Thermal Energy & Thermal Energy & HeatHeat

What is Temperature?What is Temperature?

Temperature measure of the average KE of all the particles within an object

Temperature ConversionsTemperature Conversions

oC to oF: oF = 9/5(oC) + 32oF to oC: oC = 5/9(oF – 32)oC to K: K = oC + 273K to oC: oC = K – 273

Going from Celsius to Going from Celsius to FahrenheitFahrenheit

Convert 37 oC to oF.oF = 9/5(oC) + 32oF = 9/5(37oC) + 32

= 66.6 + 32

= 98.6oF

Going from Fahrenheit to Going from Fahrenheit to CelsiusCelsius

Convert 68oF to oCoC = 5/9(oF – 32)oC = 5/9(68 – 32)

= 5/9(36)

= 20 oC

Going from Celsius to KelvinGoing from Celsius to Kelvin

Convert 100oC to K

K = oC + 273

K = 100 + 273

= 373 K

Going from Kelvin to CelsiusGoing from Kelvin to Celsius

Convert 310 K to oCoC = K – 273oC = 310 – 273

= 37oC

Thermal EnergyThermal Energy

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 EnergyThermal Energy

Which beaker of water has more thermal energy? B - same temperature, more mass

200 mL

80ºC

A400 mL

80ºC

B

Heat TransferHeat 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 TransferHeat TransferWhy 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 TransferHeat 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.

ConductionConduction

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.

ConvectionConvection

Transfer of heat through a liquid or gas through moving currents, called convection currents.

The cause of wind and weather.

RadiationRadiation 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 TransferHeat Transfer

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/(g·°C)

Heat TransferHeat Transfer

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.

Heat TransferHeat Transfer

Q = m T Cp

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

T = Tf - Ti

– Q = heat loss+ Q = heat gain

Heat TransferHeat Transfer

Calorimeter device used to

measure changes in thermal energy

Coffee cup Calorimeter

in an insulated system,

heat gained = heat lost

Heat TransferHeat TransferA 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

T = 293 K – 333 K = -40 K

Q = (0.032kg)(-40 K)(235J/kg·K)Q = -300.8 J (lost heat, negative)

Heat TransferHeat TransferHow 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= 4.184 J/g·oC

WORK:

Q = m·T·Cp

m = 230 g

T = 90°C - 12°C = 78°C

Q = (230 g)(78oC)(4.184 J/g·oC)Q = 75,061 J (gained heat, positive)