Ch. 6 Ch. 6 Thermal EnergyThermal Energy

6.1: Temperature and Heat6.1: 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

A400 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 (Cp)

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 Al 50 g Cu

• Al - It has a higher specific heat.• Al will also take longer to cool down.

Q = m T Cp

Q: heat (J)m: mass (kg)T: change in temperature (K or °C)Cp: specific heat (J/kg·K)

T = Tf - Ti

– 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

Ti = 60°C

Tf = 20°C

Q = ?

Cp = 235 J/kg·K

WORK:

Q = m·T·Cp

m = 32 g = 0.032 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

Ti = 12°C

Tf = 90°C

Q = ?

Cp= 4184 J/kg·K

WORK:

Q = m·T·Cp

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 Transfer6.2: Heat TransferHeat 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

ConductionConductionHeating 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 & ConvectionConduction & 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

           

     

     

ConvectionConvectionConvection- 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 fluidLava 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 Oil starts to lose heat by lose heat by conduction conduction

and fallsand falls

Convection CurrentsConvection CurrentsCurrents 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 EarthHeat 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

RadiationRadiationTransfer of heat to the earth – occurs

through radiationRadiation- 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

InsulationInsulationInsulation 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

The end