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What makes a 'good' microwave oven? - Coggle Diagram
What makes a 'good' microwave oven?
Properties of Microwaves
It's an electromagnetic wave
Has a frequency of 2.45 GHz and a wavelength of 0.122m.
Heating Food with Microwaves
Microwaves produced by the magnetron collide with the spinning propeller and stirrer fan.
They reflect of the fan and propeller as they are made of metal.
Microwaves move in the oven as they can reflect of metal
Can reflect of metal surfaces
Can pass through glass and plastic
Microwaves can be affected by wave properties i.e., Diffraction, Interference and Refraction.
Can be absorbed by water contained in food which allows to heat it.
Limitations of using microwave oven to heat food
Constructive and Destructive Interference
This causes food to heat unevenly as it leads to producing hot and cold spots.
In the oven chamber, microwaves are colliding with each other which suggests that the resultant electric field is the sum of electric fields at a particular point.
Constructive interference will produce an anti-node due to the resulting electric field being the greatest at that point.
At this point a hot spot is formed where food is given maximum heating.
Antinode - A point along a standing wave where maximum amplitude is produced.
Destructive Interference will occur when there is no resulting electric field at a particular point which will produce a node.
At this point a cold spot is formed on the food and no heat is given.
Node - A point on a standing wave where minimum amplitude is produced.
Effect on Metals
Reflection of microwaves of metal causes food in a metal container to remain unheated.
This occurs as microwaves reflect instead of being absorbed by the food.
This is problematic as these microwaves reflect and travel to the magnetron which can significantly damage it.
Sparks can be created if sharp metallic objects such as forks are heated with food
.
This occurs as there are more charges concentrated at the pointed end. Therefore, there is a strong electric field as charge and electric strength are directly proportional.
This cause air particles to ionise and thus enables charges to travel to the closest conductor.
This process is known as Corona Discharge.
Electromagnetic Fields
Electric Fields
A region of space surrounding point charges and charged objects.
Point charges or any charged objects experience electrostatic force when entering an electric field of another charge.
Magnetic Fields
Created by moving charged particles.
Application:
Electromagnet purely relies on the flow of electric current to produce a magnetic field.
Important Parts of Microwave Oven
Wave Guide
Guides the microwaves inside the microwave oven cavity.
Magnetron
Component of Magnetron
There is a permanent magnetic field which is perpendicular to the electric field.
Takes the shape of a hollow tube.
Consists of a cylindrical cathode and several cavities that act as anodes.
Cathode moves through the centre and out of the tube.
Electric field is located in the gap that is inside the tube.
Generates high power radiowaves
This device produces microwaves
Step-up transformer
Supplies a high voltage to the Magnetron
Increases the 120 volts A.C. to 2000 volts
Stirrer Fan
Prevents hot and cold spots to form
Moves the hot spots around so, there is more uniform heating of the food.
Reflects the waves approaching from the wave guide in different directions.
Prevents the formation of fixed standing waves.
**Safety
Avoid using any form of metals (e.g. metal containers) inside microwave oven
Don't stand too close to the oven due to exposure of radiation.
Wear safety glasses
Exposure to microwave radiation can cause painful burning to eyes.
Different uses of microwave ovens in kitchens
Reheating
Baking
Defrosting
Roasting
Boiling and Steaming
Properties of light
Frequency
Measured in Hertz (Hz)
No. of periodic oscillations occurring in a second.
Wavelength
Length between two successive points on a wave.
Measured in meters (m)
Electromagnetic wave
Consists of electric and magnetic field that travel perpendicular to each other and also to the wave propagation. (Transverse Wave)
Reflection
Diffraction
Polarisation
Refraction
Door of Oven
Consists of a Faraday Cage with a transparent window.
Designed to keep all microwave radiation in the oven and shield any exterior from radiation.