Electro-Magnetic Radiation
Intro to Electro Magnetic Waves
(or electromagnetic radiation)
(or EMR)
• Electro-Magnetic Radiation is an interesting subject in physics in part due
to the fact that we interact with EMR (electromagnetic Radiation) or
technology that makes use of our understanding of electromagnetic
Radiation to enhance our lives.
• EMR is also amazing because it is responsible for one of the integral
senses that makes up the human experience, sight! Our sense of sight,
and the beautiful multi-colored world we live in is based on our bodies
ability to make sense of a small section of EMR.
• The amount of information contained in the electromagnetic
waves/radiation that surrounds us, is astounding, and by studying it we
discovered things completely out side of human sensory perception.
Electromagnetic Waves
*In a Vacuum…
Maxwell-Faraday Equation:
Changing Magnetic field (or acceleration of a
magnetic object) => Electric Field
Electromagnetic Waves
Continued…
Maxwell-Ampere’s equation:
*approximate form
Changing Electric field ( or acceleration of a
charged object) => Magnetic field
Now what do we know?
We know that a changing electric field creates
magnetic field. We also know that a changing
magnetic field will create an electric field.
What does this mean?
It means that an alternating magnetic field OR
electric field will create self propagating
electromagnetic waves!
How does this wave behave?
Wave length=the distance over which the wave completes a
cycle.
Frequency= the number of wave lengths which pass a point per
second.
This gives a relationship between frequency and wave length:
Velocity=frequency*wavelength
Maxwell’s equations however, restrict the value of velocity to a
single value
called C, which is equal to 3*10^8 m/s
Electromagnetic Spectrum
The Particle Model of Light
You have all heard the word photon right?
It refers to the smallest piece of energy in
radiation form that can be emitted or absorbed
at a specific frequency, and strangely enough
Photons behave like particles, with no mass, a
Constant speed of C. The energy of a photon is
proportional to its frequency. In fact everything
Can be described as a wave. Google wave particle
duality if you get the chance.
How is EMR generated?
• Electro/magnetic waves/radiation can be
emitted by oscillating charges.
• Quantum and Nuclear processes (responsible
for most of the high energy radiation in our
universe) *missing mass problems and
discrete energy levels
However, we are going focus on the EMR generated by
oscillations of charged particles because… you are already
familiar with charged particles!
Where do we come into contact with
EMR?
• No brainers!
They are in the stinking names on the spectrum….
• Think a littler harder…
the heat you feel from fires, sun burns, MRI’s,
Night vision… your ideas…
• Ok impress me…
Sterilization of produce (gamma/UV rays),
determining the age of the universe (no big deal…),
chemo-therapy, setting clocks, precise measurement
Zooming in on Thermal Radiation
We are all familiar with the particle model of
matter right? Right. In case you are not,
everything that has mass is composed of, or is a
small charged particle. We also know that these
Particles are in constant motion (assuming that
the object is not at absolute zero temperature.
What does this mean… hmmm…..
ALL MATTER EMITS RADIATION!
*and we call it thermal radiation
-Thermal radiation is the electromagnetic
radiation emitted by matter, because of all the
oscillating charged particles that are its
components.
Some review…
Temperature=Measurement of the average
kinetic energy of the particles/molecules in
an object.
Heat=A form of energy transfer by changing
kinetic energy of the particles of objects.
Absolute zero= 0 degrees Kelvin
Kelvin= Celcious+273
So back to thermal Radiation…
• We know that temperature is a measurement
of how vigorously a particle oscillates…
therefore we should see higher quantities of
and greater frequencies of electromagnetic
radiation.
• So it should be possible to relate temperature
to the amount and frequency of radiation
emitted by an object… and luckily somebody
else did all the work for us!
Stefan-Boltzmann Law
The Stefan-Boltzmann Law has some pretty cool applications. It
gives us a relation between the total energy in electromagnetic
radiation emitted per unit time, the surface temperature of an object
in Kelvin, its surface area, the boltzmann constant and the objects
emissivity.
How could we then measure the temperature of an object?
What would we need?
Black Body Spectrum
A few minutes of thinking…
• You are a an engineer working for me If I gave
you a sensor that can get a reading for
temperature and a computer that can take
that reading and inform other pieces of
machinery what to do, can you think of some
possible applications of our understanding of
EMR?
Quick Problem
• Lets calculate the energy loss do to radiation
of a human body. (oh yeah! IR sensors at
border crossings are used to check for human
trafficking)
• P=A*e*(boltz)*T^4
approximate!
What Google says…
• Google says approximate surface area is
1.8m^2
• Google says an average temperature for the
surface of a human body is 306K.
• Google says the emissivity of human flesh is
.97
• Therefore, Google says that P=870W
Does that seem right? If not what are we missing?
My Demonstration
I know you all believe that this EMR stuff exists,
but I believe we take the application of our
knowledge of EMR for granted. I hope this quick
Demo will leave you with an appreciation of
sciences ability to translate information from a
medium outside of human sensory perception
into a usable, accessible form.
Questions?
We have a few minutes for you to ask questions
that came up during the lecture, The UBC
Physics Program or university in general!