Download Magnetism

Electromagnets
April
Electricity vs. Magnetism
Electricity
+ and Electric field, E caused
by electric charges,
stationary or in motion
Like charges repel;
unlike charges attract
Electric charges can be
isolated
Magnetism
North and South
Magnetic field, B
caused by electric
charges in motion
Like poles repel; unlike
poles attract
Magnetic poles cannot
be isolated
Magnetic field lines
N
S
Draw magnetic field lines around a
magnet.
N
S
Outside the magnet: mfl move from the north
pole to the south pole.
Inside the magnet: mfl move from the south
pole to the north pole.
Electromagnets
• Electric current produces magnetic field. An
electromagnet is a current-carrying coil of
wire with many loops
The Right-Hand Rule
If the wire is grasped
in the right hand
with the thumb in
the direction of the
current, the fingers
will curl in the
direction of magnetic
field.
electromagnets
I
A currentcarrying coil of
wire with many
loops is an
electromagnet.
Magnetic Field due to the electric
current
Magnetic Field due to the electric
current
Magnetic Field inside a Solenoid
A coil of several closely spaced loops is called
a solenoid.
Demonstration
• Electromagnetism
• Materials list: D-cell battery, insulated wire,
nail, compass
• Use the compass to determine whether the
nail is magnetized.
• Next, flip the battery so that the direction of
the current is reversed.
• Again bring the compass toward the same
part of the nail.
symbols of magnetic field
electromagnets
A currentcarrying coil of
wire with many
loops is an
electromagnet.
Right-hand rule practice
Electric Field
Magnetic force (Fmagnetic)
• A charge moving through a magnetic field
experiences a force.
• Fmagnetic = qvB
q is the magnitude of charge. (C)
v is the speed of charge. (m/s)
B is the strength of magnetic field. Tesla (T)
1 T is the magnetic field strength in which a charge of 1 C experiences
1 N of force when it moves at the speed of 1 m/s.
direction of magnetic force
• To determine the direction of the
force, use the right-hand rule.
• Place your fingers in the direction of
B with your thumb pointing in the
direction of v.
• The magnetic force on a positive
charge is directed out of the palm of
your hand.
Right-hand rule practice
+
+
+
-
A charge moving
through a magnetic
field follows a circular
path.
practice
• An electron moving north at 4.5 x 104 m/s
enters a 1.0 mT magnetic filed pointed
upward.
• a. what is the magnitude and direction of the
force exerted on the electron?
• b. what would the force be if the particle were
a proton?
• c. what would the force be if the particle were
a neutron?
answers
• a. 7.2 x 10-18 N west
• b. 7.2 x 10-18 N east
• c. 0.0 N
Practice
• A proton moves perpendicularly to a magnetic
field that has a magnitude of 4.20 x 10-2 T.
What is the speed of the particle if the
magnitude of the magnetic force on it is 2.40 x
10-14 N?
• 3.57 x 106 m/s
Force on a current-carrying
conductor perpendicular to
a magnetic field is
proportional to magnetic
field, current and length of
conductor.
F magnetic = BIl
B is the strength of magnetic field. Tesla (T)
I is the current. (A)
l is the length of the wire (m)
Practice
• A 4.5 m wire carries a current of 12.5 A from
north to south. If the magnetic force on the
wire due to a uniform magnetic field is 1.1 x
103 N downward, what is the magnitude and
direction of the magnetic field?
• 2.0 x 101 T, to the west
Homework
• Pages 782 and 783
• Questions 26, 30, 33, 34 and 41.