Transcript Lecture30

READING QUIZ
Electric currents create magnetic fields,
but magnetic fields cannot create
electric currents.
1. False
2. True
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Chapters 10,11,12,13,14,15
FROM LAST TIME - - 1. Permanent magnets have opposite poles: like poles repel,
opposites attract (North, South).
2. Lines of force exist around magnets (field lines).
1.
A current in a wire causes a magnetic field
outside of the wire.
Two parallel current-carrying wires exert
an attractive force on each other when the
two currents are in the same direction.
The magnetic force exerted on the moving charges
of an electric current is perpendicular to both the
velocity of the charges and to the magnetic field.
Use the right hand rule VxB to fine the direction of the Force
Use the right hand rule VxB to fine the
direction of the Force
If the index finger of the right hand points in
the direction of the velocity of the charge, and
the middle finger in the direction of the magnetic
field, the thumb indicates the direction of the
magnetic force acting on a positive charge.
The forces on each segment of a current-carrying
rectangular loop of wire combine to produce a
torque that tends to rotate the coil until its plane
is perpendicular to the external magnetic field.
The magnetic flux through the loop of wire has
its maximum value when the field lines are
perpendicular to the plane of the loop. It is
zero when the field lines are parallel to the
plane of the loop and do not cross the plane.
Force per unit length on parallel wires:
F 2k'I1I 2

l
r
K = 1x10-7 N/A2
Force on a moving charge: F = qvB,
where F is perpendicular to both v and B.
Force on a wire with current I in a perpendicular B field:
F = I LB
When a current-carrying wire is bent into a
circular loop, the magnetic fields produced
by different segments of the wire add to produce
a strong field near the center of the loop.
A current-carrying coil of wire produces a magnetic
field greater than a single loop and is proportional
in strength to the number of loops in the coil.
The two most important facts about magnets:
1. Moving a coil of wire near a magnet can cause a
current to flow in the wire.
2. Moving a magnet near a coil of wire can cause a
current to flow in the wire.
(Faraday - magnetic induction)
A magnet moved in or out of a helical coil of
wire produces an electric current in the coil.
Faraday’s Law:
Induced voltage depends on the rate of change of
enclosed magnetic flux
E = DF/t
Note: Induced voltages can cause currents to flow in a
circuit. Hence, magnetism can create electricity!
LENZ’s Law
The induced current in a loop of wire produces an a
magnetic field inside the loop that opposes the change in
the field producing the change.
Transformers
DF d

 (BA) ONE COIL
Dt dt
V1 (total) = N1 
V2 (total)  N 2 
V1 V2

N1 N 2
