Transcript Magnetic_Force

Magnetism
and its applications
Laws of Magnetism 1) Like magnetic poles repel,
and 2) unlike poles attract.
Magnetic Direction and Strength
• Law 3 - Magnetic force, either attractive or
repelling varies inversely as the square of the
distance between the poles. For example, if the
distance between two magnets with like poles is
increased to twice the distance, the repulsive force
reduces to one-quarter of its former value.
• The direction of any magnetic field is defined as
the direction the north pole of a magnet would
point if placed in the field.
• The magnetic field of a magnet points from the
north pole of the magnet to the south pole.
Earth’s Magnetic
Field
The magnetic north
pole of Earth
corresponds to the
geographic South
Pole, and the
magnetic south pole
corresponds to the
geographic North
Pole.
The Magnetic
Field
• A magnetic field exists
around any current-carrying
wire; the direction of the
magnetic field follows a
circular path around the wire.
• The Danish physicist, Hans
Christian Oersted,
discovered that a magnetic
field existed around a
conductor carrying an electric
current.
Magnetic Force
• The direction of the force on a positive
charge moving through a magnetic field
can be found using the right-hand rule.
Magnetic Force
Example - A proton moving east experiences a
force of 8.8 x 10-19 N upward due to the Earth’s
magnetic field. At this location, the field has a
magnitude of 5.5 x 10-5 T to the north. Find the
speed of the particle.
You Try!
• 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?
(1q = 1.6 x 10-19C)
• A proton traveling to the right along the x-axis
enters a region where there is a magnetic field
of magnitude 2.5 T directed upward along the yaxis. If the proton experiences a force of 3.2 x
10-12 N, find the speed of the proton.
Answers
Attraction and
Repulsion
• Two parallel current-carrying
wires exert on one another
forces that are equal in
magnitude and opposite in
direction. If the currents are in
the same direction, the two
wires attract one another. If
the currents are in opposite
directions, the wires repel one
another.
Electromagnetic Induction
• EMF – Voltage from induction (electromotive force)
• The magnitude of the induced emf depends on the velocity with
which the wire is moved through the magnetic field, the length of the
wire, and the strength of the magnetic field.
• Every time the bar crosses a line of force a current is induced.
increases in strength. The induced current in the coil must be in a
direction that produces a magnetic field that opposes the increasing
strength of the approaching field. The induced magnetic field is
therefore in the direction opposite that of the approaching magnetic
field.
• Lenz’s Law - The magnetic field of the induced current opposes the
change in the applied magnetic field.
• Faraday’s Law - If a circuit contains a number, N, of tightly wound
loops, the average induced emf is simply N times the induced emf
for a single loop.
Induction
Example: A coil with 25 turns of wire is wrapped
around a hollow tube with an area of 1.8 m2. Each
turn has the same area as the tube. A uniform
magnetic field is applied at a right angle to the
plane of the coil. If the field increases uniformly
from 0.00 T to 0.55 Tin 0.85 s, find the magnitude
of the induced emf in the coil. If the resistance in
the coil is 2.5 ohms, find the magnitude of the
induced current in the coil.
Practice – You Try!
• A single circular loop with a radius of 22 cm is
placed in a uniform external magnetic field with
a strength of 0.50 T so that the plane of the coil
is perpendicular to the field. The coil is pulled
steadily out of the field in 0.25 s. Find the
average induced emf during this interval.
• A coil with 205 turns of wire, a total resistance of
23 ohms, and a cross- sectional area of 0.25 m2
is positioned with its plane perpendicular to the
field of a powerful electromagnet. What average
current is induced in the coil during the 0.25 s
that the magnetic field drops from 1.6 T to 0.0 T?
Answers
Solenoid and Electromagnetic Coils
• The magnetic field created by a solenoid
or coil is similar to the magnetic field of a
permanent magnet.
Magnetic Domains
• A domain is a group of atoms whose
magnetic fields are aligned.
• Hard disks and magnetic tape lay down
domains as a way to store data. Unmagnetized material has domains that are
randomly oriented.
Applications Electromagnet.
THE RELAY
The relay is a device used to control a large
flow of current by means of a low voltage low
current circuit. It is a magnetic switch. When
the coil is magnetized, its attractive force pulls
the lever arm, called an ARMATURE, toward
the coil. The contact points are touched and
the large current flows through.
Electric Motor
•A MOTOR is a device which
converts mechanical energy
into electrical energy.
•EMF – Electromotive Force
Electric Generator
•In 1831 another great scientist, Michael
Faraday, pondered this question: If
electricity would produce magnetism;
can magnetism produce electricity?
Based on the research and discoveries
of Mr. Faraday, the electric dynamo was
developed. He is known as the “father of
the dynamo.”
•In order to produce an electric current,
there must be a magnetic field, a
conductor and RELATIVE MOTION
BETWEEN THE FIELD AND THE
CONDUCTOR. A GENERATOR is a
device which converts mechanical
energy into electrical energy.