Microwave Ovens and CD players

What are microwaves?

• Is another form of electromagnetic radiation (waves)

• They are smaller wave lengths compared to radio waves

Water molecules are polar because?

• Has a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom

• this makes it polar, like a tiny magnet with a positive end and a negative end

Microwaves make water molecules wiggle because?

• The oscillating electric field of the microwaves causes the water molecules to rotate back and forth to align with the field

How is heat generated from the water molecules?

• The spinning and bumping of the molecules creates friction at the molecular level, which raises the temperature of the water

• this is why microwaves heat food containing water so efficiently

Why don’t oils heat well?

• Made up of non polar molecules, which means there’s no strong positive or negative side

Why doesn’t ice heat well?

• Locked in a rigid structure

• Unable to rotate freely because they’re held in place by bonds

• even though water is polar, the microwave can’t make the molecules spin, so ice absorbs very little energy

How do we make microwaves like radio waves?

• Microwaves are just high frequency electromagnetic waves, so we can make them the same way we make radio waves

• we need a tank circuit (resonator), which uses an inductor and a capacitor

How does the tank circuit work?

• Capacitor stores energy in its electric field

• Inductor stores energy in its magnetic field

• Energy oscillates back and forth between the capacitor and inductor, creating a rapidly changing current

• This oscillation produces the alternating current needed for electromagnetic waves

How do we make the oscillation fast enough for microwaves?

• A smaller capacitor to charge and discharge quickly

• A smaller inductor to generate a magnetic field faster

What is a magnetron?

• The device inside a microwave oven that turns the oscillating current from the tank circuit into actual microwaves

• It takes electricity and produces the high frequency electromagnetic waves that heat food

How does a magnetic field affect electrons in a magnetron?

• Inside a magnetron, a magnetic field is applied perpendicular to the motion of the electrons

• the magnetic field forces the electrons to bend in a curved path instead of flying straight to the positive terminals

• this bending causes the electrons to hit specific areas (negative terminals or cavities) inside the magnetron 

Why is a magnetic field in a magnetron important?

• When the electrons hit the negative terminals, they transfer energy to the system, which helps maintain the oscillations in the tank circuits

Why can’t we put metal in a microwave?

• Metal surfaces reflect microwaves

• this is why the inside of a microwave is lined with metal - it keeps the microwaves trapped inside the oven to cook the food 

Why doesn’t the metal inside the microwave heat up?

• The metal lining in the microwave is thick and has very low electrical resistance, so very little heat is generated

• Thin pieces of metal have higher resistance, so microwaves can cause sparks or heating, which is dangerous

Why do sparks happen in a microwave?

• Occur because electric fields can become very strong near sharp points or edges on metal objects

• When the electric field is strong enough, it can ionize the air nearby and creating a spark

Why thin metal is dangerous

• Has high electrical resistance

• when a strong electric field causes current to flow through the thin metal

  • the metal heats up quickly

  • it cam melt or burn, damaging the plate

How does a convex lens focus light and what limits the spot size?

• How lens bend light so that parallel rays converge at a focal point

• Focusing the light increases the energy density at that spot, concentrating the lights power

• the smallest possible spot is limited by the wavelength of the light due to diffraction, which prevents perfect focusing

How does a convex lens project an enlarged image of an object?

• Object – The thing you want to see or project.

• Convex lens – Bends the light coming from the object.

• Focal point – The point where the light rays meet after going through the lens.

• Image – The light forms a bigger version of the object on the other side of the lens

How does a flat mirror reflect light?

• Reflects light so that the angle of incidence equals the angle of relfection

• this means the light bounces off at the same angle it hit the mirror

• reflects your face without concentrating light anywhere

How does a parabolic mirror reflect light?

• Curved bowl like shape

• it bends all incoming light rays to meet at one point (the focal point)

• thats why it acts as a lens, it can focus light or energy in one spot

• flashlight has a parabolic mirror behind the bulb, light bounces off the curved mirror and forms a tight bright beam

What is a polarizer?

• filters light based on its polarization

• only lets light waves with the electric field orientation in a certain direction pass through

• light with other orientations is absorbed or blocked

How does a polarizer work?

• Imagine unpolarized light as waves vibrating in all directions

• when this light passes through a linear polarizer, only the component of light vibrating long the polarizers axis gets through

Polarized sunglasses

Polarizing sunglasses normally block horizontally polarized light, so when you look at someone's eyes when they are wearing polarizing sunglasses, you see only vertically polarized light. If you wear polarizing sunglasses and tip your head sideways, then your sunglasses will block vertically polarized light. You will see no light coming from the eyes of other people wearing polarizing sunglasses

What is liquid crystals?

• Nematic phase:

  • Not a liquid nor a crystal, its another type of structure, the moleuclues are still randomly positioned in space and they can still move around but they are all rooughly pointint in the same direction. It has orientational order but no positional order sos therefore it’s a liquid crystal display

What can we do with liquid crystals?

• We can put it in between 2 cross polarizers 

How does a CD work?

• Data on the CD

  • The CD has a long spiral of tiny bumps (pits) and flat areas (lands).

  • The laser reads it from the inside to the outside.

• CD layers

  • Plastic layer (1.2 mm thick)

  • Metal layer (reflects the laser)

  • Protective coating (lacquer)

  • Label on top

• Reading the data

  • A laser shines on the metal layer.

  • Light reflects differently from pits and lands (some adds up, some cancels out).

  • A detector measures these changes and turns them into 1’s and 0’s.

• the laser “reads” bumps and flat spots on the CD, and these patterns are turned into digital data.