Really Cold Objects Get Hot Again

Heat

by Chris Woodford. Last updated: March 24, 2022.

Touch a radiator and it feels hot. Dip your finger in tap-water and information technology feels cold. That'south a no-brainer! Simply what if a polar bear, used to freezing Chill temperatures, touched the same things? Both might experience hot to a polar bear considering it lives in much colder conditions than nosotros exercise. "Hot" and "cold" are relative terms that nosotros can utilise to compare how things experience when they take more or less of a sure kind of energy we phone call heat. What is it, where does it come from, and how does it move effectually our globe? Let'south observe out more than!

Photograph: Now that'southward what I call estrus! The temperature of the hot rocket frazzle y'all can meet hither, during a launch of the Space Shuttle, is something like 3300°C (6000°F). Photo past courtesy of NASA on the Commons.

Contents

1. What is rut anyway?
2. What happens when something has no oestrus at all?
3. What's the difference betwixt heat and temperature?
4. How can we measure temperature?
5. How does heat travel?
   • Conduction
   • Convection
   • Radiation
6. Why do some things take longer to heat up than others?
7. Latent estrus
8. Find out more

What is oestrus anyhow?

Estrus is a shortened way of saying "rut energy." When something's hot, it has a lot of oestrus energy; when it'southward cold, it has less. Only fifty-fifty things that seem cold (such as polar bears and icebergs) have rather more estrus energy than you lot might suppose.

Objects can store rut because the atoms and molecules within them are jostling around and bumping into one another like people in a oversupply. This idea is chosen the kinetic theory of matter, because information technology describes heat as a kind of kinetic free energy (the energy things have because they're moving) stored by the atoms and molecules from which materials are made. It was developed in the 19th century by various scientists, including Austrian physicist Ludwig Boltzmann (1844–1906) and British physicist James Clerk Maxwell (1831–1879). If you're interested, here's a longer introduction to kinetic theory.

Artwork: Hotter things have more than heat free energy than colder things. That'due south because the atoms or molecules move around faster in hot things (red, right) than they do in common cold things (bluish, left). This thought is called the kinetic theory.

The kinetic theory helps us understand where the energy goes when nosotros heat something upwards. If you put a pan full of cold water on a hot stove, you're going to brand the molecules in the h2o motility around more quickly. The more heat you supply, the faster the molecules move and the farther apart they get. Eventually, they bump around then much that they break apart from one another. At that point, the liquid you lot've been heating turns into a gas: your h2o becomes steam and starts evaporating away.

What happens when something has no heat at all?

At present suppose nosotros attempt the opposite trick. Let'due south accept a jug of h2o and put information technology in the refrigerator to cool it down. A refrigerator works by systematically removing oestrus free energy from food. Put water inside a refrigerator and information technology immediately starts to lose heat energy. The more than heat it loses, the more kinetic energy its molecules lose, the more than slowly they move, and the closer they go. Soon or later, they get close plenty to lock together in crystals; the liquid turns to solid; and you find yourself with a jug of ice!

But what if you accept a super-amazing refrigerator that keeps on cooling the water and so information technology gets colder... and colder... and colder. A habitation freezer, if you have i, can accept the temperature down to somewhere between −10°C and −xx°C (14°F to −iv°F). But what if you keep on cooling lower than that, taking abroad even more rut energy? Eventually, you lot'll reach a temperature where the water molecules pretty much stop moving birthday considering they accept admittedly no kinetic energy left. For reasons we won't go into hither, this magic temperature is −273.xv °C (−459.67°F) and nosotros refer to it as accented cypher.

Photo: Ice may look cold but information technology'due south an atrocious lot hotter than absolute zip. Flick by Erich Regehr courtesy of Usa Fish & Wild fauna Service.

In theory, absolute zero is the lowest temperature anything can ever attain. In practice, it'due south virtually impossible to cool anything down that much—scientists accept tried very hard but still not actually reached such a low temperature. Amazing things happen when you get close to accented zero. Some materials, for example, can lose virtually all their resistance and become astonishing conductors of electricity called superconductors. There's a great PBS website where you can observe out lots more near accented zero and the remarkable things that happen there.

What's the difference between heat and temperature?

Now yous know about accented null, it's easy to encounter why something like an iceberg (which could be at the chilly temperature of most 3-4°C or round about 40°F) is relatively hot. Compared to absolute aught, everything in our everyday world is hot considering its molecules are moving around and they have at to the lowest degree some heat energy. Everything around us is also at a much hotter temperature than accented nothing.

You can come across there's a shut link between how much heat energy something has and its temperature. So are heat energy and temperature merely the same thing? No! Permit's get this clear:

• Heat is the free energy stored inside something.
• Temperature is a measurement of how hot or cold something is.

An object's temperature doesn't tell us how much heat energy it has. Information technology's easy to see why not if you retrieve about an iceberg and an ice cube. Both are at more than or less the same temperature but because the iceberg has far more mass than the water ice cube, information technology contains billions more molecules and a great deal more heat energy. An iceberg could fifty-fifty incorporate more than oestrus energy than a cup of coffee or a red-hot iron bar. That's because its bigger and contains so many more than molecules, each of which has some heat energy. The java and the atomic number 26 bar are hotter (have a college temperature), only the iceberg holds more than oestrus because it's bigger.

