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Heat as a Form of Energy

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SBWEP Saraburi

on 5 June 2013

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Transcript of Heat as a Form of Energy

Heat Heat and Thermal Energy You know that adding heat increases the temperature, but have you ever wondered what exactly "heat" is?

Heat is really just another word for thermal energy that is flowing. In the scientific sense, heat flows anytime there is a difference in temperature. Heat flows naturally from a warmer temperature (higher energy) to a lower temperature (lower energy). Why does this happen? What happens when you hold a chocolate bar in your hand?

Thermal energy flows from your hand into the chocolate bar and it begins to melt. This means that your body (hand) is transferring energy to the chocolate bar causing it to experience a phase change from solid to liquid. This is why Thermal Energy and Temperature. In the previous chapter, we learned that there is a direct relationship between temperature and thermal energy. If the temperature of an object or substance increases, the amount of thermal energy will also increase.

The amount of thermal energy depends on the temperature, but it also depends on the amount of matter you have. Units of Heat and Thermal Energy The joule - The metric unit for measuring thermal energy is the joule (J). This is the same joule used to measure all forms of energy.

The calorie - One calorie is the amount of energy (heat) needed to raise the temperature of 1 gram of water by 1 degree Celsius. One calorie is equal to a little more than 4 joules.

Most of you have probably noticed that food packages list "calories per serving." The unit used for measuring the energy content of the food we eat is the kilocalorie, which equals 1,000 calories. Units of Heat and Thermal Energy The British Thermal Unit - The British Thermal Unit (Btu) is often used to measure the heat produced by heating systems or heat removed by air-conditioning systems. A Btu is the quantity of heat needed to increase the temperature of 1 pound of water by 1 degree Fahrenheit. 1 Btu = a little more than 1,000 J Temperature, Mass, & Material We know that if we are heating and object its temperature increase will depend on the amount of mass, but it also depends on the type of material being heated. It takes different amounts of energy to raise the temperature of different materials.

For example, you need 4,184 J of heat to raise the temperature of 1 kg of water by 1 C, but you only need 470 J to raise the temperature of 1 kg of steel by 1 C.

Specific heat is a property of a material that tells us how much heat is needed to raise the temperature of 1kg by 1 C. Specific heat is measured in joules per kilogram per degree Celsius (J/kg C). o o o Specific Heat o Knowing the specific heat tells you how quickly the temperature of a material will changes as it gains or loses energy. What is the hottest part of this apple pie? The crust or the apple filling? Why is there a difference? Why Specific Heat Varies Why do you guys think the specific heat varies between different materials? Why is it easier to raise the temperature of steel than it is water? Consider two metals: Silver and aluminum. It is easier to increase the temperature of silver than it is aluminum. Aluminum: Specific Heat = 900 J/kg C o Energy is spread over more atoms.
Less energy per atom.
Lower temperature gain per joule. Silver: Specific Heat = 235 J/kg C o Energy is spread over fewer atoms.
More energy per atom.
Greater temperature gain per joule. o Differences in Specific Heat Silver Aluminum In general, materials made up of heavy particles (atoms or molecules) have low specific heat compared to materials made up of lighter ones. Calculating Energy Changes from Heat How could you figure out how much energy it would take to heat a swimming pool or boil a pot of water? The heat equation below tells you how much energy (E) it takes to change the temperature (T) of a mass (m) of a substance with a specific heat value (C ).

Heat Energy (J) : E = mC (T - T ) p p 1 2 Heat Transfer & What happened? Heat transfer happened. In the previous chapter, you all learned that any time there is a difference in temperature between two objects, heat flows (transfers) from the warmer object (the burner) to the cooler object (the hand).

In this section, we will learn about the 3 different ways heat is transferred between objects. Heat Conduction Heat conduction is the transfer of heat by the direct contact of particles of matter. Heat conduction only occurs between two materials with different temperatures and when they are touching. The molecules in the cocoa have a higher average kinetic energy than the molecules in the spoon.The molecules in the cocoa exchange energy with the molecules in the spoon through collisions. The molecules within the spoon spread the energy up the stem of the spoon through the intermolecular forces between them. Thermal Equilibrium As collisions continue, the molecules of the hotter material lose energy and the molecules of the cooler material gain energy. Eventually, both materials are at the same temperature. When this happens, they are in thermal equilibrium.

