Magnetic Fields
Electromagnetic Spectrum
Lecture 4
Where was the compass first used?
The first compass, which uses a magnetic
needle, was used in China
The compass was shaped like a spoon and was usually made of lodestone or
magnetite.
N
S
Most materials are not magnetic.
A common mistake is to believe that metals are magnetic –
only a very few are.
The magnetic materials you need to know are :
• iron
• steel (which is nearly all iron)
• nickel
• cobalt
Like poles repel.
North and north.
South and south
N
S
S
N
S
N
N
S
Unlike poles attract.
North and South.
N
S
N
S
magnetic monopoles do not exist
If you take a bar magnet and break it into two pieces,
each piece will again have a North pole and a South pole.
N
S
N
N
S
S N
N
S N
electric monopoles exist
S
S N
S
Every magnet has a magnetic field around it. By convention, we say
that the magnetic field lines leave the North end of a magnet and
enter the South end of a magnet.
the Earth is a large magnet,
and one with a magnetic south pole near the Earth’s geographic north pole, and
one with a magnetic north pole near the Earth’s geographic south pole
Sometimes, the Earth’s magnetic poles flip.
This happens every half-million years or so.
Magnetic South Pole
Geographic North
Pole
Magnetic North
Pole
Geographic South Pole
Magnetic North Pole
Last reversal was 780,000 years ago.
Magnetic South
Pole
When the magnetic particles from the sun, called “solar wind”,
strike this magnetosphere, we see a phenomenon called aurora
Some animals can sense the earth’s magnetic field and use it for
navigational purposes.
This ability may be related to the presence in the lobsters of the mineral
magnetite, a magnetic material used for compass needles.
magnetotactic bacteria
If he held a magnet near the slide, the bacteria would move towards the
north end of the magnet. These bacteria are able to do this because
they make tiny, iron containing, magnetic particles. It uses this magnet
as a compass to align themselves to the earth's geomagnetic field.
Oersted discovered that electric current in a wire can
deflect a magnetized compass needle,
The direction of the magnetic field is given
by the Righthand Rule
current (I) in direction of thumb
curl fingers in direction of field (B)
When a current is run
through a cylindrical coil
of wire, a solenoid, it
produces a magnetic
field like the magnetic
field of a bar magnet.
The solenoid is known
as electromagnet.
How strong is magnetic field ?
Forces due to
Magnetic Field on
moving charges
(Lorentz Force)
We know that current consists of
moving charges!!
Magnetic field excerts force on
moving charged particles;
Magnetic force
FB
qv B
The magnitude of the magnetic force is
FB
q vB sin
If the index finger of the right hand points in
the direction of the velocity of the charge, and
the middle finger in the direction of the magnetic
field, the thumb indicates the direction of the
magnetic force acting on a positive charge.
Magnetic Fields
Magnetic field B:
SI unit of magnetic field: TESLA (T)
Magnetoencephalography (MEG)
is a non-invasive technique used to measure magnetic fields generated
by small intracellular electrical currents in neurons
MAGNETIC FORCE ACTING ON A
CURRENT-CARRYING CONDUCTOR
Magnetic force
FB
IL B
The magnitude of the magnetic force is
FB
BIL
How strong is magnetic field ?
Magnetic field due to a long, straight currentcarrying wire
Magnet Coils (Solenoid) The field is stronger inside ,
weak outside
To increase the strength of an electromagnet we can do three things :
Add more turns to the coil of wire
Wrap the coil around
a soft iron core
Use a bigger electric current
DC motor
A coil is pivoted to rotate
freely inside a magnetic field.
You put a current through
the coil, which feels a torque. When the
coil has turned by 90 ,
switch current direction, so that
it continues to turn (commutator)
Summary
Magnetic Field is generated by
permanent magnet
moving charges
Currents
Effect of Magnetic Field
Force on moving charges
Force on current
Almost 200 years ago, Faraday looked for evidence that a magnetic field
would induce an electric current
He found no evidence when the current was
steady, but did see a current induced when the
switch was turned on or off.
a changing magnetic field produces an
electric current in a loop surrounding the
field called electromagnetic induction, or
Faraday’s Law
DC motor
Electric Generator
Electromagnetic Spectrum
What is Electromagnetic Spectrum?
