Download What is the direction of the magnetic field produced by this current

What is the direction of the magnetic field produced by
this current loop inside the loop?
A] upward
B] downward
A loop of copper wire is shown.
Moving the magnet up:
A] causes increasing upward B flux
B] causes decreasing upward B flux
C] causes decreasing downward B flux
D] causes increasing downward B flux
E] has no effect on the flux through the loop
A loop of copper wire is shown.
Moving the magnet up
-causes increasing upward B flux.
In what direction should the B field caused
by the induced current be?
A] up
B] down
A loop of copper wire is shown.
Moving the magnet up
-causes increasing upward B flux.
The loop current should oppose the
flux change. So the field from the
loop current should be DOWN.
What direction does the current
flow, viewed from above?
A] CW
B] CCW
A loop of copper wire is shown.
Moving the magnet up
-causes increasing upward B flux.
The loop current should oppose the
flux change. So the field from the
loop current should be DOWN.
The induced current must flow CW,
seen from above, by the RHR.
If we imagine the current loop as a
little magnet, is the N pole
A] up
or
B] down?
A loop of copper wire is shown.
Moving the magnet up
-causes increasing upward B flux.
The loop current should oppose the
flux change. So the field from the
loop current should be DOWN.
The induced current must flow CW,
seen from above, by the RHR.
N pole is down. (Field lines come
out from the N pole.) The force
between the magnet and the loop
is:
A] attractive
B] repulsive
DEMO!
A copper wire loop is shown.
Moving the loop up:
A] causes increasing upward B flux
B] causes decreasing upward B flux
C] causes decreasing downward B flux
D] causes increasing downward B flux
E] has no effect on the flux through the loop
A copper wire loop is shown.
Moving the loop up causes
decreasing downward flux.
To oppose this change (i.e. this decrease)
the field within the loop from the induced
current must point:
A] up
B] down
A copper wire loop is shown.
Moving the loop up causes
decreasing downward flux.
To oppose this change (i.e. this decrease)
the field within the loop from the induced
current must point down.
Thus, the induced current in the loop must
flow (seen from above):
A] CW
B] CCW
A copper wire loop is shown.
Moving the loop up causes
decreasing downward flux.
To oppose this change (i.e. this decrease)
the field within the loop from the induced
current must point down.
Thus, the induced current in the loop must
flow (seen from above) Clockwise, by
RHR.
More about motional EMF
A square loop is pulled through a constant B field.
What is the magnitude of the motional emf?
A] 0
B] vBL
C] 2vBL
D] vBL2
More about motional EMF
Although there is magnetic flux through the loop,
the amount is NOT changing with time. So emf = 0.
If the electric potential at b=0, what is the electric
potential at point a?
A] 0
B] vBL
C] 2vBL
D] vBL2
The magnetic field DOES act on mobile charges in the front
and trailing sides, pushing + charges up (or - charges down).
This continues for a few ns, until there is a sufficient excess
of + charges on side a to create an electric field that opposes
further charge motion. Answer B. V=EL=vBL.
This is just the “Hall Effect” !
Although there is a potential difference between a and
b, there is no emf around the loop…
An emf around a loop means “you can go downhill all
the way around, back to where you start!”
Here, the left side is not moving, so there is no magnetic force on the initially
stationary charges on that side. There is magnetic force on the charges on
the right side, pushing positive charges up.
Each charge acquires an energy = qvBL = force x distance.
That energy is then lost as the charge “slides downhill”, through the lightbulb,
heating it.
Where does the energy come from that lights the bulb?
It cannot come from the B-field, as that is unchanging while the bar is sliding.
Answer: you must use force to pull the right side at constant v.
How much force?
Ans: Power = Fv = I2R
More on Solenoids
Long solenoids have spatially uniform B
inside (from Ampere’s law)
If the current is increased linearly with
time, the B field will increase linearly with
time. In this case, the field is out of the
page (top view) and increasing with time.
If this is done, what will be the
direction of the induced E field at
point b, distance r from the axis?
On the top view:
A] up
B] down C] left
E] out of the page
D] right