Magnetism

1. MAGNETOSTATICS

2. MAGNETISM
The study of magnetic fields and their effect on materials, due
to unbalanced spin of electrons in atom.
It is readily observed every day – from the simple magnet that
attracts nails and other metals to cassette tapes to magnet-
driven trains.

3. APPLICATIONS OF MAGNETISM
Magnetism is one of the most important fields in physics. Large
electromagnets are used to pick up heavy loads.
Magnets are used in such devices as pumps, motors, and
loudspeakers.
Magnetic tapes and disks are used routinely in sound-and video-
recording equipment and to store computer data.
Intense magnetic fields are used in magnetic resonance imaging
(MRI) devices to explore the human body with better resolution
and greater safety than x-rays can provide.

4. Engineering of technology like iPods would not be possible without a
deep understanding magnetism. (credit: Jesse S., Flickr)

5. An electromagnet induces regions
of permanent magnetism on a
floppy disk coated with a
ferromagnetic material.
The information stored here is
digital (a region is either magnetic
or not); in other applications, it
can be analog (with a varying
strength), such as on audiotapes.

6. Giant superconducting magnet are used in the
cyclotrons that guide particles into targets at nearly the
speed of light. Magnetism is closely linked with
electricity.
Magnetic fields affect moving charges, and moving
charges produce magnetic fields.
Changing magnetic field can even create electric fields.
The ultimate source of any magnetic field is electric
current.

7. The Fermilab facility in Illinois has a large particle accelerator (the most
powerful in the world until 2008) that employs magnetic fields (magnets
seen here in orange) to contain and direct its beam. This and other
accelerators have been in use for several decades and have allowed us to
discover some of the laws underlying all matter.

8. NATURE OF MAGNETS
In the ancient country of Lydia, in Western Asia Minor, now Turkey, was
a city called Magnesia. The Greeks discovered that certain iron ores
found in the place could attract other pieces or iron, they called it
magnetites.
Magnetites are classified as natural magnet.

9. It is now believed that magnetism is due to the spin of electrons within
the atoms. Since the electron is a charged particle, the concept implies
that magnetism is a property of a charged particle in motion.

10. The power of attraction of a magnet depends on the
arrangement of the atoms.
All atoms are in themselves tiny magnet formed into groups
called DOMAINS.
The magnetic strength is increased if the domains are induced
to fall into line by the action of another magnet.

11. General Properties of Magnet
1. Magnets usually have two poles.
The end of the magnet which points north when magnet is
free to turn on a vertical axis is the north-seeking pole, simply
the N pole.

12. The opposite end which points south is the south-seeking pole
or S pole.
Magnets come in many shapes and sizes, but each has at least
two poles.
If you cut a magnet into pieces, every piece will still have at
least two poles.

13. North and south poles always
occur in pairs. Attempts to
separate them result in more
pairs of poles.
If we continue to split the
magnet, we will eventually get
down to an iron atom with a
north pole and a south pole—
these, too, cannot be separated.

14. 2. Like Magnetic poles repel and unlike poles attract

15. 3. Permanent magnets are magnets made from alloys of
cobalt and nickel. These magnets retain their magnetism for a
long time.

16. 4. Other metals like iron can be magnetized by Induction.
When a piece of iron nails touches a permanent magnet, the
nails becomes a magnet. It retains in this condition for as long
as it is within the magnetic field. The nail is a temporary
magnet and its magnetism is described as induced
magnetism.

17. MAGNETIC FIELDS
10 ⁻⁹ to 10⁻⁸ gauss – the magnetic field of the human brain
0.25–0.60 gauss – the Earth's magnetic field at its surface
25 gauss – the Earth's magnetic field in its core
50 gauss – a typical refrigerator magnet
100 gauss – an iron magnet
1500 gauss - within a sun spot
600–70,000 gauss – a medical MRI machine

18. Earth’s Magnetic Field

19. Earth’s Magnetic Field
The strength of the magnetic field at the Earth's surface at this time
ranges from less than 30 microtesla (0.3 gauss) in an area including
most of South America and South Africa to over 60 microtesla (0.6
gauss) around the magnetic poles in northern Canada and south of
Australia, and in part of Siberia.
The Earth's core, however, is hotter than 1043 K, the temperature at
which the orientations of electron orbits within iron become
randomized. Therefore the Earth's magnetic field is not caused by
magnetised iron deposits, but mostly by electric currents (known as
telluric currents).

20. The magnetic field pattern of Earth is similar to the pattern that would
be set up by a bar magnet placed at its center. An interesting fact
concerning Earth’s magnetic field is that its direction reverses every few
million years.

21. Electric current is the source of all magnetism.
(a) In the planetary model of the atom, an electron orbits a nucleus,
forming a closed-current loop and producing a magnetic field. (b) Electrons
have spin (pictured as rotating charge) forming a current that produces a
magnetic field.

22. (a)The B field of a circular current loop is similar to that of a bar magnet.
(b) A long, straight wire creates a B field with field lines forming circular loops.
(c) When the wire is in the plane of the paper, the B field is perpendicular to
the paper.

