Heat Flow Flashcards
When more energy is added to a system than is lost, its temperature …
… increases. Conversely, when more energy is lost than is added, its temperature decreases.
If you want to maintain the boiling temperature of the water for a long period of time (perhaps to infuse some tea), how should you set the stove’s heat to consume the least amount of energy?
Turn the heat as low as possible to maintain
100 °C
the rate of heat loss to the environment needs to be equal to the rate of energy gain from the stove under the pot.
any heat flow greater than the minimum required for boiling is a waste — that extra energy goes towards converting water to steam
How can you bring a fresh pot of cold water to a boil while consuming the least energy?
Turn the heat to maximum.
it will always take the same amount of energy to heat the water to 100 if no heat energy is lost to the environment
While the water is heating, it is constantly losing heat energy to the environment. If the heating takes more time (at a lower power level), the water is spending more time losing energy to the environment
Heat energy will always flow from a warmer place to a cooler one, and the greater the temperature difference…
… the faster it flows
A near-boiling pot of water will lose heat to the environment at a much faster rate than a warm pot of water.
At room temperature, it takes 10 minutes for a pot of water to cool from 100 to 60 after the stove is turned off.
How long will it take for the pot to cool from 60 to 20?
More than10 minutes
Is more or less heat delivered to the environment in this slow cooldown than the faster cooldown with the heater off?
It takes more than 10 minutes to cool down to 60
More heat is delivered to the environment.
You’ll need hot water in an hour or so, but in the meantime, you don’t need the pot of boiling water you just heated up.
You want to save as much energy as possible. What’s the best approach?
Turn off the heat until you need hot water.
If you turn the heat off completely, it will still be losing heat to the environment, but it will spend the longest time at the lowest temperature, minimizing the heat loss to the environment
To consume the tea as warm as possible, should you add cold milk immediately after you make it, or right before you want to drink it?
Right after you make it
if you cool down the tea immediately, its rate of heat loss will be minimized, compared to if you leave it at a high temperature for longer
While ice melts at 0, aluminum melts at 660. A droplet of molten aluminum is suspended in water boiling on a hot burner.
What happens to the aluminum?
It cools down.
The water will not transfer any heat energy to the aluminum since heat energy only flows from a hotter place to a colder place.
putting molten aluminum in the water will cause heat to flow from the aluminum to the water — cooling the aluminum and heating the water
A cup of warm water is suspended in a large pot of water held at a steady boil.
Will the water in the cup ever boil?
No
Transfer of energy
When more energy is added to a system than is lost, its temperature increases. Conversely, when more energy is lost than is added, its temperature decreases
