RefrigerationEnergy for Heat Transfer

1st Law of ThermodynamicsWhenever heat flows into or out of a system,

the gain or loss of thermal energy equals the amount of heat transferred.

You don’t get energy from nothing. The amount of energy in the beginning is equal to

the amount of energy at the end, but it may be in a different form.

2nd Law of ThermodynamicsHeat never spontaneously flows from a cold

substance to a hot substance. In the summer heat from flows from the warm

outdoors into your cool house. Your ac does not cool the outside. The outside makes your house warmer.

In the winter heat flows from your warm house to the cold air outside

The cold air does not make your house colder Your house makes the outdoors warmer

3rd Law of ThermodynamicsNo system can reach absolute zero

natures lowest possible temperature where all molecular motion ceases

Physicists have been within less than a millionth of 1 Kelvin, but never 0K!

RefrigeratorsFacts

Cooking does not destroy poisons in food.To preserve food it must be kept cold (between

32oF and 40oF) and dry.Drawbacks

Cost associated with purchase and operationEnergy supply needed to operate

Work is done in order to move thermal energy from low to high.

If you unplug the fridge, it no longer stays cool.

°C x 9/5 + 32 = °F

(°F - 32) x 5/9 = °C

°C + 273 = K

Liquid pumped into the cooling unitIt is forced through tiny openings causing it to evaporate and

draws heat from its surroundings. The gas is directed outside the cooling unit to the coils on the

back. The gas condenses, releasing its heat to the surrounding air

(outside the refrigerator)

Energy is Needed for a Phase Change Whenever a substance changes phase,

an energy transfer occurs. ○ Heat of Fusion: amount of energy needed

to change a substance from a solid to a liquid (melt)

○ Heat of Vaporization: amount of energy required to change a substance from a liquid to a gas (boil)

SolidLiquid Gas

Energy is absorbed when change of phase is in this direction

Energy is released when change of phase is in this direction

Practice Problems:The heat of fusion of water is 335 kj/kg.

Is energy absorbed or released when ice melts? How much energy must be absorbed to melt 1 kg

of ice?335 kj

Is energy absorbed or released when water freezes?

How much energy must be released to freeze the water back into ice?

335 kj

There is no temperature change during a phase change!

Energy is going into the change

in phase not into eating up the

substance!