Keane pours a ribbon of milk into his still coffee. The white ribbon swirls, thins, and fades until the whole cup is one even tan.
Nobody stirred. Could the reverse happen while he watches: the tan un-mixing back into black coffee and a white ribbon?
The second law gives the test. A process happens on its own when it leaves energy and matter more spread out.
Milk spreading through coffee leaves matter more spread out. So the spreading happens on its own.
The reverse would gather the milk back into one ribbon. That ending is more concentrated.
A process that would leave energy and matter more concentrated never happens on its own.
So the prediction takes two steps. Picture the ending the process proposes. Then ask: more spread out, or more concentrated?
Energy follows the same test as matter. Heat spreading from a hot thing into cooler surroundings passes the test; heat gathering itself back fails it.
A bag of frozen peas sits on a 22 °C counter. Will heat spread from the kitchen air into the peas on its own?
Picture the ending: the air's energy shared into the cold peas as the peas warm.
That ending leaves the energy more spread out, more evenly shared across air and peas.
Yes. The ending is more spread out, so the warming happens on its own.
Could the thawed peas re-freeze themselves on the same 22 °C counter, with no device running?
Picture the ending: the peas' energy gathered back out into the warmer air, leaving the peas frozen.
That ending concentrates energy away from the colder peas into the hotter air, a more concentrated, less likely arrangement.
No. That ending is more concentrated, so it never happens on its own.
Smoke curls from a just-blown-out birthday candle into the kitchen air. Will the smoke spread through the kitchen on its own?
Could that spread smoke later gather itself back into a thin column above the candle's wick, on its own?
Jola says: 'A cold metal ladle left in a pot of hot soup will warm on its own, because the soup's energy spreading into the ladle leaves the energy more spread out.' Is she correct?
A warm phone lies on a cold stone windowsill. Which of the following happens on its own?
A tray of cold soda cans sits in a 30 °C garage, with nothing running. Which of the following happens on its own?
A pinch of chili oil has spread evenly through a pot of broth. Which of the following statements is correct?