Artwork: An iceberg is much colder than a loving cup of java merely information technology contains more oestrus energy because it's then much bigger.

How can we mensurate temperature?

A thermometer measures how hot something is, not how much heat energy it contains. 2 objects at the same temperature are as hot, but one can comprise a lot more than oestrus energy than the other. We can compare the temperatures of dissimilar things using two common (and adequately arbitrary) scales chosen Celsius (or centigrade) and Fahrenheit, named for Swedish astronomer Anders Celsius (1701–1744) and German language physicist Daniel Fahrenheit (1686–1736).

In that location's too a scientific temperature scale called the Kelvin (or absolute scale), named for British physicist William Thompson (later Lord Kelvin, 1824–1907). Logically, the Kelvin scale makes much more than sense to scientists because it runs upwardly from accented aught (which is also known every bit 0K, without a degree symbol between the zero and the M). You'll see lots of Kelvin temperatures in physics, merely you won't find weather forecasters giving you temperatures that way. For the record, a reasonably hot day (xx–30°C) comes in at something similar 290–300K: you just add 273 to your Celsius figure to convert to Kelvin.

How does heat travel?

One thing y'all've probably noticed about heat is that it doesn't generally stay where yous put it. Hot things get colder, cold things go hotter, and—given enough time—most things eventually end up the same temperature. How come?

In that location's a basic law of physics called the second law of thermodynamics and it says, substantially, that cups of coffee always go cold and water ice creams e'er melt: heat flows from hot things toward cold ones and never the other manner around. You never see coffee boiling all by itself or ice creams getting colder on sunny days! The 2nd law of thermodynamics is also responsible for the painful fuel bills that drop through your letterbox several times a year. In brusk: the hotter y'all make your home and the colder it is outside, the more heat you lot're going to lose. To reduce that problem, yous demand to empathize the three different ways in which heat can travel: chosen conduction, convection, and radiation. Sometimes you'll encounter these referred to equally three forms of heat transfer.

Conduction

Conduction is how heat flows between two solid objects that are at different temperatures and touching one another (or betwixt two parts of the aforementioned solid object if they're at different temperatures). Walk on a stone floor in your blank feet and it feels cold because rut flows chop-chop out of your body into the floor by conduction. Stir a saucepan of soup with a metal spoon and you'll soon have to observe a wooden one instead: heat travels quickly along the spoon past conduction from the hot soup into your fingers.

Animation: When you agree an iron bar in a fire, heat travels forth the metal past conduction (red arrow). Why? Atoms at the hot finish motion more speedily as they absorb the burn's heat. They gradually laissez passer their energy further along the bar, eventually warming the whole matter upwards.

Convection

Convection is the primary way heat flows through liquids and gases. Put a pan of common cold, liquid soup on your stove and switch on the heat. The soup in the bottom of the pan, closest to the heat, warms upwards quickly and becomes less dense (lighter) than the cold soup above. The warmer soup rises upward and colder soup up to a higher place it falls down to take its place. Pretty soon you've got a apportionment of oestrus running through the pan, a bit similar an invisible heat conveyor, with warming, ascent soup and cooling, falling soup. Gradually, the whole pan heats up. Convection is likewise one of the ways our homes heat up when nosotros turn on the heating. Air warms upward higher up the heaters and rises into the air, pushing common cold air down from the ceiling. Earlier long, there's a circulation going on that gradually warms upwards the entire room.

Animation: How convection pumps heat into a saucepan. The pattern of warming, ascension soup (red arrows) and falling, cooling soup (blue arrows) works similar a conveyor that carries heat from the stove into the soup (orange arrows).

Radiation

Moving picture: Infrared thermal images (sometimes called thermographs or thermograms) prove that all objects give off some oestrus energy by radiations. In these two photos, you can see a rocket on a launch pad photographed with a normal photographic camera (in a higher place) and an infrared thermal camera (below). The coldest parts are majestic, blue, and black; the hottest areas are red, yellow, and white. Photo past R. Hurt, NASA/JPL-Caltech, courtesy of NASA.

Radiation is the third major way in which heat travels. Conduction carries estrus through solids; convection carries heat through liquids and gases; but radiation can carry heat through empty space—fifty-fifty through a vacuum. Nosotros know that much merely considering we're alive: about everything we do on Earth is powered by solar radiation beamed toward our planet from the Dominicus through the howling empty darkness of space. But there's plenty of heat radiation on Earth too. Sit near a crackling log burn down and you lot'll feel heat radiating outward and burning your cheeks. You're not in contact with the fire, so the estrus's not coming to you by conduction and, if you're outside, convection probably isn't carrying much toward y'all either. Instead, all the heat you feel travels by radiation—in directly lines, at the speed of light—carried past a type of electromagnetism called infrared radiation.

Why practise some things take longer to heat up than others?

