ShmoopTube

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heat transfer thermal energy never chills out my absolute zero [mumbles]

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Well is there anything better than a campfire on a cold night the [People gather round a campfire]

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smell the warmth the cooking of gooey marshmallow goodness

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whenever you light up a campfire it's pretty obvious that there's a lot of [Men appear in basketball attire]

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heat going on we have to be careful with fire though we can't just go around and

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willy-nilly burning stuff down Scouts don't give out merit badges for arson [Scouts appear by a campfire]

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but when we're sitting six feet away from the fire well how does all that

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heat get to us turns out that heat moves around in a few different ways which

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shows just how complicated heat can be dead hot there well let's do a quick [Man burns hands on fire]

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refresher about what heat is in the first place well heat is the transfer of

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thermal energy from one system to another and that transfer part is really

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important if energy isn't moving well we don't have any heat you may have [Woman with thermal energy t-shirt sits down]

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experienced heat transfer if you've ever touched a pan on the stove you may think

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that pan is hot well you may also think I am not smart for touching that hot pan [Man touching pan]

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but what do we mean when we say something is hot different hot an object

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is hot if it has a lot of thermal energy and we measure the amount of thermal

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energy in an object by taking its temperature orally whenever possible

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thermal energy comes from wiggling okay physicists might not use the term [Molecules vibrating]

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wiggling but all the molecules and atoms that make up an object are constantly [Baby jumping on trampoline]

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vibrating like a sugared up two-year-old the more they wiggle the more thermal

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energy the object has there are three different temperature scales that are

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used right?

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what well those are scales you know anyway here in the US and a few small [Map of globe appears]

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countries like Belize and the Bahamas we use the Fahrenheit scale the rest of

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the world uses Celsius because well it's a simpler scale to understand with [People protesting on road]

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Fahrenheit water freezes at 32 degrees and boils at 212 why is there a hundred

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eighty degrees spread between those two points

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but with Celsius water freezes at zero degrees and boils at 100 it just makes

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more sense doesn't it but we can convert Fahrenheit to Celsius

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with little math first we subtract 32 from the temperature in Fahrenheit then

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we multiply that number by 5 and then divide the result by 9 here's the

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equation in all its glory though there are the two temperature [Thermometer rises]

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scales that we use in day-to-day life but scientists use a different one

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called Kelvin, Kelvin is just like Celsius except zero in Kelvin is zero as

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in no thermal energy no wiggling no vibrating nothing and it turns out that [Molecules stop vibrating and stay still]

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absolute zero is just about equal to negative 273 degrees Celsius which means

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that water freezes at 273 degrees Kelvin and boils at 373 degrees so how do we

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convert from Celsius to Kelvin well we just add 273 you know whatever we have [Boy scout appears in classroom]

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in Celsius way easier than messing around with all that fahrenheit stuff so

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let's get back to how heat actually moves around now people can move around [Scout stood by campfire]

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in all sorts of ways we can drive we can fly we can do the worm yeah but heat [Man doing the worm and cheerleaders scream]

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moves in three ways conduction convection and radiation

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well conduction is a method that we're all familiar with whether we know it or

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not conduction is the transfer of heat between two systems that are in direct [Conduction definition appears]

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contact with each other say that we're trying to be old school scouts by doing

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some blacksmithing yep there used to be a merit badge for blacksmithing but it [Blacksmith hitting iron]

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was discontinued in the 1950s because well who needs a blacksmith anymore I

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mean I order all my horseshoes from Amazon free shipping and everything but [Scout opens van]

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imagine a blacksmith sticking a piece of metal into a fire to get it all hot and

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bothered well what happens at a molecular level here well the

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temperature of the fire is definitely higher than the temperature of the

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metal when the fire comes into contact with the metal the fire sends heat to [Man discussing heat and metal in a field]

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the metal bar the particles on the bar start getting their groove on shaking

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and vibrating like crazy in fact there's basically a molecular

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mosh pit going on and as these molecules slam into other molecules that aren't [Molecules vibrating rapidly]

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touching the fire well those particles also start to move faster that keeps on

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going down the length of the metal it's like basically a giant molecular chain

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reaction you might have experienced conduction in action if you've ever used [Scouts roasting marshmallows]

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a metal skewer to roast a marshmallow maybe a prop the skewer on a log for a

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minute or two with one end in the fire then try to pick it up and that might

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have helped one of your fellow Scouts get a merit badge for first aid when [Scout burns hand on fire]

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they treated your second-degree burn on your fingers their conduction doesn't

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have to involve fire if you've got an electric stove the burner conducts heat

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to a pan because they're in direct contact well that's the key part of [Pan on top of a stove]

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conduction objects have to be all touchy-feely with each other but some

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materials are better at this whole conduction thing than others which is

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why in MIT's exist oven mitts protect our soft tender hands [Man wearing oven mitts]

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because they're made of material that doesn't conduct heat very well these

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materials are called insulators just like insulation in your home most metals

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are good at conducting heat because in addition to atoms that get excited

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metals also have free electrons you they're just bouncing around Footloose [Electrons travelling round a shell]

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and fancy-free going from atom to atom that extra energy from the free

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electrons gets the whole vibration thing going even quicker in insulators though

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the electrons aren't going anywhere they're stuck to the atoms like clingy [Boy attached to woman's leg]

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little brothers so energy doesn't move around very much at all and the best

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insulation of all is a vacuum nope not a dyson we're talking about the absence of [Vacuum appears]

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matter no molecules at all like in outer space that's how thermoses work they've

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got a vacuum in between the inner wall and the outer wall it's not a perfect

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vacuum there are a few molecules of gas popping around but the fewer the

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particles the fewer chances for them to get

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they're all worked up but when we're talking about gases how does heat move

