Thermal Energy PS CH 5

Temperature Measure of average kinetic energy of all of the particles in an object How hot or cold and object is Ex. 40 degrees The S.I. Unit for temperature is Kelvin Measured from absolute zero The theoretical lowest temperature possible Cannot actually be achieved Does not include the word “degrees” Zero Kelvin (0 K) is absolute zero Most of the world, and science, uses Celsius Based on water’s boiling and freezing points Water freezes at 0 degrees C and boils at 100 degrees C We use the Fahrenheit scale Strange scale… Not sure how it was defined. Water freezes at 32 degrees F and boils at 212 degrees F

Temperature Conversions Not easily converted from one to another Need 2 different equations and a bit of algebra To get to Kelvin, easiest to first go to Celsius Equations: °F°C: °C=(°F−32)x 5 9 °CK: 𝐾=°C+273 How would we find the equations for K °C and °C °F? What about from °F to K and vice versa?

Example The average temperature for the human body is 98.6 °F. What is this in Celsius and Kelvin? 37°C 310 K

Problem #1 The lowest possible temperature in the universe is 0K, also called absolute zero. What is this in °C and °F? -273°C -459°F Worksheet w/8:00 partner.

Thermal Energy Not the same as temperature, nor is it the same as heat, but related to both The sum of the kinetic and potential energies of all of the particles that make up the object. Recall that all matter is made up of atoms Atoms move spontaneously (without input of energy) The movement of the atoms (KE) determines the thermal energy of that object Can be transferred to other objects or the environment Cannot be created or destroyed Can change forms Measured in Joules (J)

Thermal Energy and Temperature Temperature affects the thermal energy of an object or system If the temperature is increased, the particles move faster and TE increases If the temperature is decreased, the particles move slower and TE decreases

Heat The transfer of TE from one place to another due to a temperature difference Thermal energy moves spontaneously from higher temperature to lower temperature

Specific Heat The specific heat (C) of a material is the amount of energy needed to raise the temperature of 1 kg of the substance by 1 °C. Water has a very high specific heat Requires a lot of energy to change the temperature Makes it a good coolant for vehicles Also results in ice forming on the surface, but little change lower

Changes in Thermal Energy The change in thermal energy of an object depends on the object’s specific heat, the amount of temperature change, and the how much of the object there is. Q=mΔTC Where: Q is the change (quantity) in thermal energy, m is the mass of the object, ΔT is the change in temperature, and C is the specific heat of the object Derive the units… Or just think about it…

Example How much energy is needed to raise the temperature of 100 kg of water (C=4200 J/kg°C) from 10°C to 20°C? 4.2x106J

Practice #1 How much energy is needed to raise the temperature of 10 kg of copper (specific heat= 400J/kg°C) by 5°C? 20,000 J

Practice #2 8x104J of energy are added to 10 kg an unknown substance, and causes its temperature to increase 10°C. What is the substance? Concrete

Practice #3 (Be Careful…) What is the change in thermal energy that will cause 1 kg of steel (specific heat 500J/kg°C) initially at 30°C to go to a final temperature of 25°C? -2500 J Negative because the object cooled Expect something related to this on the test…

Interesting 1 Calorie (as in the calories from food) is the amount of energy needed to raise the temperature of 1 kg of water by 1°C. Calories were originally calculated by burning the foods and seeing how much they raised the temperature of 1 kg of water. Now they use averages of known values based on previous measurements collected by this method Saves time, but not exactly accurate This method is called calorimetry A calorimeter is a device used to measure the specific heat of objects Once the researcher has that, they can calculate the change in thermal energy, and convert this to calories

3Methods of Heat Transfer Conduction-Transfer of energy due to collisions between particles that make up the substance Convection-transfer of energy in a fluid (air or liquid) due to the density difference between cold and hot fluid Radiation- Transfer of energy by electromagnetic waves

Conduction Most effective in thermal conductors Best in solids Materials that heat moves easily through Best in solids In solids, atoms are close together When heated, atoms move faster and bump into one another Good thermal conductors Metal spoon in hot soup The spacing of atoms in liquids and gases make them poor conductors Called thermal insulators Objects that do not easily transfer energy by conduction

Convection More energetic particles move from one place to another As an object is heated, the particles move around more Increases the volume of the object, and thus decreases the density Less dense materials rise, while more dense materials sink Works best in liquids and gases, poorly in solids Warm air rises (air near ceiling is warmest) As it rises, it cools and then sinks It is replaced by rising hot air Same thing occurs in liquids (boiling water) Creates convection currents

Convection currents and Weather Convection in the atmosphere is responsible for wind, clouds, and tornadoes As the cooler air rapidly sinks, it moves across the surface As the warm air rises, it carries moisture, and forms clouds If there are rapid convection currents, they can gain enough rotation to form a tornado High and low pressure areas on weather map

Radiation The only transfer that does not require matter Can move through space Good for us, or wouldn’t be any “us” Radiation can interact with particles Due to particles in liquids and gases being far apart, radiation usually passes through Because it interacts with more of the particles, it can transfer more energy Best in gases, okay in liquids, poor in solids Solids block a lot of the radiation, so only the surface is affected Then moves by conduction

Thermal Insulators Insulators prevent rapid transfer of energy Slows it down Insulators are the opposite of conductors If heat readily conducts, it is a poor insulator Often, pockets of air are used as air is a very poor conductor Thermoses, winter coats, home insulation

Thermodynamics Thermodynamics is the study of the relationships between TE, heat, and work 1st Law of Thermodynamics-The total energy of a system is constant. Energy can be transferred, but cannot be created or destroyed. Restatement of Law of Conservation of Energy Energy can be transferred by heat (transfer of TE) or by doing work on the system

2nd Law of Thermodynamics Energy spontaneously moves from areas of greater concentration to areas of less concentration Hot objects heat cool objects The reverse cannot happen without doing work (inputting energy) on the system

Thermal Energy Transformations As with any energy, it can be transformed, but not created or destroyed Engines burn fuel to create mechanical energy (goal is KE) Solar collectors gather radiation and convert it to potential energy Read more in section 3, but don’t spend tons of time.