The inside of a refrigerator sits at about 3 °C in a kitchen at about 22 °C, hour after hour.
Heat flows spontaneously from hotter to colder. So how does heat end up leaving the cold interior for the warmer kitchen?
On its own, heat would only leak the other way: from the 22 °C kitchen into the 3 °C interior.
Yet the interior stays cold, because the refrigerator keeps moving heat out of it, from colder to hotter.
The refrigerator's compressor is where that work comes from. While the compressor runs, it does work, and that work drives heat out of the cold interior into the warmer kitchen.
With the compressor off, no work is supplied. Kitchen heat then slowly leaks in, and the interior warms.
You have seen that energy crosses a closed system's boundary in exactly two ways: as heat, or as work.
Here both doors are busy at once. Heat is pumped out of the interior, and work supplied from outside makes the pumping possible.
A window air conditioner moves heat from a 24 °C bedroom into 35 °C outdoor air. What makes that flow possible?
The flow runs from a colder place to a hotter place.
Moving heat from colder to hotter requires work supplied from outside.
The air conditioner's compressor does that work while it runs.
Work supplied from outside, by the running air conditioner, makes the flow possible.
A camper keeps milk in a plain metal box inside a 28 °C tent, with no power supply. Will heat flow out of the 10 °C milk into the hotter tent air?
That flow would run from colder to hotter.
Moving heat from colder to hotter requires work supplied from outside, and the box has none.
No. Without work, heat only leaks the spontaneous way: from the hot tent air into the milk.
A drinking fountain chills its water below the hallway's temperature. What makes the heat flow out of the cooler water into the warmer hallway air possible?
Jola says: 'An ice-rink chiller keeps the rink frozen inside a warm arena only while its machinery does work.' Is she correct?
An insulated lunch bag holds 10 °C yogurt in a 28 °C classroom. Nothing supplies work. Can heat keep flowing out of the yogurt into the warmer room?
A heat pump warms a house in winter by moving heat from the colder outdoor air into the warmer indoor air. Which of the following statements is correct?
A refrigerator's interior sits at 3 °C in a 22 °C kitchen. Why does the refrigerator need a powered compressor at all?
Which of these heat flows requires work supplied from outside?