In an attempt to keep my cup of tea warm, I’ve been placing a saucer over the top. Is this keeping the liquid warm … or is the saucer drawing heat via convection? Your friend, (Mrs.) Earl Grey.
For a really detailed analysis, I think we’d need to know the material the saucer is made of as well as dimensions. For a general guess, I’d say that you’re keeping it warmer since the amount of heat lost due to dissipation in the air is going to be much greater than the amount of heat a small saucer could draw in.
Without the saucer, the tea would get cool because the higher energy molecules would eventually bounce right out of the cup, leaving only lower energy molecules.
By placing a saucer over the top, you are preventing the complete escape of these higher-energy molecules. they bounce off the saucer, and some of them head back down into the tea, keeping the average level of molecular activity (i.e. the temperature) higher than it would have been without the saucer.
However, in bouncing off the saucer, some energy is lost to the saucer, heating it a little.
So your OP is not in fact an either/or scenario. Both activities are occuring. Eventually, the tea, saucer, and surroundings, barring any outside interference, will be the same temperature. What the saucer does is stave off the inevitable for a longer period of time.
Exactly what scotandrsn said – both. Naturally some heat transfers to the saucer. Much more heat would be lost to the air without the saucer in place.
Heat losses to your teacup occur four different ways:
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Radiation - the teacup radiates heat in a straight line towards anything colder than it. Keep large black objects (especially anything cryo-cooled) out of the tea’s “line of sight” and you’ll be fine on this score.
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Conduction - the tea warms the teacup because the two are in direct contact; the teacup warms the saucer because they’re in direct contact, and so on. The amount of heat transferred depends on material properties, but changes linearly with the temperature difference. Once the teacup reaches the same temperature as the tea, you’ll only lose as much heat from the tea (into the teacup) as the teacup loses to the saucer, which shouldn’t be much.
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Convection - Convection is when a moving fluid transfers heat; it’s similar to conduction. The liquid tea is transferring heat to the teacup by conduction (primarily) but because the tea is also moving around a little bit, there’s a small amount of convection there. A much more significant convection loss would be the cool air in the room, which moves past the teacup and leaches heat from it (slow) but which also moves across the top surface of the tea and cools it (possibly faster). The more the air in the room is moving around – e.g. if there’s a fan in your parlor – the faster your tea will cool.
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Phase Change - if the tea is steaming, then very small pieces of very hot tea (hotter than the average temperature of the tea as a whole) are “boiling off” and the average heat energy in any given particle of tea is dropping. Imagine a room full of people, and every five minutes you come in and take the three tallest people out of the room; the average height in the room keeps dropping. That’s how your tea is cooling off.
If you place your saucer on top, you can almost eliminate the losses from (3) and reduce the losses from (4), although when you pick your saucer up, you’ll find hot water has condensed on the underside of it and the saucer is slightly warm. As long as your tea is in a room that is cooler than the tea itself, the tea will eventually cool down to ambient temperature, and nothing you can do short of sealing it into a Dewar flask will stop it.
I’m sure Miss Manners would not approve, but if you want to keep your tea hot for a few more minutes at the expense of your social reputation ( :rolleyes: ), go for it.
The saucer is definately keeping the tea warmer; the biggest heat loss from an open cup would be through evaporative cooling. Restrict that and the tea will stay warmer longer than pretty much anything else you could do to it in a regular cup. When it’s cold here (-10C) and I have a cup of coffee on my dashboard -truck hasn’t warmed up yet - it will go from piping hot to cool in under 5 minutes. If I put anything over the top, it will stay acceptably warm 10 minutes or so.
You guys kill me. I love your detailed explanations.
The cup and the saucer are heavy white ceramic. From the Pottery Barn Suppertime collection, if you must know. Affordable! Good-looking! Oven-, freezer-, microwave-safe!
Opinions. Opinions. Opinions.
A science question suggests a real science experiment! (Mrs.) Earl Grey.
Two identical cups and saucers and two identical cover saucers. Make tea in both cups in the same way and cover.
After 15 or 20 minutes taste each and let us know which stayed warmer/hotter.
Short of an electric cup warmer to set you tea on you could make a cup cover similar to a tea cozy used on the tea pot.
Two identical cups and saucers and two identical cover saucers. Make tea in both cups in the same way and cover.
After 15 or 20 minutes taste each and let us know which stayed warmer/hotter.
Now, wait just a minute. …
Wouldn’t the wife of Earl Grey be Countess Grey?
Or…Make a 2 cup pot of tea. Prewarm the pot with hot water. Brew your tea with the pot covered with a tea cozy or towel. Drink a small cup at a time. When I do this, the last cup is still able to burn the roof of my mouth as easily as the first cup did!
Lady Grey an even better tea.
Continuing the hijack, there were several Earl Greys, they best known of whom was Prime Minister of Britain from 1830-1834:
http://www.npg.org.uk/live/search/a-z/sitG.asp
And they have a picture of the 2nd Countess Grey