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### Second Law of Thermodynamics

The Second Law of Thermodynamics is one of three Laws of Thermodynamics. The term "thermodynamics" comes from two main words "thermo," meaning heat, and "dynamic," meaning power. So, the Laws of Thermodynamics are the Laws of heat power. All things in the universe are affected by Laws of Thermodynamics. The Second Law of Thermodynamics states heat naturally tends to flow from a hot object to a cold object. Work has to be done to get heat to flow from cold objects to warm objects.

Suppose we dropped a hot rock into cold water. Wouldn't it be odd if the water turned to ice and the rock got hotter? Instead, the rock cools as the water warms. We would be in a different universe otherwise.

#### Question

How does the Second Law of Thermodynamics help in the working of a refrigerator?

Before refrigerators, there were iceboxes. A block of ice placed in a cabinet along with perishable foods absorbed the heat from the food and the interior of the icebox. This kept food cold but was inconvenient. We needed a constant resupply of ice and we had to deal with water from the melting ice.

In modern fridges, a compressed liquid is pumped through tubes ("coils") inside the fridge. The fluid turns into a gas inside the coils. Just as melting ice absorbs heat in an icebox, evaporating fluid absorbs heat in modern refrigerators. The advantage is that the gas is recycled. A compressor on the exterior of the fridge turns the gas back into liquid to be used again. However, nothing is free. We no longer resupply ice to keep the fridge cold, but we have to supply electrical power.

Boats and fridges leak. If we were sitting in a canoe in the middle of a lake and water started to accumulate in the bottom of the canoe, we have little choice but to bale the water. Since boats sit in water, not on top of water, we cannot simply poke a hole in the bottom of the boat to drain it. Water flows downhill, therefore we have to supply work to lift the water from the bottom of the boat in order to dump it over the side.

A fridge contains a cold interior sitting in the middle of a warm room. Heat leaks through even the best insulation. To keep the interior cold, we cannot simply open the fridge door. Heat naturally flows from warm objects to cold objects; therefore, we have to supply work to pump the heat from the cool interior of the fridge into the warm room. A compressor and a pump on the exterior of the fridge supply the work. However, understanding them involves different laws of thermodynamics.

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Thanks to Mohit Ghai for suggesting this topic.