PHYS 142 Lecture 17

1
A mass oscillates displaying the Simple Harmonic
Motion plotted in the position vs time graph at
right.
Position
time
1) 2) 3) 4)
Velocity
Which of the velocity vs time graphs best
represent its motion?
Velocity
Velocity
Velocity
2
A positively charged sphere is set spinning as
shown at left.
1)A magnetic field is set up with a North Pole
at the spheres top. 2)A magnetic field is set
up with a South Pole at the spheres
top. 3)The magnetic field lines form closed
concentric circles centered on the spin
axis. 4)Since the charge stays in a fixed
position, no magnetic field is generated.
3
Oxygen O8
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d
Iron Fe26
4
If a N pole moves towards a loop, both in the
plane of the page, in what direction is the
induced current? 1) clockwise 2)
counter-clockwise 3) no induced current
5
If a conducting coil is rotated as shown, in the
B field pointing right, in what direction is the
induced current? 1) clockwise 2)
counter-clockwise 3) no induced current
6

• Wire 1 (length L) forms a 1-turn loop,
• and a bar magnet is dropped through.
• Wire 2 (length 2L) forms a 2-turn loop,
• the same magnet is dropped through.
• Compare the induced EMFs for both.
• 1) ? 1 lt ? 2 2) ? 1 ? 2 3) ? 1 gt ? 2

Compare the magnitude of the induced currents in
these two cases. 1) I 1 lt I 2 2) I 1
I 2 3) I 1 gt I 2
7
A bar magnet is held above the floor and dropped.
In A, there is nothing between the magnet and
the floor. In B, the magnet falls through a
copper loop. In which case will the magnet fall
faster? 1) A 2) B 3) same speed
for both
S
S
A
B
N
N
copper
8
Answers to Multiple Choice questions Slide 1
(3) The steepest slopes in the top plot must
correspond to the maximum speeds. Slide 2
(2)South Pole at the spheres top. Right hand
rule for circulating charge points thumb
down. Slide 4 (3) no induced current The
angle between the B field lines and the normal to
the loop is always 90 degrees. Since cos(90)
0 the flux is always zero and therefore not
changing. Slide 5 (1) clockwise Flux will be
increasing. Current will oppose increase, so
induced B field will point to the left. The
Right Hand Rule indicates a CW current. Slide 6
(1) ? 1 lt ? 2 ? N ??/?t. Both loops have
same ??/?t but loop 2 has N 2.
(2) I 1 I 2 I V/R. Loop 2 has twice
the EMF of loop 1, but also twice the
resistance since it
is twice as long. Slide 7 (1) A When the
magnet is above the loop, an induced current will
produce a N-pole at its top of the loop,
repelling the magnet. Once below, the induced
current will produces an N-pole at the loops
bottom, attracting the magnets S-pole. Or,
think about energy. When an induced current
flows its energy has to come from somewhere. It
must come from the kinetic energy (½mv2) of the
falling magnet, i.e., it must fall more slowly.