Ch. 6 – Thermal Energy
I. Temperature and Energy Transfer
(p. 158 – 170) Temperature Thermal Energy Heat Transfer
A. Temperature
Temperature measure of the
average KE of the particles in a sample of matter
Thermometers rely on expansion
B. Temperature Scales
Fahrenheit and Celsius most familiar to you Celsius used often in science – why? Kelvin, based on Celsius and absolute zero Absolute zero = temperature at which an
object’s energy is the lowest Celsius – Kelvin Conversion
K = oC + 273 ? K = 32oC
K = 32oC + 273 = 305K
C. Energy Transfer
Molecules of warmer substance are moving faster than molecules of colder substance
As these substances come in contact with each other, energy is transferred from warm to cold objects because of collisions of particles
Example: ice cube in a warm hand
D. 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
D. Thermal Energy
Which beaker of water has more thermal energy? B - same temperature, more mass
200 mL
80ºC
A400 mL
80ºC
B
E. 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
E. 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
A B
F. Thermal Energy Transfer
Conduction
Convection
Radiation
G. Conduction
Conduction = movement of heat energy by direct contact Good conductors =
material through which energy can be easily transferred as heat
Ex: metals, saltwater Opposite of conductor
= insulator – slows the transfer of energy as heat
Ex: cork, wood, glass, air
H. Convection
Convection is the movement of gases or liquids from a warmer spot to a cooler spot
A lava lamp displays convection as heated wax expands and rise and cooler wax falls
I. Radiation
Electromagnetic waves - require no matter to travel through; ex: light, IR, UV
Electromagnetic waves transfer energy = radiation
When sunlight hits earth, its radiation is absorbed or reflected
Darker surfaces absorb more of the radiation and lighter surfaces reflect the radiation
J. Thermal Energy Summary
Room is warmed by radiation
K. Heat Transfer
Specific Heat (C) 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) or J/(g·°C)
K. 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
K. Heat Transfer
q = m C T
q: heat (J)m: mass (kg)C: specific heat (J/kg·K) T: change in temperature (K or °C)
T = Tf - Ti
– q = heat loss+ q = heat gain
K. Heat Transfer
Calorimeter device used to
measure changes in thermal energy
Coffee cup Calorimeter
in an insulated system:
heat gained = heat lost
K. Heat Transfer
A 32.0-g silver spoon cools from 60.0°C to 20.0°C. How much heat is lost by the spoon?
GIVEN:
m = 32.0 g
Ti = 60.0°C
Tf = 20.0°C
q = ?
C = 235 J/kg·K
WORK:
q = m·C·T
m = 32 g = 0.032 kg
T = 20°C - 60°C = – 40°C
q = (0.032kg)(-40°C)(235J/kg·K)q = – 301 J
K. Heat Transfer
How much heat is required to warm 230 g of water from 12.0°C to 90.0°C?
GIVEN:
m = 230. g
Ti = 12.0°C
Tf = 90.0°C
q = ?
C= 4184 J/kg·K
WORK:
q = m·C·T
m = 230 g = 0.23 kg
T = 90°C - 12°C = 78°C
q = (0.23kg)(78°C)(4184 J/kg·K)q = 75,000J
K. Heat Transfer A 0.400 kg sample of glass requires 3190 J for its
temperature to increase from 273 K to 308 K. What is the specific heat for this type of glass?
GIVEN:
m = .400 kg
Ti = 273 K
Tf = 308 K
q = 3190 J
C= ? J/kg·K
WORK:
q = m•C•T
T = 308 K – 273 K = 35 K
(3190 J) = (0.400 kg)(35 K)C
Kkg
JC
35400.
3190
C= 228 J/kg·K
K. Heat Transfer
How much energy would be absorbed by 550 g of copper when it is heated from 24°C to 45°C?
GIVEN:
m = 550 g
Ti = 24°C
Tf = 45°C
q = ?
C= 0.38 J/g·K
WORK:
q = m·T·Cp
T = 45°C - 24°C = 21°C
q = (550g)(21°C)(0.38 J/g·K)q = 4,400 J
CCu= 0.38 J/g·K
Assignments
HomeworkFinish Thermal Energy WS – Work on it now so you can get some help!