Thermal Energy

Heat Heat: transfer of thermal energy from one object to another because of temperature differences Heat flows spontaneously from hot objects to cold objects Why does a cold drink become warm?

Temperature Temperature: measure of how hot or cold an object is compared to a reference point Reference points for Celsius are freezing and boiling points of water Reference point for Kelvin is absolute zero which equals 0 K

Temperature Temperature is related to average kinetic energy of particles KE of particles is due to random motion through space As an object heats up the particles move faster = higher KE of particles = higher temperature

Thermal Energy Thermal energy = total potential and kinetic energy of all the particles in an object Depends on the mass, temperature and phase (solid,liquid,gas) of an object

Thermal Energy Example Cup of tea Temp of 110°F KE of particles 80 J Thermal Energy lower Tea pot full of tea Temp of 110°F KE of particles 80 J Thermal energy higher

Thermal Contraction and Expansion As temp decreases the particles move slower causing less collisions Balloon outside on a cold day Thermal Expansion As temp increases the particles move faster and spread apart Balloon brought in from cold into warm room Gases expand more than liquids and liquids expand more than solids Thermal expansion is used in thermometers to show temperature

Measuring Heat Changes Calorimeter is used to measure changes in thermal energy Uses the principle that heat flows from hotter to cooler objects until they reach the same temperature

Specific Heat Specific Heat: Amount of heat needed to raise the temperature of 1 gram of material by 1 degrees Celsius The lower the specific heat the more its temperature rises Unit is joules per gram per degree Celsius J/g°C

Specific Heat Q = m x c x T Q = heat absorbed by material (Joules) m = mass of material (grams) c = specific heat (J/g °C) T = change in temperature

Specific Heat An iron skillet has a mass of 500.0 grams. The specific heat of iron is 0.449 J/g °C. How much heat must be absorbed to raise the skillet’s temperature by 95 degrees Celsius? Q = m x c x T Q = (500) (0.449) (95) Q = 21, 327.5 J

Specific Heat In setting up an aquarium, the heater transfers 1,200,000 joules of heat to 75,000 grams of water which has a specific heat of 4.18 J/g °C. What is the increase in the water’s temperature? 1,200,000 = (75,000) (4.18) (T) Change in temp = 3.8 °C

Specific Heat 34,000 J of heat is absorbed by a 750 g iron skillet when it’s temperature rises from 25°C to 125°C. What is the specific heat of the iron skillet? 34,000 = (750) (c) (100) 34,000 = 75,000 (c) 0.45 J/g°C = c

Heat Transfers Conduction Convection Radiation

Conduction Conduction: Occurs within a material or between materials that are touching Collisions between particles transfer thermal energy without any overall transfer of matter Conduction is slower in gases than in liquids or solids because the particles are farther apart Conduction is faster in metals because the electrons are free to move around

Conduction Thermal conductors: material that conducts thermal energy well Wire rack in oven Pots and pans Aluminum and copper Not all thermal conductors are hot Tile floors Feel cold because they are transferring thermal energy rapidly away from your skin

Conduction Thermal insulators: material that conducts thermal energy poorly Air Wooden spoon Wool Plastic foam cups Use trapped air to slow down conduction

Convection Convection: transfer of thermal energy when particles of a fluid move from one place to another Fluids = liquids or gases Convection currents: fluid circulates in a loop as it heats up and cools down

Radiation Radiation: transfer of energy by waves moving through space All objects radiate energy. As the temperature increases, the rate at which it radiates energy increases

Thermodynamics Study of conversions between thermal energy and other forms of energy Zeroth law: two systems in thermal equilibrium with a third system are in thermal equilibrium with each other

Thermodynamics 1st law: Energy is conserved 2nd law: Thermal energy can flow from colder objects to hotter objects only if work is done on the system 3rd law: Absolute zero cannot be reached