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Electro-Magnetic Radiation

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Electro-Magnetic Radiation

- 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.

Maxwell-Faraday Equation:

Changing Magnetic field (or acceleration of a magnetic object) => Electric Field

Maxwell-Ampere’s equation:

*approximate form

Changing Electric field ( or acceleration of a charged object) => Magnetic field

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!

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

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.

- 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!

They are in the stinking names on the spectrum….

- No brainers!
- Think a littler harder…
- Ok impress me…

the heat you feel from fires, sun burns, MRI’s, Night vision… your ideas…

Sterilization of produce (gamma/UV rays), determining the age of the universe (no big deal…), chemo-therapy, setting clocks, precise measurement

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…..

-Thermal radiation is the electromagnetic radiation emitted by matter, because of all the oscillating charged particles that are its components.

Temperature=Measurement of the averagekinetic 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

- 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!

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?

- 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?

- 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!

- 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?

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.

We have a few minutes for you to ask questions

that came up during the lecture, The UBC

Physics Program or university in general!