Download Magnets - HRSBSTAFF Home Page

Electricity and Magnetism
Physics 12
16.1 in textbook
Magnetic Force
• Magnetic forces have properties that are
similar to electric and gravitational force in
that:
– The force varies following an inverse square
relationship due to the separation of poles
– The force varies directly with the strength of the
poles
• However, since we don’t have magnetic
monopoles there is no equation for
magnetic force
Magnetic Forces are…
• Similar to electric forces because:
– Like poles repel and unlike poles attract
– Both are strong forces (unlike weak
gravitational force)
• Different to electric forces because:
– Can’t have a monopole (like a single charge)
– Magnets don’t discharge when touched (like
conductive charging)
Magnetic Field Lines
• It is possible to map a magnetic field in a
manner similar to that of electric or
gravitational fields
Magnetic Field Lines
• Arrows go into the South poles and away
from North poles
• Lines are perpendicular to poles
• Increase in lines means increase in pole
strength
What happens if you break a
magnet in half?
• You will get 2 magnets with North and
South poles
• You always have to have dipoles; never
single poles (monopole)
Common Magnets
If the Earth had a North
“North pole” this is what
would happen with a
magnet.
• Compass needle
(points towards Earth’s
North magnetic pole)
• ***Note: What we
consider the “North Pole”
in the Arctic is really a
South pole… explain!
• North poles repel North
poles so if the compass
needle points there it
must be South!
Magnetic Materials
• Materials that are strongly magnetic:
- they can be turned into magnets
– called ferromagnetic materials (iron, steel,
nickel, cobalt)
• Materials that are not strongly magnetic
can show signs of magnetism but they are
small and usually unnoticeable.
Domain Theory
• On a small scale, ferromagnetic materials are actually
made up of tiny regions known as domains. Each
domain behaves like a tiny magnet with a North and
South pole.
• In an unmagnetized piece of iron, these domains are
arranged randomly pointing in all directions. The
magnetic effects end up cancelling each other out.
Magnetic Domains
• If the domains are aligned, the material will
be magnetic
Diamagnetism
• All materials are diamagnetic in nature.
Diamagnetism is a very weak form of
magnetism, exhibited only in the presence of
a very strong external magnetic field. It is
difficult to observe in everyday life.
• Materials like ferromagnetic materials have
magnetic properties that overshadow the
diamagnetic ones.
• Diamagnetic is used to refer to non-magnetic
materials like water, wood, plastic, etc.
Magnetic Domains
• Some materials will easily align the domains in the
presence of a magnetic field; however they will
usually return to a random arrangement after the
field is removed; these are called temporary
magnets (like iron) – kind of like induction for
charges
• Other materials will not easily align the domains,
however once aligned they will remain aligned;
these are called permanent magnets (like steel)
• Even permanent magnets can “demagnetize” if
heated above a certain point (Curie point – see
page 755)
• If you heat a magnet and then “freeze” it by cooling
it to room temperature, you get a more permanent
magnet.
Electric Charges and Magnetic
Poles
• In 1819, Hans Oersted noticed that a current
carrying wire (moving charged particles or
electrons) would create a magnetic field
– when a compass needle was near a current the
compass needle deflected
• He also noticed that static charges did not create
this.
• This was the first step into the realm of
electromagnetism.
Magnetic Field around a Current
• It was observed that a compass needle placed
near a straight current carrying wire will align
itself so that it is perpendicular to the wire.
Magnetic field lines are actually CIRCLES
around the wire. The direction of the magnetic
field can be found using the first right hand
thumb rule.
• The idea that all magnetic fields are a result of
electric currents supports the idea that North and
South poles must always exist in pairs since an
electric current will always produce both.
Right Hand Thumb Rule 1
-
Used to find polarity in a straight wire or
single coil
Steps: a) Point your thumb in the direction of the
current (positive flow).
• Curl your fingers
–
(as if making a fist)
• Fingers will point in
the direction of the
magnetic field
Right Hand Thumb Rule
Example 1
• The picture below shows a cross-sectional
view of a wire carrying a current away
from you (into the page – note the x).
Sketch the magnetic field around the wire.
Solution
• Using the first RHT rule, make a slightly
open fist with your thumb pointed outward.
Point your thumb into the page (note the
x). Your fingers will curl around
the wire in a clockwise
pattern, shown below.
Example 2 – Draw in the magnetic
field (is it into or out of the page?)
Magnetic Field of a CurrentCarrying Coil
• If you take a straight wire (like in previous example) and
form a single loop, the first RHT rule can be applied to
show that the field inside the loop is the same direction
everywhere. The field outside the loop is in the opposite
direction.
Magnetic Field in a Coil
• We can use RHT
Rule 1 to determine
the magnetic field in a
coil of current carrying
wire
• We end up with a
strong magnetic field
inside the soil of wire
Coil
• The field here is into the page inside the loop
and out of the page outside the loop.
• Note: The field lines are more concentrated
inside the loop… so the field is stronger here.
• Think about how the field should get weaker
the further from the current (distance
increases, field intensity decreases).
• Inside the loop, the field is uniform unlike
outside the loop.
Worksheet
• Check Your Understanding 1, 3
• Question 1
Solenoid
• Solenoid: A coil of wire containing many
loops.
• The solenoid behaves as a magnet, with a
North pole at one end and South pole at
the other.
• This is called an ELECTROMAGNET.
Solenoid
• To increase the strength of the
electromagnet:
– add more loops
– increase the current
– use a ferromagnetic core inside the coil (so
the domain is aligned)
Solenoid
Second Right Hand Thumb Rule
• Used to find polarity in a solenoid
• Steps:
a) wrap your fingers around the coil in the
direction of the + current flow
b) your thumb will point toward the North pole
(or the direction of the magnetic fields INSIDE
the coil)
Notes
• Field lines are always CLOSED loops
• Field lines go from North to South poles
Example 1 – Which pole is
North?
Example 2
Example 3
Which compasses are pointing in the
wrong direction?
Worksheet
• Remaining questions
• Check: 2
• Question: 2, 4
Practice Problems
• Section Review
– Page 767 question 2 (read pages 766 – 767)