last lecture
Electromagnetic waves – oscillating electric and magnetic
fields
c = 3x10
8m/s or 186,282 miles/sec
Radios –
using the “tank” circuit to emit and receive
electromagnetic waves of a specific frequency
- AM and FM radios
- Ionosphere reflection
Cell phones –
using low power transmitters in “cells”, frequency
can be reused.
c =
How does a microwave oven work?
Which of the following can a microwave
oven heat efficiently?
A) Ice cube B) Dried fish Microwaves are light, with wavelengthin the range of 1 mm – 1m.
- Can a microwave cook any type of food ?
- How does it cooks inside-out, not just on the surface ?
- Why do they have turntables?
- Why do Warnings say ‘Don’t put metal objects inside’ ?
- When set to “defrost”, what does the microwave do ?
- Why does it have a screen mesh/glass window ?
- Can it heat ice or steam?
Physics-related Questions about Microwave Ovens
The parts of a microwave oven
Microwaves are sent into the cooking chamber
Jul 19 1894 - Sep 8 1970
Some historical facts about microwave ovens
In 1945, Percy Spencer, working for Raytheon to develop magnetrons for active radar signals, noticed a chocolate bar in pocket melted while standing in front of an operating magnetron.
He then tested popcorn in front of the magnetron and it quickly popped all over the room.
In 1947, Raytheon made the first microwave oven – Radarange (6 ft tall, 3000 W power $3K) but did not sell well. In 1965, a countertop version was developed for $495.
How does a microwave oven cook food?
Electric field Magnetic field
Microwave ovens selectively make liquid water hot. Most food,
even "dry" foods, have
water
in them.
Water (H2O) is a polarmolecule with 2 hydrogen
atoms being more positive than the single oxygen
atom
-
+
In liquid water, the molecules are in constant motion and are normally
What happens if water molecules are in an electric field?
E
The molecules will experience a
torque
from
the electric field, and become aligned with
direction of the field.
Therefore, water molecules are oriented by the electric field.
What will happen to the molecules if the directionof the electric field is flipped rapidly (1 billion times/second)?
What happens to water molecules in an oscillating electric field?
- The polarwater molecules follow the oscillation of the electric field. - They collide more frequently with the molecules (water and other)
around them
- The molecules move faster and faster, and the temperature increases (heating)!
This is how a microwave
oven cooks food!
oscillating electric field
time
Question:
Most microwave ovens use a wavelength of 12 cm. how
many times does a water molecule flip in one second?
Can a microwave cook any type of food ?
Physics Questions about Microwave Ovens
Microwave can only cook effectively foods with sufficiently amount of liquid water.
How does it cook inside-out, not just on the surface?
At 2.45 GHz, microwaves can penetrate the food and excite the water molecules inside.
In contrast, traditional ovens heat food from the outside, relying entirely on thermal conduction/convection to carry heat in food. Why do they have turntables?
Because light, and microwaves, reflect and interfere! Microwaves are reflected by the oven wall, forming “standing waves” in the cooking chamber.
Standing waves – waves that oscillate in place
Nodes –zero amplitude (no oscillation) at all times
Anti-nodes– locations with maximum amplitude
In side a microwave oven, the electromagnetic waves also form
standing waves from reflections at the walls.
Why do Warnings say “Don’t put metal objects inside” ?
The rapidly changing electric and magnetic fields of the microwaves induce currents in the metal, generating a large amount of heatingif the resistance is high.
Never put aluminum foil or plates with metal lining inside a microwave oven.
When set to “defrost”, what does the microwave do ??
Defrosting is hard for the microwave to do since water molecules in the ice are not free to rotate. In defrosting, a few loose water molecules will be quickly heated, and they heat up their surroundings by conduction and convection, making liquid in little pockets.
On defrost mode, the oven switches on/off the microwaves to allow heat conduction from liquid water to melt ice and subsequently heat the liquid water.
Why does it have a screen mesh/glass window ?
The glass door with screen mesh prevents microwaves from leaking. The mesh size should be much smaller than the wavelength: d << λ. If d ≈ 5 inches, the microwaves will escape and hurt people in front of it.
Can it heat ice or steam?
In ice, molecules are locked into bonded arrangements, and can’t break loose to flip-flop, therefore, cannot be heated by microwaves.
Of steam, water, and ice, only
liquid water
is heated by microwave
In steam (water vapor), molecules are far apart. They flip back andforth, but don’t collide with other water molecules frequently enough to increase their kinetic energy, therefore, no heating is generated.
Are microwaves harmful?
The only thing microwaves can do is vibrate water molecules. - No adverse effects on the food cooked.
