Chapter 5 Thermal Energy Section 1: Temperature, Thermal Energy, and Heat Section 2: Conduction, Convection, and Radiation Section 3: Using Thermal Energy

Section 1: Temperature, Thermal Energy, and Heat All matter is made of tiny particles—atoms or molecules These particles are in constant, random motion Because they are in motion, these particles have kinetic energy Temperature - the measure of the average kinetic energy of all the particles in that something Temperature scales When we talk about the air temperature or the temperature of something, we typically use a scale based on the physical properties of water The Fahrenheit and Celsius scales Fahrenheit scale - 180o between the freezing and boiling points Celsius scale - 100o between the freezing and boiling points A 1o change on the Celsius scale = a 1.8o change on the Fahrenheit scale

Section 1: Temperature, Thermal Energy, and Heat The Kelvin Scale In science the Kelvin temperature scale is used Based on the concept of absolute zero Absolute zero – temperature at which the random motion of the particles in something ceases 0 K = absolute zero = 273oC Note: that the degree symbol is not used when expressing temperature on the Kelvin scale Converting Between Temperature Scales Converting Fahrenheit to Celsius: Equation: Example: convert 10 oF to oC Solution:

Section 1: Temperature, Thermal Energy, and Heat Converting Celsius to Fahrenheit: Equation: Example: convert 20 oC to oF Solution: Converting Celsius to Kelvin: Converting Kelvin to Celsius: Note: There is no direct conversion from Fahrenheit to Kelvin, you must first convert the temperature in Fahrenheit to Celesius and then convert Celsius to Kelvin.

Section 1: Temperature, Thermal Energy, and Heat Thermal energy – the sum of the kinetic and potential energies of all the particles in something Energy is transferred by collisions between particles Particles within something exert an attractive force on each other; this is the source of potential energy Thermal energy and temperature are related: When the temperature of an object increases, the average KE of the particles increases So, as the temperature increases the thermal energy increases Thermal energy and mass: As long as the temperature does not change, when the mass of an object increases its thermal energy increases Heat – thermal energy that flows from something at a high temperature to something at a lower temperature Heat always flows from hot to cold (2nd Law of Thermodynamics) Specific heat – the amount of heat required to raise the temperature of 1-kg of a substance 1oC or 1K Different materials have different specific heats

Section 1: Temperature, Thermal Energy, and Heat The amount of thermal energy changes when heat flows into or out of an object The heat flow, or change in thermal energy can be calculated: The equation: Q can be positive or negative If heat flows into an object its temperature increases, so Q is positive If heat flows out of an object its temperature decreases, so Q is negative Example: a 0.05-kg silver spoon is heated so that its temperature increases from 20oC to 60oC. What is the change in the thermal energy of the spoon? Solution 𝐐=𝐦𝐜∆𝐓 Where: Q = change in thermal energy (J) m = mass of the material (kg) c = specific heat of the material (J/kgoC) T = change in temperature

Section 2: Conduction, Convection, and Radiation The 2nd Law of Thermodynamics states that energy (heat)can only move in one direction: from an object or substance at a higher temperature to an object or substances at a lower temperature. This movement of energy is called heat transfer. There are three methods of heat transfer:   Conduction – the transfer of energy through matter by the direct contact of particles Example: holding an ice cube in your hand. Heat flows from your hand into the ice. Result: your hand gets cooler and the ice starts to melt. On the molecular level, particles within one substance will collide with each other and so transfer energy. In the case of two different substance, particles from each substance collide with each other and transfer energy Conduction can occur in solids, liquids, and gases, but solids are generally better conductors than liquids or gases, and metals are better conductors than nonmetals (wood, plastic, glass)

Section 2: Conduction, Convection, and Radiation Convection – the transfer of energy by the motion of the heated particles in a fluid Remember, a fluid is any substance that flows, so a fluid can be either a liquid or a gas In convection, the more energetic fluid particles move from one location to another, and carry energy with them As the particles in the fluid move faster, they get farther apart, and the density of the “hot” fluid is less than the density of the surrounding fluid. So, the mass of the “hot” fluid is constant but its volume increases Because the “hot” fluid is less dense, it rises. As it rises it starts to lose heat. As the fluid continues to lose heat it contracts, the volume decreases, and the density increases. Eventually, the fluid loses enough heat that its density is great enough to cause it to sink. This rising and sinking action is a convection current Examples: the flow of magma in plate tectonics, ocean currents, lava lamps

Section 2: Conduction, Convection, and Radiation Radiation – the transfer of energy by electromagnetic waves Example: energy from the Sun When radiation strikes a material, some of the energy is absorbed, some is reflected, and some is transferred through the material Unlike conduction and convection which require a medium in order to transfer energy, radiation requires no medium

Section 3: Using Thermal Energy Heating systems create and control thermal energy and move it from one place to another Types of heating systems include forced-air, radiator, electric, and solar heating Solar collector – a device that transforms radiant energy from the Sun into thermal energy Thermodynamics – the study of the relationships between thermal energy, heat, and work 1st Law of Thermodynamics – if the mechanical energy of a system is constant, the increase in thermal energy of that system equals the sum of the thermal energy transfers into that system and the work done on that system The 1st Law says that in a system total energy is constant but can change form when work is done on the system 2nd Law of Thermodynamics – energy spontaneously spreads from regions of higher concentration to regions of lower concentration Or: energy flows from objects or regions at a higher temperature to objects or regions at a lower temperature Heat engine – a device that converts some thermal energy into mechanical energy Internal combustion engine – a heat engine that burns fuel inside a set of cylinders Refrigerators and air conditioners do work to transfer thermal energy. They reduce the temperature of a system by moving energy from one place to another.