Transcript Magnetism - Cuero ISD

Magnetism
Pre-AP Physics
Chapter 20
Magnets and Magnetic Fields
• A magnet will attract objects containing
iron.
• A magnet has two poles, North and South.
• The end that points north on a compass is
truly the north-seeking pole.
• Magnets exert forces. Like poles repel
and opposite poles attract.
• Magnetic poles are not like electric
charges in the fact that electric charges
can be isolated; magnetic poles cannot.
• Only iron, cobalt, nickel, and gadolinium
show strong magnetic effects. They are
said to be ferromagnetic.
• There is a magnetic field, B, surrounding
the pole of a magnet. This field can be
represented by field lines just as electric
fields were represented by electric field
lines.
• The direction of a magnetic field is the
direction that the north pole of a compass
needle would point if placed in it. Thus
field lines will point away from north and
toward south.
•
• The Earth is a huge magnet. Its
geographic north pole is a south pole
magnetically and vice versa. It is called
the north magnetic pole.
• The Earth’s poles do not coincide with the
magnetic poles. There is a magnetic
declination, or angle, between the
magnetic north and the true north
directions. It varies from 0o to 25o
depending on the point of reference.
Electric Currents Produce
Magnetism
• In 1820, Oersted found that when a current flows
through a wire, a compass needle will be
deflected when placed near the wire.
• Electric current produces a magnetic field.
• A right-hand rule will help you remember the
direction of field lines when considering a
current-carrying wire.
• Grasp the wire in your right hand with the thumb
pointing in the direction of conventional current
flow. The fingers then point in the direction of
magnetic field.
I, current
This method also
works for a
circular currentcarrying wire.
• Another right hand rule helps determine
the directions of magnetic force,
conventional current and magnetic field.
• The right hand rule tells you the direction a
magnetic field (B) will move an already moving
charge.
• Your right hand needs to be open, with your thumb
pointing at 90 degrees to your fingers.
• The fingers point in the direction of the magnetic
field (which always points from the north pole of a
magnet to the south pole).
• The thumb points in the direction a positive charge
is moving. These charges could be a single charge,
a bunch of charges (known as current, I), or a wire
being pushed (a wire has charges in it, so moving a
wire moves the charges in the wire). All of these
examples are all v (thumb).
• The palm points in the direction the magnetic field
moves the charge. This is the Fmag, or magnetic
force. Fmag MUST BE the direction B moves the
charge (or wire) NOT an external force. A current
flowing in a wire, due to an external voltage
supply is not Fmag; it is the direction of v (moving
charges). However, the direction the current
carrying wire moves due to B IS Fmag (your
palm). Also, when a wire is moved into B, the
charges in the wire (q) move due to B, causing an
induced current in the wire (Fmag).
• Two notes: 1) If the charge isn't moving, there is
no Fmag. 2) The Right Hand Rule works only for
positive charges (protons and conventional
current, which flows from positive to negative). If
you are asked to find Fmag for an electron or
negative current, use your left hand.
Force on an Electric Current in a
Magnetic Field
• Since an electric current exerts a force on a
magnet field, then by Newton’s third law, a
magnet should exert a force on a currentcarrying wire.
• The electric force will be perpendicular to the
magnetic field direction and the current direction.
• F = I l B sinθ where I is current, l is the length
of wire in the field, B is the magnitude of the field
and θ is the angle between the current and the
magnetic field.
• Note: Maximum force is obtained when θ equals
90o and zero when θ equals 0o.
Units of magnetism
• The SI unit for magnetic field is tesla (T)
which is also known as a weber per meter
squared.
• Another unit is the gauss (G) which is
defined as 10-4 T = 1 G
Force on a moving electric charge
• F = qvB sin θ where q is the charge, v is
the velocity of the charge and B is the
magnetic field. Θ is the angle between v
and B. The force is greatest at θ = 90o.
• If the particle moves in a circular path,
r = mv/qB.