Unlike materials can store more or less rut depending on their internal atomic or molecular structure. Water, for example, can store huge amounts of rut—that'due south one of the reasons we employ it in cardinal-heating systems—though it as well takes a relatively long time to heat up. Metals let rut pass through them very well and heat upward quickly, but they're non so good at storing heat. Things that shop heat well (like water) are said to take a high specific oestrus chapters.

The idea of specific oestrus capacity helps u.s. empathise the difference between heat and temperature in another way. Suppose you place an empty copper saucepan on top of a hot stove that's a certain temperature. Copper conducts heat very well and has a relatively depression specific heat capacity, so it heats up and cools downward extremely quickly (that's why cooking pots tend to have copper bottoms). Just if you fill the same pan with water, it takes far longer to heat up to the same temperature. Why? Because yous need to supply much more heat energy to raise the temperature of the h2o past the same amount. Water's specific heat capacity is roughly 11 times higher than copper's, and so if yous have the same mass of water and copper, it takes 11 times as much energy to heighten the temperature of the water by the same number of degrees.

Chart: Everyday materials have very unlike specific heat capacities. Metals (blue) accept low specific estrus capacities: they conduct heat well and store it badly, so they feel cold to the bear upon. Ceramic/mineral materials (orange) have higher specific heat capacitors: they don't conduct oestrus likewise as metals, store it better, and feel slightly warmer when yous touch them. Organic insulating materials (green), such every bit wood and leather, carry heat very poorly and store information technology well, so they feel warm to the touch. With very high specific oestrus capacity, h2o (yellow) is in a class of its own.

Specific estrus capacities can aid y'all understand what happens when you estrus your home in different means in winter-fourth dimension. Air heats up relatively rapidly for two reasons: first, considering the specific heat capacity of air is near a quarter of h2o'due south; 2d, because air is a gas, information technology has relatively fiddling mass. If your room is freezing and you turn on a fan (convection) heater, you'll find everything seems to warm up very quickly. That's because yous're essentially just heating up the air. Plow off the fan heater and the room will absurd down pretty fast too because the air, past itself, doesn't accept much ability to store heat.

Photo: The wooden spoon feels much warmer than the metal 1, fifty-fifty though both are the same temperature. The metallic spoon conducts rut more than readily from your hand, and it's this that makes it feel colder.

And so how do you go your room actually warm? Don't forget that there isn't but air in information technology that y'all need to heat up: at that place's solid furniture, carpets, curtains, and lots of other things likewise. Information technology takes much longer to heat these things upwardly because they're solid and much more massive than the air. The more common cold, solid objects you accept in your room, the more than heat energy you lot take to supply to heat them all upward to a particular temperature. Y'all'll need to heat them up using conduction and radiation as well as convection—and that takes time. But, because solid things store oestrus well, they also accept time to cool downwardly. So, providing you accept decent insulation to stop heat escaping from the walls, windows, and so on, once your room has reached a certain temperature, information technology should stay warm for some time without your having to add together any more heat.

Latent heat

Does more estrus e'er make college temperature? From what we've said so far, you might be forgiven for thinking that giving something more rut always makes its temperature rise. Generally that'southward truthful, but not ever.

Suppose you have a lump of ice floating in a pan of water and y'all place it on your hot stove. If y'all stick a thermometer in the ice-water mixture, you'll find information technology's around 0°C (32°F)—the normal freezing bespeak of h2o. But if you keep heating, you'll find the temperature stays the aforementioned until pretty much all the ice has melted, even though y'all're supplying more heat all the time. It'southward nearly as though the ice-h2o mixture is taking the heat you lot're giving it and hiding it abroad somewhere. Oddly plenty, that'south exactly what's happening!

Artwork: Normally things get hotter (their temperature rises) as you supply more oestrus energy. That doesn't happen at the points when things cook (modify from solid to liquid) and vaporize (plough from liquid to gas). Instead, the energy you supply is used to change the state of matter. The energy doesn't vanish: information technology'southward stored equally latent heat.

When a substance changes from solid to liquid or from liquid to gas, it takes energy to alter its state. To plow solid ice into liquid water, for example you have to push the h2o molecules inside farther apart and break apart the framework (or crystalline structure) that holds them together. Then while ice is melting (in other words, during the change of state from solid water to liquid water ice), all the oestrus energy y'all supply is being used to carve up molecules and none is left over for raising the temperature.

The heat needed to change a solid into a liquid is called the latent oestrus of fusion. Latent means subconscious and "latent rut of fusion" refers to the hidden heat involved in making a substance change state from solid to liquid or vice-versa. Similarly, you lot demand to supply oestrus to change a liquid into a gas, and this is called the latent heat of vaporization.

Latent heat is a kind of energy and, although it may seem to be "hidden," information technology doesn't vanish into thin air. When liquid water freezes and turns back to ice, the latent estrus of fusion is given off again. You tin see this if you cool water systematically. To start with, the temperature of the water falls regularly as you remove heat energy. But at the bespeak where liquid water turns to solid ice, yous'll find water freezes without getting any colder. That's because the latent heat of fusion is existence lost from the liquid equally it solidifies and it's stopping the temperature from falling so apace.

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Source: https://www.explainthatstuff.com/heat.html

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