Chapter 10 Energy

Energy What is energy? The potential to do work What is work? Change in self or surroundings Both work and energy are in units of joules (J) What are the forms of energy? Kinetic, Potential, Chemical, Electrical, Nuclear, Sound, Electrochemical, Thermal What is the formula for nuclear energy?

E = m c2

Energy calorie The law of conservation of energy Non-SI unit of energy 1 calorie of energy can raise the temperature of 1 gram of water by 1o C 1 calorie = 4.184 joules 1 cal = 4.184 J 1 Calorie = 1kcalories = 1000 calories The law of conservation of energy Energy cannot be created out of nothing, but it can be changed from one form to another Where does electrical energy come from?

Thermal Energy What is thermal energy? How is thermal energy measured? Internal kinetic energy How fast the particles are moving or vibrating How is thermal energy measured? thermometer

Heat Heat: transfer of thermal energy Thermal energy flows from high to low (high temperature objects transfer thermal energy to objects with lower temperature) Thermal energy is transferred by conduction, convection or radiation

First Law of Thermodynamics The change in the internal energy of a system is equal to the heat added to the system and the work done on the system ΔU = q + w

Second Law of Thermodynamics Heat always from high temperature to low temperature.

Entropy The state of disorder of the particles. Particles are more organized at lower temperatures, therefore less entropy Particles are less organized at higher temperatures, therefore greater entropy Solids Liquids Gasses

Conduction: direct contact

Convection: requires a fluid (liquid or gas)

Radiation: via electromagnetic waves

Radiation: via electromagnetic waves

Change in thermal energy The amount of change in thermal energy can be estimated by determining the change in temperature. Change in Temp = final temp – initial temp ΔT = Tf – Ti also ΔT = T2 – T1

Questions?

Check for understanding 1 calorie = _______ joules A typical 8.0 oz soda could have 110 Calories. That is equivalent to ________ joules. What is the change in temperature of a soda taken from a refrigerator (at 4o C) and consumed (normal body temp = 37o C)? In Antarctica, the temperature can drop from a high in the summer of 288 K to a low of 183 K in the winter. Calculate the temp change from summer to winter. 4.184 460000 33o C -105 K

Heat Capacity When energy is put into or taken away from an object, it can increase or decrease the object’s internal kinetic energy. Not all objects change temperature at the same rate as energy is added or taken away.

Heat Capacity

Specific Heat Capacity The specific heat capacity is the amount of heat per unit mass required to raise the temperature by one degree Celsius. Specific heat can be measured in units of joules per gram oC (J/goC) q = m x C x ΔT C is the specific heat, q is the heat required and ΔT is the change in temperature.

Heat Capacity How much energy is absorbed when 88.0 g of water is heated from 5.00o C to 37.0o C?  q = m x C x ΔT C (of H2O) = 4.184 J/ g oC m = 88.0 g C = 4.184 J / g x C ΔT = 37o C - 5o C = 32o C) q = (88.0g)( 4.184J/go C)(32oC) q = 1.18 x 104 J or 11800 J

Latent Heat What do you expect to see as you add energy to a substance?

Latent Heat Latent heat is the heat released or absorbed by a chemical substance during a change of state that occurs without a change in temperature a change of state, e.g. the melting of ice or the boiling of water

Heat of Vaporization (ΔHvap) Problem Solving Specific Heat (Cp) Latent Heat of Fusion (ΔHfus) Heat of Vaporization (ΔHvap) Water (H2O) 4.18 J/g °C 334 J/g 2257 J/g How much heat (energy transfer) is required to melt 100. g of H20?  Q = m x ΔH Q = 100g x 334 J/g = 33400 J = 33.4 kJ

Review How is internal kinetic energy measured? Thermometer What units are used for temperature? Celsius, Kelvin, Fahrenheit the transfer of thermal energy is Heat What are the units for heat Joules What are the units for specific heat? J/goC

Review What is a calorie? Unit of measure for energy & work The energy required to raise the temp of one gram of water by 1oC What device did we use in the lab to measure heat? Calorimeter How is specific heat calculated? Heat/(mass x temperature change)

Chapter 10 (in a nutshell) 1 cal = ____ joules 4.184 3 Cal = ____ joules 12552.

Thermal expansion In general, if the temperature of a substance increases, so does its volume. This phenomenon is known as thermal expansion. The notable exception to this rule is water

Thermal expansion The most common thermometers use a glass tube containing a thin column of mercury, colored alcohol, or colored mineral spirits. When the thermometer is heated, the volume of the liquid expands. The change in length of the liquid column is proportional to the temperature.

Thermal expansion

isobaric process a thermodynamic process in which the pressure stays constant The addition or removal of energy or any work done on or by the system can alter the total internal energy

isobaric process ΔU = q - w U = internal energy, q = heat, and w = work If the material is able to expand or contract, work is equal to change in volume, w = p x Δv, where p is pressure and v is volume Therefore ΔU = q – (p x Δv)

Heat Engines systems that perform the conversion of thermal energy to mechanical work. A heat source generates thermal energy that brings a substance (e.g. air, water) to a high temperature The heated substance expands and generates work in the engine and transfers the remaining energy as heat to the colder "sink"

Heat Engines

Heat Engines Combustion Engines

Heat Engines Combustion Engines https://www.youtube.com/watch?v=DHUwFuHuCdw https://www.youtube.com/watch?v=wUFmO1vv-oU https://www.youtube.com/watch?v=m64ClfsXe_4

Isovolumetric process Also an isochoric process A thermodynamic process in which the volume remains constant Since the volume remains constant, no work can be done by expansion or contraction Therefore ΔU = q

Adiabatic process a process that occurs without the transfer of heat ΔU = w w = p x Δv Therefore ΔU = -(p x Δv)

Adiabatic process Adiabatic heating occurs when the pressure of a gas is increased from work done on it by its surroundings, e.g. compressing a piston Adiabatic cooling occurs when the pressure of a substance is decreased as it does work on its surroundings.

Refrigeration (adiabatic heating and cooling)

Air conditioning

Santa Ana winds

Efficiency of Heat Engines A measure of how well an engine operates is given by the engine’s efficiency In general, efficiency is a measure of the useful energy taken out of a process relative to the total energy that is put into the process.

Efficiency of Heat Engines The net work of a heat engine can be calculated by comparing the energy put into the work generating part of the engine to the energy put into the cold sink

Efficiency of Heat Engines The efficiency of the heat engine can be calculated by comparing the net work to the amount of energy that had to be put into the engine to do that amount of work