Thermal equilibrium occurs when two objects have the same temperature. Does heat flow during thermal equilibrium? Why or why not? Thermal Conductors
& Insulators Heat conduction can happen in solids, liquids, and gases. Solids make the best conductors of heat because their particles are packed closely together. The tighter the particles, the more collisions occur. In a gas, the particles are very spread out and fewer collisions occur. This is why many materials used to keep things warm have air pockets. Thermal Conductors Materials that conduct heat well are called thermal conductors. The word "conductor" is also used to describe a material's ability to conduct electrical current. In general, good electrical conductors are also good thermal conductors. Copper Gold Thermal Insulators Material that conducts heat poorly are called thermal insulators. These thermal insulators are also poor conductors of electrical current. Wood Styrofoam Using a Vacuum to Insulate Heat conduction only happens if there are particles to collide with another. Heat conduction does not occur in the vacuum of space. One way to create a great insulator on Earth is to make a vacuum. A vacuum is void of everything, even air. Convection When you watch water boiling in a pot, you can see convection occurring. Bubbles form on the bottom and rise to the top. The hot water at the bottom rises to the top and forces cooler water to sink. This circulation carries heat through the water. This heat transfer process is called convection. Convection is the transfer of heat through the motion of matter such as air and water. Natural Convection Fluids expand when they heat up. Since expansion increases volume but not mass, the density of the liquid is decreased. The cooler surrounding fluid maintains a higher density. Thus, the warmer (less dense) fluid floats to the top and the cooler (more dense) fluid sinks to the bottom. This is natural convection. Forced Convection In some houses a boiler heats water and then pumps circulate the water to rooms. Since the heat is being carried by a moving fluid (matter), this is another example of convection. However, since the fluid is being forced to flow by the pumps, this is called forced convection. Heat transfer, winds, & currents Have you ever looked into the sky and seen a large bird like a buzzard or a hawk soaring higher into the sky without flapping its wings? The bird is riding a thermal. A thermal is a convection current in the atmosphere. A thermal forms when a surface like a blacktop highway absorbs solar radiation and emits energy as heat. The air nearest the road becomes warmer and less dense. The warm air rises and cool air drops to be heated by the warm asphalt. Giant Convection Currents There are giant convection currents in the atmosphere. They for as a result of the temperature differences between the equator and the poles. The warm air rises & flows from the equator to the poles and cool air sinks & flows from the poles to the equator. What is air that is flowing? When air flows horizontally from an area of high density and pressure into an area of low density and pressure, we call the flowing air "wind". Global Wind Cells & Ocean Currents When warm air is moving towards the poles, it doesn't make it all the way there due to Earth's rotation. In fact, the combination of global convection and Earth's rotation sets up a series of wind patterns called global wind cells in each hemisphere. These cells play a large role in shaping weather patterns.

The oceans are also affected by the global wind patterns and Earth's rotation. They cause the surface ocean currents to move in large circular patterns. The ocean currents deeper beneath the surface move slower than surface currents and are driven by temperature and density differences in the ocean. Ocean currents play a big role in heating & cooling some parts of Earth. Thermal Radiation If you are out on a sunny day, you have felt the warmth of the Sun. Heat from the Sun is transferred to Earth by thermal radiation. Thermal radiation is electromagnetic waves produced by objects because of their temperature. All objects with a temperature above absolute zero emit thermal radiation. Ahhh!!!! Too much thermal radiation!!! Where does thermal radiation
come from? Thermal radiation comes from the thermal energy of atoms. As you know, an increase in temperature means an increase in thermal energy. Because the Sun is extremely hot, its atoms emit lots of thermal radiation. Unlike convection & conduction, thermal radiation can travel through the vacuum of space. All the energy Earth receives from the Sun comes from thermal radiation. Space isn't stopping me! Objects emit & absorb radiation All objects with a temperature above absolute zero emit thermal radiation. Objects also absorb radiation. If not, all objects would continue to emit the radiation until their temperature decreased to absolute zero. When an object absorbs, its temperature goes up. When an object emits, its temperature goes down. The temperature adjusts until there is a balance between radiation absorbed & radiation emitted. Are some surfaces are better
absorbers than others? Black surfaces absorb almost all the thermal radiation that falls on them. A silver mirror surface reflects most thermal radiation, absorbing very little. In general, surfaces that are good absorbers are also good emitters. http://www.youtube.com/watch?v=wr8Z4SCETPs&safe=active Real World Applications Specific Heat Heat Transfer Using Heat Transfer to Stop Hypothermia Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as 35.0 °C (95.0 °F)

If left untreated, a person can and will die from hypothermia. Fighting Hypothermia With Heat Transfer You cannot allow this person to lose any more heat!!

Conduction: you must insulate the person from having contact with anything cold (i.e. the ground, wet clothing)

Convection: provide shelter or some other barrier to keep the wind off of the person

Radiation: make certain that all parts of the body are covered (don't forget the head!!) in order to prevent any more emission of body heat How to get more heat out of your fire!! A fire warms your body through thermal radiation and convection. The emitted radiation moves horizontally in all directions of the fire. Your body will absorb the radiation emitted in your direction, but if you know how to use your brain you can get more heat through reflection! How does it work!?! The fire is emitting thermal radiation, but you only absorb the radiation emitted in your direction. If there is a wall behind the fire, the wall will reflect some of the radiation back in your direction giving you even more radiation to absorb and thus, more heat! Snow as an insulator?
You must be joking! While it may seem odd to bury yourself in snow for warmth, the deep snow actually stays around 32 degrees. Even when the air temperature is much colder. How does this work?!? Solid ice is a poor insulator, but snow has a lot of air pockets....even compressed snow. These air pockets reduce conduction. The barrier of the walls prevents convection (cold wind hitting you) and the walls will reflect any thermal radiation that is emitted within the snow cave/igloo. I can make fire from ice....Can you? By taking a piece of ice (the clearer the better) and fashioning it into a lens, you can focus the thermal radiation from the Sun and generate enough heat to make dry kindling smolder and begin to burn! If you have a magnifying glass, you can use it and save some time. Understanding Specific Heat allows us to explore space. Approximately 24,300 LI-900 tiles were installed on each space shuttle to allow them to withstand the tremendous amounts of heat they experience at lift off and when re-entering Earth's atmosphere.

LI-900 has a density of 144.2 kg/m . It is made of pure silica fibers, but 94% of its volume is air so it is incredibly strong but also light.

These tiles are able to withstand temperatures as high as 2,300 F due to their specific heat level of 1,675 J/kg * K. LI-900 Tiles have a very high specific heat 3 o Wrapping a hypothermia victim
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