ElectroMagnetic Radiation Intro to Electro Magnetic Waves or

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= 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. 8 m^2 • Google says an average temperature for the surface of a human body is 306 K. • Google says the emissivity of human flesh is. 97 • Therefore, Google says that P=870 W 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!