Transcript PHYS 142 Lecture 17

A mass oscillates displaying the
Simple Harmonic Motion plotted in
the position vs time graph at right.
1)
Which of the
velocity vs time
graphs
best represent
its
motion?
2)
3)
4)
A positively charged sphere is
set spinning as shown at left.
1)A magnetic field is set up with a North Pole
at the sphere’s top.
2)A magnetic field is set up with a South Pole
at the sphere’s 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.
Oxygen
O8
1s
2s
2p
Iron
3s
26
Fe 3p
4s
3d
4p
5s
4d
If a N pole moves towards a
loop, both in the plane of
the page, in what direction
is the induced current?
1) clockwise
S
2) counter-clockwise
3) no induced current
N
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
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,
#1
the same magnet is dropped through.
#2
Compare the induced EMF’s for both.
1)
1<2
2)
1 = 2
3)
1 > 2
Compare the magnitude of the induced currents in these
two cases.
1) I 1 < I 2
2) I 1 = I 2
3) I 1 > I
2
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
N
N
copper
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 sphere’s 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 <  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 loop’s bottom, attracting the magnet’s 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.