line segment connecting the charges
⇒
B
is the right answer.
2. An electron is placed in a uniform electric field directed toward the right side of the page. What is the direction of the
force that field exerts on the electron?
(A) toward the right;
(B)
toward the left;
(C) upwards;
(D) downwards;
(E) perpendicular to the plane of the page.
Remember the electric field direction is defined as the direction a
positive
charge would be pushed. Thus the electron,
that has a
negative
charge, will be pushed in the opposite direction: bf to the left.
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 Spring '08
 Turner
 Physics, Charge, Magnetism, right answer, Electric charge

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