Video Transcript
A conducting loop is moved so that
half of its area is within a uniform magnetic field, directed out of the plane of
the diagram shown along the axis of the loop. Then, the loop moves from being
partially within the magnetic field to being completely within the magnetic
field. Is the current through the loop
clockwise or counterclockwise?
The question asks what the
direction is of the current induced in the conductive loop due to the motion of the
loop into a uniform magnetic field. Initially, only half of the loop is
located within the magnetic field, but recall that the loop is moving. The motion of the loop in the
magnetic field results in a magnetic force acting on free electrons in the loop. The forces on the free electrons
act to produce a net motion of the free electrons around the loop.
The answer to the question depends
on which direction around the loop the free electrons move in, as the direction of
the current in the loop is opposite to the direction of the net flow of the free
electrons. When charged particles move in a
magnetic field, the direction of the induced current can be determined using
Fleming’s right-hand rule. Let us use this rule with the
loop.
The direction of the magnetic field
is out of the plane of the screen. We see then that when the direction
of the motion of the loop is toward the right, the direction of the induced current
is toward the bottom edge of the loop. Recall that the direction of motion
of free electrons is opposite to the direction of current. Free electrons are therefore pushed
toward the top edge of the loop.
When free electrons move toward the
top edge of the loop, the concentration of negative charge is greater at the top
edge of the loop and less at the bottom edge of the loop. The greater concentration of
negative charge at the top edge of the loop repels free electrons at the top edge of
the loop, which therefore move toward the left. The lesser concentration of
negative charge at the bottom edge of the loop attracts free electrons at the bottom
edge of the loop, which therefore move toward the right.
We see then that the net flow of
free electrons is directed counterclockwise. The direction of the current in the
loop is opposite to the direction of the net flow of free electrons. So the current induced in the loop
is clockwise.
