Download Direction of Field Symbol

Chapter 20 Magnetism
Magnets
 The ends of a bar magnet are called poles
 Like poles repel and unlike poles attract
 Regardless of their shape, all magnets have a north and south
pole
Magnetic Fields
 Magnetic Field lines point from the north pole to the south pole
of the magnet
 The north pole of a compass needle always points in the direction of
the field (from North to South)
Magnetic Field of the Earth
 The Earth’s geographic North pole is actually the magnetic
south pole
 The north pole of a compass points towards geographic north
and since opposites attract, we know that the Earth’s geographic
pole is magnetic south
Magnetic Force
 A charge moving through a magnetic field experiences a force
Fmagnetic  qvB sin 
q= magnitude of charge
v= speed of charge
B= Strength of the magnetic field (measured in Tesla, T)
θ= angle between v and B (F=0 if θ=0)
A second Right-Hand Rule
 Of course, force is a vector!
 To find the direction of the magnetic force use another right
hand rule
 Fingers point in direction of the field
 Thumb points in direction of v
 Palm points in direction of magnetic force
Conventions for
direction
of
field
WARNING: The right
Direction
of Field
Symbol
Into the page
X
Out of the
page
hand rule is for the
direction of the force
acting on a POSITIVE
CHARGE.
To find the direction of
the force acting on a
negative charge, you’ll
have to use the rule and
change the sign!
Examples
Direction of F
Direction of v
Direction of B
Sign of Charge
Out of the page
East
North
+
Into the page
East
North
-
Out of the page
West
South
+
Into the page
West
South
-
South
West
Into the page
+
South
West
Out of the page
-
East
North
Out of the page
+
South
Out of the page
East
-
Out of the page
South
West
-
Into the page
west
North
+
Path of a charge in a magnetic field
 The path of a charged particle moving perpendicular to a
magnetic field is a circle (p.595)
 The magnetic force acting on the particle acts like the
centripetal force
mv
Fmagnetic  qvB 
r
2
Magnetic Field of a wire
 Moving charges produce magnetic
fields
 If there is a current moving through
a wire, a magnetic field is produced
around the wire
o I
B
2r
 I is current, r is perpendicular to
wire
 µo=4π x 10-7 Tm/A
Magnetic Field of a wire
 The “Right Hand Rule” for the magnetic field
 Point your thumb in the direction of the current and curl
your fingers in the direction of the field
Force on a current carrying wire
 A magnet exerts a force on a current-carrying wire
Fmagnetic  IlB sin 
 I= current
 l= length of wire
 B= magnitude of magnetic field
 Θ is the angle between the direction of current and the
magnetic field
 If current is parallel to B, F=0 (F=0 if θ=0)
The Right-Hand Rule revised
 Of course, force is a vector!
 To find the direction of the magnetic force use another right
hand rule
 Fingers point in direction of the field
 Thumb points in direction of I
 Palm points in direction of magnetic force
I
Force between two current carrying
wires
 Two current-carrying wires exert a force on each other
 If the currents are moving in the same direction the wires attract each
other
 If the currents are moving in opposite directions, the wires repel

 7 Tm 
 2.0 x10
 I1 I 2 l
 o I1 I 2 l 
A
F

2L
L




2.0x10-7 Tm/A= µo/2π
I= current
l= length of wire
L= distance between wires
Electromagnetic Induction (Ch 20)
 Michael Faraday discovered
the phenomenon of
electromagnetic induction
 A changing magnetic field
can produce an electric
current (induced current)
 B must be changing for this
to work
Moving a magnet through
a coil of wire produces a
current
Magnetic Flux
 Magnetic Flux is
proportional to the number
of field lines passing through
some area
 The angle θ is the angle
between B and a line drawn
perpendicular to the surface
Magnetic Flux  Φ B  BAcosθ
 If θ is 90, no lines pass
through the area, so flux is 0
 Unit for flux is the Weber
(1Wb= 1 Tm2 )
Faraday’s Law of Induction
 Recall what electromagnetic induction is. A changing
magnetic field induces a current
 Faraday’s Law mathematically:
 B
Induced Emf (Voltage)   N
t
 N represents the number of loops in the wire
 ΔΦB is the change in magnetic flux
Lenz’s Law
 B
Induced Emf (Voltage)   N
t
 The negative sign indicates that the induced current’s
magnetic field is always opposite to the original change in
flux
 Changing flux induces an emf, which induces a current
 That current then produces its own magnetic field
 That magnetic field points in the opposite direction of the
change in flux
Lenz’s Law
Direction of the magnetic field produced by the induced
current?
 A. Down
 B. Up
More Practice with Lenz’s Law
In which direction is the current induced in the coil for each situation?
a. Current produced will be counterclockwise to produce a field that points out
of the page
b. The area decreases, so flux decreases. Current will be clockwise to produce
A field that points into the page
c. Initially flux is out of the page. Moving the coil means the flux decreases.
Induced current will be counterclockwise to produce a field out of the pge
d. Field lines and surface are parallel so there is no flux, so no current is induced
e. Flux will increase to the left so the current will be counterclockwise to produce a
Field to the right
Induced EMF for moving conductor
 What if the magnet is stationary and the wire is moved
instead?
 This is called motional emf
Induced Emf (Voltage)  Blv
 B= magnetic field
 l= length of wire
 v= speed
Sample Problem p. 655 #15
 B= 0.450 T
 R= 0.230 Ω
 v= 3.40 m/s
Calculate the force required to pull the loop from
the field at a constant velocity of 3.4 m/s
How do we get force?
Fmagnetic  IlB
 We have l, B what’s I?
 Ohm’s Law I= V/R.
 We have R…what’s V??
 Law of induction!:
Induced Emf (Voltage)  Blv
 V=Blv=0.5355 V
 I=V/R=0.5355V/0.230 Ω= 2.33 A
 F=IlB= (2.33A)(0.350m)(0.450 T)= 0.367 N