Thermal Energy & Heat

What is Temperature? Temperature  measure of the average KE of all the particles within an object

Temperature Conversions o C to o F: o F = 9/5( o C) + 32 o F to o C: o C = 5/9( o F – 32) o C to K: K = o C K to o C: o C = K – 273

Going from Celsius to Fahrenheit Convert 37 o C to o F. o F = 9/5( o C) + 32 o F = 9/5(37 o C) + 32 = = 98.6 o F

Going from Fahrenheit to Celsius Convert 68 o F to o C o C = 5/9( o F – 32) o C = 5/9(68 – 32) = 5/9(36) = 20 o C

Going from Celsius to Kelvin Convert 100 o C to K K = o C K = = 373 K

Going from Kelvin to Celsius Convert 310 K to o C o C = K – 273 o C = 310 – 273 = 37 o C

Thermal Energy  the total energy of the particles in a material  KE - movement of particles  PE - forces within or between particles due to position  depends on temperature, mass, and type of substance

Thermal Energy Which beaker of water has more thermal energy?  B - same temperature, more mass 200 mL 80ºC A 400 mL 80ºC B

Heat Transfer Heat  thermal energy that flows from a warmer material to a cooler material Like work, heat is...  measured in joules (J)  a transfer of energy

Heat Transfer Why does A feel hot and B feel cold? 80ºC A 10ºC B  Heat flows from A to your hand = hot.  Heat flows from your hand to B = cold.

Heat Transfer Heat is always transferred from hot to cold. Insulators slow the transfer of heat due to air pockets. Conductors easily allow the transfer of heat, like metals. Heat is transferred by conduction, convection, and radiation.

Conduction Heat is transferred due to objects touching each other or through collisions. Occurs best in solids. Heat continues to be transferred until both objects reach the same temperature, called a thermal equilibrium.

Convection Transfer of heat through a liquid or gas through moving currents, called convection currents. The cause of wind and weather.

Radiation Transfer of heat through electromagnetic radiation (light from stars or light bulbs). Transferred in all directions. No contact required! Dark or dull objects absorb more than light or shiny objects do.

Heat Transfer Specific Heat (C p )  amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin  units: J/(kg·K) or J/(g·°C)

Heat Transfer Which sample will take longer to heat to 100°C? 50 g Al50 g Cu Al - It has a higher specific heat. Al will also take longer to cool down.

Heat Transfer Q = m   T  C p Q:heat (J) m:mass (g)  T:change in temperature (K or °C) C p :specific heat (J/g·K or J/g.o C)  T = T f - T i – Q = heat loss + Q = heat gain

Heat Transfer Calorimeter  device used to measure changes in thermal energy Coffee cup Calorimeter  in an insulated system, heat gained = heat lost

Heat Transfer A 32-g silver spoon cools from 60°C to 20°C. How much heat is lost by the spoon? GIVEN: m = 32 g T i = 60°C T f = 20°C Q = ? C p = 235 J/kg·K WORK: Q = m·  T·C p m = 32 g = kg  T = 20°C - 60°C = – 40°C  T = 293 K – 333 K = -40 K Q = (0.032kg)(-40 K)(235J/kg·K) Q = J (lost heat, negative)

Heat Transfer How much heat is required to warm 230 g of water from 12°C to 90°C? GIVEN: m = 230 g T i = 12°C T f = 90°C Q = ? C p = J/g· o C WORK: Q = m·  T·C p m = 230 g  T = 90°C - 12°C = 78°C Q = (230 g)(78 o C)(4.184 J/g· o C) Q = 75,061 J (gained heat, positive)