A lesson in heat (and the study of it) Chapter 12 Thermodynamics A lesson in heat (and the study of it) Chapter 12

Kinetic Energy and Atoms Everything is made of moving atoms/molecules Moving things have kinetic energy Kinetic energy causes “warmth” More warmth means more energy More energy/motion means more warmth

What makes a hot body hot? The kinetic-molecular theory: Matter is made up of tiny particles that are always in motion There is a direct relationship between the speed in which the molecules move and the temperature. The warmer it is, the faster the particles move. Think about the molecules in a solid as if they were held together by springs. The springs are the electromagnetic forces that keep the solid together. Thermal energy, then, is the overall energy of motion of the particles that make up an object. Warmer objects have more thermal energy than colder bodies.

Temperature and Heat If the temperature outside is 90 degrees – why do some things feel hotter than others? If the temperature outside is 30 degrees – why do some things feel colder than others?

Heat The energy that transfers from one object to another because of a temperature difference between them ALWAYS goes from high temp to low temp; warmer objects to cooler objects Substances do not contain heat – they contain internal energy

Thermal Energy and Temperature: How can you determine an object’s “hotness” (keep it to Physics!!!)? Temperature!!! A property of an object Depends on the average kinetic energy of the molecules in the object (not the number of particles). Think of it like this – A 1kg object and a 2kg object can have the same temperatures. The thermal energy in an object is proportional to the number of particles it has, but its temperature is not.

Temperature How hot or cold something is compared to a standard Measured by degrees on a scale – Celsius, Fahrenheit or Kelvin Celsius: 0-100 Kelvin: Based on absolute zero; Tc +273 What is absolute zero?

Thermal Equilibrium The temperatures of different objects in contact with each other will always seek to equal out.

Equilibrium and Thermometry Understanding thermometers at a molecular level: Your body is hotter than the thermometer, meaning the thermal energy of your body is higher causing the molecules in your body to move faster. When you put the thermometer in your mouth, energy is transferred from the skin of your mouth to the thermometer by a process called conduction. The thermal energy of your skin goes down and of the thermometer increases.

Examples Making a snowball Holding a cup of hot chocolate

Thermal Expansion When temperature increases, all forms of matter will expand (one exception). Different materials expand at different rates. It’s easiest for gases to expand – why? How does this affect roads and buildings? Thermostats

Thermometers How do you go about measuring temperature? If you think you are sick, you mom might take your temperature, but what causes the liquid in the thermometer to rise? Matter expands when its temperature increases and contracts when it decreases. Place mercury, alcohol, or even water in a glass tube with a scale on the side and you can measure temperature by the expansion and contraction of the liquid.

Measuring Heat Specific Heat (Unit of heat – calorie): Defined as the amount of heat required to raise the temperature of 1 gram of water by 1°C. Kilocalorie =1000 calories ( this is what’s actually used to measure food); they just write it as Calorie (capital c). SI unit for heat – joules ( 1 calorie = 4.184 J)

Expansion of Water The exception – ice cold water contracts when heated (at first), then it begins to expand. 4°C is the magic number. As water freezes and gets below

Conduction Transfer of thermal energy through direct contact of particles. All matter is composed of molecules in constant motion, so all matter can “conduct” heat Think about a soup spoon

Heat Conductors Materials that conduct heat well Solids are better than liquids or gases – why? Which metals are the best conductors? Which is #1?

What are some poor conductors? Metals have “loose” outer electrons, which allows them to conduct heat better than other materials. What are some poor conductors? Wood, plastic, glass, fiberglass, paper Also called what?

Questions If you pulled a wooden spoon and a metal spoon out of the freezer – which one is colder? Is snow a good conductor or an insulator? It’s not a source of heat, but it prevents heat from escaping Do insulators keep heat in or keep cold out?

Convection Transfer of energy in fluids by the movement or flow (currents) of particles in a substance. Solids do not “flow”, but liquids and gases do.

Convection currents produce winds. As air expands – it cools When air is compressed – it heats up

Radiation Transfer of energy through electromagnetic waves, goes through gas and atmosphere Since there are no fluids (like air and water) in space, convection is not possible for transferring the heat from the sun.

Radiation brings heat to our planet and convection works to transfer the heat once it’s inside our atmosphere. When radiation strikes, some of the energy is absorbed, some is transmitted, and some is reflected (the atmosphere surrounding the Earth acts to reflect some of the Sun’s radiant energy. Otherwise we would burn up.

Types of Radiant Energy – Electromagnetic Waves http://www.colorado.edu/physics/2000/waves_particles/ http://imagers.gsfc.nasa.gov/ems/waves3.html Objects at low temperatures emit long wavelengths Objects at high temperatures emit short wavelengths Infrared waves are extremely short, and are absorbed by our skin producing a heat sensation Heat = infrared radiation

Emission of Radiant Energy Good reflectors are poor absorbers Light colored objects reflect more light and heat than dark colored objects Good absorbers are also good emitters Poor absorbers are poor emitters

Specific Heat The amount of energy required to raise the temperature of a unit mass, one temperature unit. Measured in J/kg * K Heat gained or lost by an object as its temperature changes depends on the mass, the change in temperature and the specific heat of the substance. Q = mC∆T (C is NOT celsius here)

Change of State Three states are solids, liquids, and gases. Melting point is the temperature at which a substance goes from a solid to a liquid. Boiling point is the temperature at which a substance goes from a liquid to a gas. Amount of energy needed to melt 1kg of a substance is the heat of fusion. (Q=mHf) Thermal energy needed to vaporize 1kg of a liquid is the heat of vaporization. (Q=mHv)

First Law of Thermodynamics The total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it. Just a restatement of the law of conservation of energy – energy cannot be created nor destroyed, but changed into other forms Device to convert thermal energy to mechanical energy is a heat engine. Refrigerators remove thermal energy from a colder body and add it to a warmer body through mechanical energy. A heat pump is a refrigerator that can be run in two directions.

The Second Law of Thermodynamics: Entropy: measure of the disorder in a system or how much particles want to move from high pressure to low pressure. 2nd Law: natural processes go in a direction that maintains or increases the total entropy of the universe. Predicts that heat flows spontaneously only from a hot body to a cold body.

Newton’s Law of Cooling The rate of cooling of an object depends on how much hotter the object is than the surroundings A warm home will lose heat to the cold outside at a greater rate when there is a larger difference between the inside and outside temperatures The rate of cooling is proportional to the temperature difference between the object and its surroundings Rate of cooling ~  T

Newton’s Law of Cooling The larger the temperature difference of an object the faster it cools. 60° C Temperature 40° C 20° C time

The Greenhouse Effect Your assignment – write a 3 paragraph summary about Global Warming and the Greenhouse Effect using your textbook and one external source (must be cited). Discuss the causes, effects, and future predictions about this phenomena How can we decrease Global Warming? How is global warming similar to the inside of your car heating up? Due Friday