GCSE: Extension Work A2 Level :Resonance Cooking with Microwaves
Cooking with Microwaves • The microwave oven is well known as a fast, quick and efficient way of cooking food. • Its invention has changed the way we cook in the home.
Care when using the term ‘microwave’! • Watch how you express the term ‘microwave’ in your answers! • Distinguish clearly between • ‘microwaves’ the waves – the photons of electromagnetic energy and • microwave ovens – the devices that cook using that energy
Cooking with Microwaves • The microwaves are produced by a magnetron - a high-powered vacuum tube that generates coherent (all ‘in step’ and of one wavelength) microwaves. • When you have the oven on a low setting you can hear the magnetron being switched on and off - delivering bursts of microwaves instead of a full stream of them. • The fan, that you can hear when the oven is on, is there to cool the magnetron - without it running the oven would overheat.
Cooking with Microwaves • The microwave oven cooks food by using high frequency electromagnetic waves (part of the electromagnetic spectrum - so their speed is 3 x 108 m/s in a vacuum) called microwaves at 2.45 GHz. • These are absorbed particularly well by water, and fat and sugar to a lesser extent.
Cooking with Microwaves • They get absorbed by the surface of the food and penetrate about 1cm into food before it is all absorbed. • The food is then hot on the surface and the heat is then conducted deeper into the cooler part of the food.
Cooking with Microwaves • The wavelength of mircowaves are between 1 millimetre and 30 centimetres. • Try calculating the wavelength of microwave oven microwaves! You should KNOW the wave equation c = speed of electromagnetic radiation = 3 x 108 m/s f = frequency of the waves = 2.45 GHz = 2.45 x 109 Hz
Cooking with Microwaves c = fl where c = speed of electromagnetic radiation = 3 x 108 m/s f = frequency of the waves = 2.45 GHz = 2.45 x 109 Hz • = wavelength of the microwaves • = c/f = 3 x 108 / 2.45 x 109 = 0.12 m or 12 cm ANS
Cooking with Microwaves • Most food stuffs contain water. • Microwaves are most readily absorbed by water molecules (although fats and sugars also absorb too!). • These molecules then vibrate more quickly and therefore rise in temperature.
Cooking with Microwaves • Some science books say that the frequency of microwaves used in a microwave oven is the 'natural frequency' of water and resonance is the mechanism that causes the high amplitude vibration. • That is not the case – the chosen value of the frequency is not the resonant frequency of the water molecule.
Cooking with Microwaves • The vibration is down to the push and pull of the electric and magnetic forces from the wave that cause the vibration. • Water molecules (and sugars) are polar (one part is negative compared to the other).
Cooking with Microwaves • The electromagnetic waves have an electric field that 'pulls and pushes' charged particles such as the water molecule. • This causes the water molecules (that are attached together by hydrogen bonds into chains) to twist and turn. • Their vibrational and rotational energy increases - so their temperature rises.
Cooking with Microwaves • The frequency used in microwave ovens (2.45 GHz) is a sensible but not unique choice. • Waves of that frequency penetrate well into foods of reasonable size so that the heating is relatively uniform throughout the foods.
Cooking with Microwaves • Leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications - the frequency was chosen in part because it would not interfere with existing communication systems.
Cooking with Microwaves • Using a frequency that water molecules responded to strongly (as in a resonance) would be a serious mistake--the microwaves would all be absorbed by water molecules at the surface of the food and none would be able to reach molecules deeper in the food.
Cooking with Microwaves • Conduction is a slow process. • So the food would not cook very well in the centre if the resonant frequency was used.
Cooking with Microwaves The 2.45 GHz frequency was chosen because • it is absorbs well enough in liquid water so that the waves maintain good strength (even deep inside a typical piece of food). • Higher frequencies would penetrate less well and cook less evenly.
Cooking with Microwaves The 2.45 GHz frequency was chosen because • Lower frequencies would penetrate better, but would be absorbed so weakly that they wouldn't cook well. • Also leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications - if lots of different frequencies were chosen it would cause a problem in communications..