SOURCES OF ENERGY AND HEAT TRANSFER Flashcards
What is conduction?
The direct transfer of energy from one solid body to another. Vibrating molecules from one surface will strike the relatively still molecules of another, thereby transferring energy.
By far the most efficient method of heat transfer.
Conduction
Examples of heat transfer through conduction.
Searing a steak Crisping the bottom of pizza Cooking scrambled eggs Making grill marks on a burger Sautéing onions
What is convection?
The transfer of one solid body to another through the intermediary of a fluid–that is, liquid or gas.
Is convection efficient?
Moderately, though in cooking its efficiency depends greatly on the way the fluid flows around the food.
The motion of fluid if referred to as what?
Convection patterns.
What is the general rule about air speed flowing over a given surface? Explain.
The faster it flows, the more energy it can transfer. Still air will rapidly give up its energy, but with moving air, the energy supply is constantly being replenished by new air being cycled over a substance such as food.
What are convection oven fans designed for?
To keep air moving around at a good clip to promote faster, more even cooking.
Will agitating oil when deep frying lead to foods that crisp and brown more efficiently?
Yes
Examples of heat transfer through convection.
Steamed asparagus Boiling dumplings in stock Deep-frying onion rings BBQing a pork shoulder The top of a pizza baking in an oven
What is radiation?
Transfer of energy through space via electromagnetic waves.
Does radiation require a medium to transfer?
No
An important fact to remember about radiant energy.
It decays.
Why does radiant energy decay (get weaker)?
By the inverse square law–the energy that reaches an object from a radiant energy source is proportional to the inverse of the square of its distance.
Example of radiant energy decaying.
Hold your hand 1 foot away from the fire, then move it two feet away. Even though you’ve doubled your distance, the fire will feel only about one-quarter as hot.
Examples of radiant heat transfer.
Roasting a pig on a spit next to hot coals
Toasting garlic bread under the broiler
Getting tan from the sun
Broiling some marinated salmon
Most of the time, in cooking, are all three methods of heat transfer used? Example.
Yes, to varying extents. Burger on the grill uses radiant heat (fire), convection (lid) and conduction (grill grate).
The only other standard method of energy transfer we commonly use in the kitchen.
Microwaves
Microwaves have the unique ability to what?
Penetrate through the exterior of food when heating it.
Just like heat, microwaves are a form of what?
Electromagnetic radiation.
When microwaves are aimed at an object with magnetically charged particles, those particles do what?
Rapidly flip back and forth, creating friction.
Microwaves can pass through most solid objects to a depth of what?
At least a few centimeters or so; not a relatively slow transfer of energy - better than the conduction, convection, radiation.
Most people can hold there hand in a 200ºF oven for how long?
30 seconds or so.
Water over what temp generally becomes painful to the touch?
135ºF
Water at what temp is hot enough to scald you?
180ºF
Water at what temp will blister and scar you?
212ºF
How does a lower temperature of water burn you while a higher temperature oven will simply be uncomfortable?
There are many more molecules in a cup of water than in air. Therefore, the water has more energy and heats up your hand much more rapidly.
Does food cook faster in a dry or moist oven? Why?
A moist oven has denser air and more energy, cooking the food faster.
Will boiled foods cook faster than baked or roasted food?
Yes