• The electromagnetic spectrum is the complete
spectrum or continuum of light including radio
waves, infrared, visible light, ultraviolet light, X-rays
and gamma rays
• An electromagnetic wave consists of electric and
magnetic fields which vibrates thus making waves.
Wavelength, Frequency and Energy
Since all electromagnetic waves travels at the same speed,
wavelength and frequency have an indirect relationship.
Electromagnetic wave with a short wavelength will have a high
frequency and electromagnetic wave with a long wavelength
will have a low frequency.
electromagnetic wave with a high frequency has high energy
and with low frequency low energy
Properties of waves include
•Speed (c)
• Frequency (f)
•Wavelength (
Speed (c), frequency (f) and wavelength ( ) are related in
the formula
xf=c
Electromagnetic Waves
• Speed in Vacuum
–300,000 km/sec
–3 108 m/s
• Speed in Other Materials
–Slower in Air, Water, Glass
Electromagnetic waves travel through empty space!
Radio waves
•Low frequency
•Used in many devices such as radio,
wireless devices, etc.
f ~ 100 MHz
λ~1m
Medical applications of radio waves
RADIO WAVE
Signal Transmission
MAGNETIC RESONANCE
The only radiation patients are exposed to
by nuclear magnetic resonance imaging is
very low energy radio waves.
Microwaves
• Longer than light and
infrared, shorter than radio
• First used in radar, now used
in communication, medicine
and consumer use
(microwave ovens)
f~ 1-10 GHz
λ~ 1-10 cm
Medical applications of microwaves
Physiotherapy
Microwave diathermy
The heat energy obtained from the wave is used for giving relief to the patient
Infrared waves
• Invisible electromagnetic waves
that are detected as heat
• Can be detected with special
devices such as night goggles
• Used in heat lamps
• Higher energy than microwaves
but lower than visible light
14
f~10
Hz
λ~ 10-300 m
38
Medical applications of infrared
images in thermography
The human body gives out
infrared radiationn that
can be detected by
sensors.
A pyroelectric thermometer
measures body temperature by determining the
amount of infrared radiation emitted by
the eardrum and surrounding tissue.
Visible Light
• The portion of the electromagnetic spectrum that
human eyes can detect
• red, orange, yellow, green, blue, violet
• Red is the lowest frequency and violet is the highest
frequency
15
f~10
Hz
λ~ 400-700 nm
Medical applications of visible
light
Visible light in optical fibres
for medical uses
UltraViolet
• waves with frequencies slightly
higher than visible light
•Most humans are aware of the effects of
UV through the painful condition of
sunburn.
•uses: food processing & hospitals to kill
germs’ cells, fluorescence Microscopy:
14
10
f~ 7.5 x
-3x
λ~ 10-400 nm
16
10 Hz
Medical applications of ultraviolet
Ultraviolet lamps are used to sterilize
workspaces and tools used in medical
facilities.
X-Rays
• High energy waves
• First discovered by
Roentgen
• Used in medicine, industry
and astronomy
• Can cause cancer
16
20
10 -10 Hz
f~
-8
-12
λ~ 10 -10 m
Medical applications of X rays
COMPUTED TOMOGRAPHY
PLAIN FILM RADIOGRAPHY
STOMACH
RADITION TREATMENT
CONTRAST RADIOGRAPHY
Gamma rays
• Highest energy
• Blocked from Earth’s surface by
atmosphere
20
>10 Hz
f
-12
λ<10 m
Medical applications of Gamma rays
It can be used to treat cancer. These
high energy rays are directed at
cancerous tumours to kill cancer cells.
Tracers Used in Nuclear Medicine –
the tracer is injected into the patient.
The radiation emitted from the patient
is detected using a gamma camera.
Sterilization - medical instruments
(e.g. syringes). Gamma rradiation not
only kills cells, it can also kill germs or
bacteria.