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Heat

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Cassie Henderson-Moulton

on 26 April 2016

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Transcript of Heat

Heat naturally flows from objects of higher temperature to objects of lower temperatures.
Heat, for a long time, was just a measure of
something
that moved from warmer objects to cooler ones. Then, Count Rumford discovered that motion produces heat as well. He connected the idea that motion dictates heat. As dictated by the
kinetic theory of matter
:
Matter is composed of tiny particles called atoms and molecules. These particles are always moving. This random motion of atoms and molecules is present in different degrees in solids, liquids and gases.
Heat
Temperature
To tell how warm or cold and object is, we measure its temperature. Using a thermometer, a colored liquid(usually alcohol) either expands or contracts creating a temperature reading. On the celsius thermometer, 0 degrees is when water freezes and at 100 degrees water boils. (In Fahrenheit: water freezes at 32degrees and boils at 212 degrees).
Absolute Zero
In principle, there is no upper limit to heat. Matter may change into various states, but more heat/energy can always be added. However, substances can only become so cold - due to how gas contracts when it is cooled.
So, What is Heat?
Heat: the thermal energy transferred from one substance to another because of a temperature difference between the two substances.
The Laws of Thermodynamics
When thermal energy transfers as heat, it does so without a net loss or gain. Energy lost in one place is gained in another - therefore we apply these two laws to energy:
First law: Whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred.
Second law: Heat never spontaneously flows from a cold substance to a hot substance.
Third law: No system can reach a temperature of absolute zero.
Heat Transfer
Conduction: heat conduction occurs by means of particles in a solid, mainly electrons. An example would be to heat one end of a nail, the heat moves up and through the entire nail even though its only being heated on one end. Metals are good conductors. Things that don't conduct heat well or at all are called insulators.
Thermal Energy
The warmer an object is, the faster its particles move, meaning that they have more kinetic energy. Also: the warmer an object is, the more thermal energy it contains. Thermal energy in a substance is the total energy of all of its atoms and molecules. Thermal energy consists of all the potential and kinetic energy of the particles in a substance.
Chapter 8
Temperature tells us about the random motion of atoms and molecules in a substance.
Pressure: is defined by the amount of force exerted on a given area. Pressure goes down as gases contract when cooled, and math shows that the coldest temperature a gas can ever get to is -273 degrees. This is called absolute zero. At absolute zero molecules have lost all kinetic energy, and no more energy can be taken from a substance. It can't become any colder. It was by learning this that the Kelvin absolute zero scale was created.
Important: matter does not contain heat. Matter contains thermal energy. Heat can be thought of as: thermal energy in transit.
So what about cold, then?
There is really no such thing as cold. If you feel cold it's because you are loosing thermal energy (in the form of heat).
A unit of heat is called calorie. A calorie is the amount of heat needed to raise the temperature of 1g of water by 1 degree celsius. We measure the energy of our foods/fuels by measuring the amount of energy released when they are burned. (We use kilocalorie or Calorie as our unit).
Entropy
Natural Systems tend to disperse from concentrated and organized-energy states toward diffuse and disorganized states.
Consider: this is why our coffee or tea (that tastes so great when nice and hot) eventually get cold. There is no way to add more heat into my coffee cup unless I add more by putting it in the microwave. Think: processes that disperse energy tend to occur without an input of more energy (think of the wind blowing, it just does it). The opposite holds true as well: processes that result int he concentration of energy tend not to occur unless energy is added
Entropy: The measure of energy dispersal is a quantity known as
entropy
. More entropy means more spreading of energy. This is also known as "order leads to chaos."
Specific Heat
Different substances have different capacities for storing thermal energy. Different materials require different amounts of thermal energy for the same increase in temperature. Each substances has its own specific heat capacity:
The specific heat capacity of any substance is defined as the quantity of heat required to change the temperature of a unit mass of the substance by 1 degree celsius.
Convection: a way that liquids and gases transfer heat is convection, which is a transfer of heat by motion of a fluid - or currents. Warmer fluid moves away from the heat source and cooler fluid moves toward the heat source and is warmed.
Radiation: thermal energy from the Sun travels through space, our atmosphere and to our surface. Heat is transferred not with conduction or convection but though radiation. We call this radiant energy. It takes the form of electromagnetic waves. (This is part of the light we see). The longer the wavelength, the cooler the substance. The shorter the wavelength the hotter the substance.
Homework
Read Chapter 8. Complete pages 150-153. Review 1-20; Think and Explain 1-16; Think and Solve 1-2; Multiple Choice 1-10.
Due: Thursday 4/28.
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