23. Magnetic Field of Force
The SI unit of magnetic field is the tesla (T), also called the weber (Wb)
per square meter (1 T = 1 Wb/m2).
The unit of B is: B = T = Wb/m2 = N/C.m/s = N/A.m
The cgs unit of magnetic field is the gauss (G).
1 T = 10⁴ G

24. Properties of Magnetic Field Lines
1. The direction of the magnetic field is tangent to the field line at any
point in space. A small compass will point in the direction of the field
line.
2. The strength of the field is proportional to the closeness of the lines. It is
exactly proportional to the number of lines per unit area perpendicular
to the lines (called the areal density).
3. Magnetic field lines can never cross, meaning that the field is unique at
any point in space.
4. Magnetic field lines are continuous, forming closed loops without
beginning or end. They go from the north pole to the south pole.

25. Magnetic fields exert forces on moving charges
The magnitude of the magnetic force F on a charge q moving at
a speed v in a magnetic field of strength B is given by
where θ is the angle between the directions of v and B. This
force is called the Lorentz force. This is how we define the
magnetic field strength B —in terms of the force on a charged
particle moving in a magnetic field. The SI unit for magnetic field
strength B is called the tesla (T).

26. Magnetic fields exert forces on
moving charges. This force is one
of the most basic known.
The direction of the magnetic
force on a moving charge is
perpendicular to the plane formed
by v and B and follows right hand
rule–1 (RHR-1). The magnitude of
the force is proportional to q , v , B
and the sine of the angle between
v and B.

27. Charges and Magnets
There is no magnetic force on static charges. However, there is
a magnetic force on moving charges. When charges are
stationary, their electric fields do not affect magnets.
But, when charges move, they produce magnetic fields that
exert forces on other magnets. When there is relative motion,
a connection between electric and magnetic fields emerges—
each affects the other.

29. Magnetic Force on a Current-Carrying Conductors
Magnetic force on current-carrying
conductors is used to convert electric
energy to work. Motors are the most
common application of magnetic force
on current-carrying wires.
Motors have loops of wire in a
magnetic field. When current is passed
through the loops, the magnetic field
exerts torque on the loops, which
rotates a shaft. Electrical energy is
converted to mechanical work in the
process.

31. The torque found in the preceding example is the maximum. As the coil
rotates, the torque decreases to zero at θ = 0 . The torque then reverses its
direction once the coil rotates past θ = 0 . This means that, unless we do
something, the coil will oscillate back and forth about equilibrium at θ = 0 .
To get the coil to continue rotating in the same direction, we can reverse the
current as it passes through θ = 0 with automatic switches called brushes.

32. Meters are very similar to motors but only
rotate through a part of a revolution. The
magnetic poles of this meter are shaped
to keep the component of B perpendicular
to the loop constant, so that the torque
does not depend on θ and the deflection
against the return spring is proportional
only to the current I .

33. The magnetic field strength (magnitude) produced
by a long straight current-carrying wire, where I is
the current, r is the shortest distance to the wire,
and the constant μ0 = 4π × 10−7T ⋅ m/A is the
permeability of free space. Since the wire is very
long, the magnitude of the field depends only on
distance from the wire r , not on position along the
wire.
Field lines form circular loops centered on the wire.
Right Hand Rule 2 states that, if the right hand
thumb points in the direction of the current, the
fingers curl in the direction of the field.

34. Ampere’s Law and Others
Each segment of current produces a magnetic field like that of a long
straight wire, and the total field of any shape current is the vector sum
of the fields due to each segment.
The formal statement of the direction and magnitude of the field due
to each segment is called the Biot-Savart law. Integral calculus is
needed to sum the field for an arbitrary shape current. This results in a
more complete law, called Ampere’s law, which relates magnetic field
and current.
Ampere’s law in turn is a part of Maxwell’s equations, which give a
complete theory of all electromagnetic phenomena.

35. A solenoid is a long coil of wire (with many turns or loops, as opposed
to a flat loop). Because of its shape, the field inside a solenoid can be
very uniform, and also very strong. The field just outside the coils is
nearly zero.
(a) RHR-2 gives the direction of the magnetic field inside and outside a
current-carrying loop.
(b) Detailed mapping shows the B field is similar to a bar magnet.

36. (a)Because of its shape, the field inside a solenoid of length l is remarkably
uniform in magnitude and direction, as indicated by the straight and
uniformly spaced field lines. The field outside the coils is nearly zero.
(b) This cutaway shows the magnetic field generated by the current in the
solenoid.
The magnetic field
strength inside a
solenoid is
B = μ0𝑛𝑙

37. A toroid is often used to create an almost uniform magnetic field in
some enclosed area. The device consists of a conducting wire wrapped
around a ring (a torus) made of a non-conducting material.
The first toroid was invented in 1830 by the physicist Michael Faraday.
He noticed that the change in the magnetic field resulted in the voltage
in a wire. This phenomenon is known as Faraday’s law of induction.
Toroid inductors are used in medical devices, telecommunications,
musical instruments, industrial controls, refrigeration equipment,
ballasts, electronic clutches, electronic brakes, in the aerospace &
nuclear fields, in air conditioner equipment and in amplifiers.

38. The solenoid is
cylindrical, while the
shape of the Toroid is
circular, just like a
doughnut.
The magnetic field
generated in the
solenoid is outside it,
while the magnetic
field generated in the
Toroid is inside the
core.

40. THANK YOU!