Ch. 5 - EnergyCh. 5 - Energy

II. Thermal Energy(p.134-137, 141-144) Temperature Thermal Energy Heat Transfer

A. TemperatureA. Temperature

Temperature measure of the

average KE of the particles in a sample of matter

Measuring TemperatureMeasuring Temperature

A thermometer is a instrument for measuring temperature.

Uses a standard for comparison.Examples: Celsius, Fahrenheit, and

Kelvin Scales

Celsius Scale (Celsius Scale (ooC)C)

Based on freezing point (0oC) and boiling point (100oC) of water

Standard metric scale for measuring particle temperature

Kelvin Scale (K)Kelvin Scale (K)

Using absolute zero as a standardAbsolute zero is the temp. where

kinetic molecular motion stopsConversion: (273.15+ oC)= Kelvin

Temperature vs. HeatTemperature vs. Heat

Temperature: A measure of average kinetic

energy of an object Use standard scales: (K), (oC) Temp. is not a measure of the

total amount of energy in an object

Temp. vs. HeatTemp. vs. Heat

Heat: Energy given off or absorbed by

an object Measures the motion of molecules

in an object Measured in Calories

B. Thermal EnergyB. Thermal 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

B. Thermal EnergyB. Thermal Energy

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

200 mL

80ºC

A400 mL

80ºC

B

Thermal Energy Cont.Thermal Energy Cont.

Total energy of the particles in a material

A sum of both kinetic and potential energy

A more massive object has more thermal energy at the same temperature than a less massive object.

C. Heat TransferC. Heat Transfer

Heat thermal energy that flows from

a warmer material to a cooler material

Like work W=F x d, heat is... S.I. unit is Calories Often measured in joules (J) a transfer of energy

C. Heat TransferC. Heat 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.

C. Heat TransferC. Heat 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/(kg·°C)

Specific Heat Cont.Specific Heat Cont.

The ability of a substance to absorb heat energy (specific heat)

Different substances absorb heat at different rates (land versus water)

The greater the mass of an object the more heat it absorbs

C. Heat TransferC. Heat 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.

C. Heat TransferC. Heat Transfer

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

C. Heat TransferC. Heat Transfer

Calorimeter device used to

measure changes in thermal energy

Coffee cup Calorimeter

in an insulated system,

heat gained = heat lost

C. Heat TransferC. Heat 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

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

C. Heat TransferC. Heat 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= 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

Forms of Heat TransferForms of Heat Transfer

Three forms of Heat Transfer:

(6.1- pgs 152- 158) Conduction

Convection

Radiation

ConductionConduction

Conduction involves the transfer of heat through direct contact

Heat conductors conduct heat well, insulators do not

ConvectionConvection

Takes place in liquids and gases as molecules move in currents

Heat rises and cold settles to the bottom

Creates weather patterns and ocean currents

Fluid ConvectionFluid Convection

Currents are formed be the movement of liquids and gases from warmer to cooler areas.

RadiationRadiation

Heat transferred through space

Energy from the sun being transferred to the Earth

Thermal Pollution (5.3- 138-140)Thermal Pollution (5.3- 138-140)

Occurs when waste heat significantly changes the temperature of an environment

Warmed water from factories decreases dissolved oxygen in water