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ConcepTest #21:
ConcepTest #22:
A particle has a net positive charge, but
it experiences no net electric force.
A conducting sphere initially has no net
charge. A positively charged rod is then
brought close to the sphere. The sphere
is then connected to ground. The rod is
then removed, and then the connection
to ground is broken. After these steps,
what is the net charge on the sphere?
Which of the following statements must
be true? Hold up as many cards as
necessary.
1. There are no other charges nearby.
2. If there are other charges nearby,
then they must all have a sign
opposite that of the first particle.
3. If there are other charges nearby,
then they must all have a sign the
same as that of the first particle.
4. If there are other charges nearby,
there must be precisely the same
number of positive charges as there
are negative charges.
5. None of the other statements is
necessarily true.
1. Positive
2. Negative
3. No net charge
4. Not enough info
ConcepTest #23:
Consider the following directions for the
deflection of an electron:
a) An electron is flying to the right
through an electric field that points
down as shown. What direction
does it deflect as it passes through
the electric field?
1. Up
4. Left
2. Down
5. Into the page
3. Right
6. Out of the page
(back of card) No Direction
b) An electron is flying to the right
over the North end of a bar magnet as
shown. What direction does it deflect
as it passes over the bar magnet?
N
S
ConcepTest #24:
The electric potential at point A is
30 V, while at point B it is 80 V. A
+2 C charge moves from A to B,
subjected only to electrostatic
forces.
If the charge has 120 J of kinetic
energy at A, what is its kinetic
energy at B?
1. 0
4. 120 J
2. 20 J
5. 170 J
3. 70 J
6. 220 J
ConcepTest #25:
Particle A has charge +Q and mass M.
Particle B has the same charge +Q, but
twice the mass, 2M.
DV
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●
The particles begin at rest and are
accelerated through the same potential
difference DV.
1. Particle A
The particles then enter a region of
uniform magnetic field B directed out
of the page. Ignore the repulsive
interaction between the two charged
particles.
Which particle has the largest radius
of curvature in the region of the
magnetic field?
2. Particle B
●
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●
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●
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3. Same radius
4. Not enough info
ConcepTest #26:
A long straight current carrying wire, length 2L,
is oriented along the y-axis as shown. A small
piece of the wire at (0,y) is indicated. A field
point P is on the x-axis at a distance xP.
L
P
Which
of the following best represents

dB the magnitude of the contribution to the
___,
magnetic field due to the small piece of wire?
(xP,0)

Id 
(0,y)
1.
0
0
Ix P dx
4.
4 ( x P2  y 2 )3 / 2
Iydx
0
4 ( x P2  y 2 )3 / 2
Ix P dy
0
5.
4 ( x P2  y 2 )3 / 2
Iydy
0
3.
4 ( x P2  y 2 )1 / 2
Ix P dy
0
6.
4 ( x P2  y 2 )1 / 2
2.
–L
ConcepTest #27:
A metal ring drops downward
into and then through a region of
magnetic field pointing into the
page as shown. The magnetic
field is uniform and points into
the page in the boxed region, and
is zero outside the boxed region.
For each of the following cases,
consider the following choices for
the direction of the current in the
ring:
v
X X X X X X
X X X X X X
X X X X X X
X X X X X X
X X X X X X
X X X X X X
(a)
(b)
1. Clockwise
2. Counterclockwise
3. No current
X X X X X X
X X X X X X
X X X X X X
X X X X X X
X X X X X X
X X X X X X
(c)
4. Not enough info
(d)
Try it Yourself:
A uniform magnetic field points into
the page as shown. A disk made out
of a conducting material that lies in
the plane of the page is rotated
clockwise.
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Calculate the magnitude of the
induced e in the disk. Where on the
disk is the potential higher?
Explain how, even though the flux
through the disk is constant, e is still
induced in the disk.