Microwave Oven

Ravi P. Agrahari (Science & Technology)

What are Microwaves?

• Microwaves are a form of electromagnetic energy, like light waves or radio waves

• Microwaves are used extensively in communications• such as to relay long-distance telephone signals, television programs

and computer information across the earth or to a satellite in space.

• Good for transmitting information because it can penetrate haze, light rain and snow, clouds, and smoke.

• Also used in radars and in detecting speeding cars.

• Microwave has become most familiar as the energy source for cooking food.

Electromagnetic radiation exists in a range of frequencies called the electromagnetic spectrum. Each frequency has a specific wavelength and as the frequency decreases, the actual length of the wave gets longer

Radiation Type Frequency Range (Hz) Wavelength Range

Gamma rays above 3 x 1019 < 10-12 m

X-rays 3 x 1017 - 3 x 1019 1 nm - 1 pm

Ultraviolet Radiation 7.5 x 1014 - 3 x 1017 400 nm - 1 nm

Visible Spectrum 3.8 x 1014 - 7.5 x 1014 750 nm - 380 nm

Infrared Radiation 1011 - 3.8 x 1014 25 um - 2.5 um

Microwaves 108 - 1012 1 mm - 25 um

Radio waves 104 - 108 >1 mm

Table 1: Frequency and Wavelength Range of Each Radiation Type in the Electromagnetic Spectrum.

History

• Invented accidentally by Dr. Percy LeBaron Spencer. • While testing a magnetron during work, he discovered

the candy bar in his pocket melted.• Experimented with other food products (popcorn and

eggs), and realized microwaves can cook foods quickly. • At 1947, 1st commercial microwave oven produced

(called Radarange)- Mostly used by restaurants, railroad cars, ocean liners and military.

• Improvement and refinements made by 1967, 1st domestic microwave oven produced.

How the Oven Works

• Electricity from the wall outlet travels through the power cord and enters the microwave oven through a series of fuse and safety protection circuits

• These circuits include various fuses and thermal protectors that are designed to deactivate the oven in the event of an electrical short or if an overheating condition occurs

• When the oven door is closed, an electrical path is also established through a series of safety interlock switches

- Sensing that all systems are set to go, the signal activates triac, producing a voltage path to the high-voltage transformer. - The high-voltage transformer along with a special diode and capacitor arrangement increases the typical household voltage from ~115 volts to ~3000 volts

- The magnetron converts the high voltage in to the microwave frequency for cooking- The microwave energy is transmitted into a waveguide- The waveguide feeds the energy to the stirrer blade and into the cooking area - When the door is opened, or the timer reaches zero, the microwave energy stops.

Magnetron

Electrons from a hot filament would

travel radially to the outside ring if it were not for the magnetic field.

The magnetic force deflects them as

shown and they tend to sweep around the circle. In doing so, they “pump” the natural frequency of the cavities.

The currents around the resonant

cavities cause them to radiate electromagnetic energy at that resonant frequency.

How Foods Get Cooked

• The microwaves that penetrate the food have an electric field that oscillates 2.45 billion times a second, a frequency that is well absorbed by polar liquid molecules such as water, sugars, fats and other food molecules.

• Water interacts with the microwave: - flipping its orientation back and forth very rapidly - bumping into one another and producing heat, cooking the food. • Glass, paper, ceramic, or plastic containers are used in

microwave cooking because the microwaves pass through them• Metal reflects microwaves - Unsafe to have metal pans/aluminum foil in oven, may damage oven

Health Hazards

• It is known that microwave radiation can heat body tissue the same way it heats food.

• Exposure to high levels of microwaves can cause a painful burn - Ex. the lens of the eye ~ exposure to high levels of microwaves can cause

cataracts.

• Microwave oven used low level of microwaves, within the region of non-ionizing radiation

• Still uncertain in the effects of humans from long term exposure to low level of microwaves

- Still experimenting

• Best to stay a way (an arm’s length) in reducing exposure to microwaves

Thank you….