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within them and same question with water it moves differently in fluids and yeah [Scout discussing gases in vacuum]

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fluids aren't just things that are wet anything that flows is a fluid in air

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well yeah gases can definitely flow like carbon dioxide flow and off dry ice heat

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is transferred within fluids using a method called convection let's think

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about a car on a hot sunny day like when you come out from a Sunday matinee and [White car drives and parks outside theater]

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realize that the inside of your car is about 5 million degrees if you open the

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door and look carefully you can actually see the hot air from your interior [Man and woman walk up to car]

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rising and shimmering as it escapes but when a fluid gets hot its molecules

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freak out just like they do in a solid but in a fluid the molecules aren't in

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any kind of rigid matrix their weight less attached to each other so as they [Thermometer rises]

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vibrate they get farther and farther apart from each other which makes them

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less dense and that's density in terms of the amount of mass per volume not

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density like my brain first thing in the morning before I've had coffee when a [Scout holding a cup of coffee]

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fluid loses density it rises because it's lighter than the part of the fluid

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that's colder and denser well that's how a hot air balloon works the heated gas

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in the balloon is less dense than the air outside of the balloon so it's able [Hot air balloon rises into the air]

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to float within a fluid the less dense fluid rises and gravity does its thing

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on the colder material pulling it down that motion creates circulation if we

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have a heat source warming liquid or gas from the bottom the matter close to the

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heat rises which makes the colder matter sink bringing it closer to the heat

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source which makes it lose density and rise up around and around and round and

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round it goes well there's another way convection can occur forced convection

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yeah forced convection happens when an outside force pushes a liquid around

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there are fancy-pants convection ovens that use this method there's a fan [A chicken appears in an oven]

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inside the oven which makes the air circulate keeping the temperature nice

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and even inside another way create forced convection is to chug a soda then [Girl chugs a soda bottle at the bar]

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give yourself a few seconds to generate that mighty belch that's dying to come

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out if chances are the air inside your guts

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is warmer than whatever room you're in so as you exhale you're creating [Girl belches at the bar]

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convection disgusting convection but still well the key difference between

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conduction and convection is movement in conduction only heat moves around from

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one object to another but in convection matter moves along [Man licks a frozen lamp post]

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with the heat okay one more method of heat transfer and it's the scariest

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sounding one radiation yeah now we tend to think of radiation as the thing that [Godzilla appears in Tokyo]

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made Godzilla but good news Godzilla isn't real

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also not all radiation is nuclear radiation, radiation just means energy

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that's transferred by waves or by rays of particles well anything that has a

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temperature gives off electromagnetic waves and matter can absorb those waves [A rock appears with electromagnetic waves]

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too you've seen this radiation if you've ever seen one of those weird infrared [Scout appears in car garage]

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images like this guy this cat we can see how much heat he's given off compared to

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the colder background and we can also see how much colder his nose is than the

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rest of his body...... all right another example of heat transfer by radiation [Sun beaming and ship travelling in the ocean]

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comes from the Sun well the Sun is about 93 million miles away from us but it

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creates a huge amount of heat, heat like around 6000 degrees Kelvin and the

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electromagnetic radiation generated by this thermal energy is what heats our [Electromagnetic radiation appears from the sun]

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planet as hot as the Sun is there's no way for convection to transfer that heat

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throughout the solar system that's because most of outer space is of vacuum

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remember convection needs fluid there's no fluid in space because well there's [Scout wearing astronaut helmet discussing vacuums in space]

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no anything in space it's a vacuum so it's the electromagnetic waves that get

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the atmosphere and everything else all worked up the thermal energy transfer [Man sitting on beach and is burnt by sun]

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doesn't come from direct contact like conduction or from matter moving around

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as it gets excited like convection it's all about the waves here people like I

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said before there's nothing better than a nice toasty [Scout standing by campfire]

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the campfire and we already talked about how conduction can turn roasting a

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marshmallow into a second-degree burn right but would there be convection [Scout burns hand roasting marshmallow]

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going on here and how about radiation well what do we need for convection well

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for one we need a heat source and I'm pretty sure this pile of burning logs

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meets that requirement well then we need fluid and a melting marshmallow well it

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doesn't count but air sure does the air molecules that

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are coming into direct contact with the fire are experiencing conduction but as [Scout discussing air molecules by campfire]

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they rise well the cooler air sinks and then it

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heats up plus the warm air rises and excites some of the cooler air around it

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oh yeah we're 2 for 3 and radiation well if you've ever been camping on a cold

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night you've probably held your hands up to warm them by the fire I'm going to [Scout warming hands by the fire]

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assume you're smart enough to not actually touch the fire and convection

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is all about rising and sinking air it doesn't really do the whole side to side

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thing when you put your hands up to something hot you're benefitting from

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the electromagnetic waves radiating from the heat so we've definitely got [Scout warming hands on fire with electromagnetic waves]

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radiation going on here as well don't worry though it's highly unlikely

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the campfire will turn you into some grotesque irradiant mutant we think and

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the campfire shows how heat can be transferred in all three ways at the [Heat transfer ways from the campfire]

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same time which is usually the case in fact even snuggling up to someone

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involves the conduction of body heat convection going on in the air [Man and woman cuddling on sofa]

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surrounding you and radiation coming from well pretty much everything well

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there's no getting away from heat which is a good thing and there's no getting

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away from physics either even in the scouts there's a merit badge for physics [A scout merit badge appears]

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nuclear physics as a matter of fact which might seem kind of sinister after

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all why do the scouts need nuclear physics I don't worry about it it's all

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part of our plan to take over the world with pinky in the brain and rule with an [Scout discussing nuclear physics and explosion occurs]

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iron fist just remember our motto be prepared yeah be prepared for the Scout

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uprising yes people be very prepared you

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