Ch. 6 Thermal Energy

6.1: Temperature and Heat Temperature  measure of the average kinetic energy of the particles in a sample of matter

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

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

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

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.

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/(kg·°C)

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.

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

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

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 Q = (0.032kg)(-40°C)(235J/kg·K) Q = – 301 J

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 = 4184 J/kg·K WORK: Q = m·  T·C p m = 230 g = 0.23 kg  T = 90°C - 12°C = 78°C Q = (0.23kg)(78°C)(4184 J/kg·K) Q = 75,061 J

6.2: Heat Transfer Heat flows from hot to cold.  If you hold something cold, heat flows from hand to object.  If you hold something hot, heat flows from object to hand Conduction- transfer of thermal energy through matter by the direct contact of particles  Occurs because particles are in constant motion  KE transferred as particles collide

Conduction Heating of metal pan-  Particles in handle of pan move slowly  Fast moving particles from the bottom bump into slower particles and speed them up  Occurs until all particles move the same speed Conduction works best in solids- especially metals- because particles are close together

Conduction & Convection Metals- good conductors-because electrons move easily & transfer KE to nearby particles Fluid- any materials that flows Convection- transfer of energy in a fluid by the movement of heated particles Convection currents transfer heat from warmer to cooler parts of a fluid.

Convection vs. Conduction-  Conduction involves collisions and transfers of energy.  Convection involves movement of the energetic particles from one location to another

Convection Convection- results in changes in density  As particles move faster, they get farther apart  Fluid expands as temperature increases  Larger volume = smaller density  Decreasing density causes the rise of the warmer fluid Lava Lamp-  Cool oil = dense = sits on the bottom  Warmer oil = less dense than alcohol & rises  As it rises, it loses energy through conduction Causes decrease in density = sinking

When oil is cool Oil is warm, so it rises Oil starts to lose heat by conduction and falls

Convection Currents Currents in which warm portions of the fluid move through the substance- convection The warm portions transfer energy to the cool section through conduction

Heat Transfer on Earth At equator- earth experiences the most heat from the sun.  Result: evaporation of water and large accumulations of clouds.  As the water vapor rises, it cools and condenses, forming rain After the rain = dry air  Dry air causes moisture to evaporate, drying out the ground – causes desert Convection currents create deserts and rain forests over different regions of Earth

Radiation Transfer of heat to the earth – occurs through radiation Radiation- the transfer of energy by electromagnetic waves. The waves travel through space even without matter

Radiant Energy – Energy transfer by radiation When radiation strikes a substance:  Some energy is absorbed  some is reflected  some is transmitted through the material Amount of energy that is absorbed, reflected and transmitted depends on:  Type of material  Dark absorb more energy than light

To control the flow of heat: Use clothing, blankets, layers of fat, fur, etc. Insulator- material that does not allow heat to flow through easily Gases – like air- are good insulators because:  Gas particles are very far apart & can’t transmit energy through conduction.  If the gas is also held in place, particles can’t move around and warm up the rest of the gas

Insulation Insulation is made of fluffy materials containing pockets of trapped air – prevents heat loss Thermos- vacuum layer between 2 layers of glass  Vacuum contains few particles so conduction & convection don’t occur. Thermos- coated in aluminum  Reflects electromagnetic waves that would either heat the substance or allow the substance to cool

Picture altered from How stuff works.com

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