Transcript Document
Objectives:
• Heat and Thermal Energy • Specific heat • Calorimetry
•
Which of the following is/are forms of energy?
•
A
– Temperature •
B
– Heat •
C
– Cold •
D
– Gasoline •
E
– Light
•
A
– T
emperature
- is a measure •
B
–
Heat
- energy which transfers among particles •
C
–
Cold
- is only the absence of heat •
D
–
Gasoline
- Potential Energy •
E
–
Ligh
t - Electromagnetic
energy
, same as radiation
HEAT
•
What does it mean to “heat up some water” really mean?
•
What does it mean to “chill some water” ?
•
When an object “lost all its heat”?
•
“Heat up some water”
– energy is being transferred into water. Its molecules are moving faster or gaining kinetic energy. • “
Chill some water ”
– heat is transferred out of the water. Its molecules are moving slower or losing kinetic energy.
• • “
losing all its heat ”
- Objects contain no heat only when the molecules lose all their available energy at 0 Kelvin, or (-273 ° C ) . Objects that are frozen, or at 0 ° C still contain heat.
Heat
•
Thermal Internal energy-
the overall energy of motion of the particles that make up an object. •
Heat
- is the energy that flows between two object as a result of temperature difference.
• • Symbol =
(Q)
is used to denote heat • (
Q)
> 0 Heat is positive when thermal energy is added to a system •
(Q)
< 0 Heat is negative when thermal energy is removed from a system. • The
SI unit
of energy is joule
J
SPECIFC HEAT
SPECIFIC HEAT
•
Specific Heat
-The quantity of energy (heat) that must be added to substance to raise the temperature of a unit mass one temperature unit. • How does specific heat describe a substance ’s response to heat?
Specific Heat
• A substance with a high specific heat capacity requires a lot of thermal energy to show a given change in temperature.
• Water is such a substance.
• • • Ex. change 1kg of water • by 1 ° C need to add 4186 J, 1kg of silver 234J
Specific Heat
• Specific heat relates
heat, mass
, and
temperature
• Definition of specific heat symbol
c
•
Specific heat = heat / ( mass x temperature change)
• c = Q / (
m
∆T ) • SI units: J/(kg K) = J (Kg ° C )
Specific Heat
• Calculating the Amount of Heat Entering and Leaving a System • Δ
T
=
T final –T inital
Specific heat
• A 1.0 g sample of copper is heated from 25.0
° C to 31.0
° C. How much heat did the sample absorb?
• Specific heat ( c ) of copper is . 0.385 g/J ° C
Specific Heat
• A 1.0 g sample of copper is heated from 25.0
° C to 31.0
° C. How much heat did the sample absorb?
• Q =
mc ∆T
• Q = (1.0 g)(0.385 g/J ° C)(6.0
° C) • Q = 2.3 J • Specific heat of copper in Kg is 385 J/k
gK
and 1g =001
Kg When heat enters a system, ΔT is positive and so is Q
Calorimetry
•
What is thermal equilibrium?
Calorimetry
• Objects reach thermal equilibrium when they reach the same temperature. • In
calorimetry
two bodies at different temperatures are brought in contact with each other and exchange heat until a common temperature is reached •
Calorimetry
is used to measure specific heat ( c ) .
Calorimeter ( insulated flask)
Calorimetry
•
Conservation of energy calorimetry
• Calorimeter, is insulated flask and measures changes in thermal energy. • Operation of Calorimeter depends on the conservation of energy in an isolated, closed system. • Energy cannot leave or enter an isolated system.
Calorimetry
• • Conservation of energy E w +E B ∆E w + ∆E B = 0 or Q W + Q B = 0 =
constant
• ( ∆E B = ∆E W )
Block water
Calorimetry
• In a calorimeter the change in thermal energy is equal to the heat transferred because no work is done.
• ∆U = Q = mC∆T • Increase of energy of Water = decrease in energy of Block • m w C w ∆T w + m B C b ∆T b = 0
Calorimetry
Difference between temperature ∆T = T f -T i ∆T is positive when Temperature increses T f >T i ∆T is negative when Temperature decreases T f < T i m w C w (T f –T wi ) + m B C b (T f –T i B ) = 0
• What basic principles does calorimetry rely on ?
• 1- The final temperatures of the block and water are equal since the system is in thermal equilibrium • The total energy of the system is conserved
•
Don ’t forget Pre – Lab tomorrow on Calorimetry
• A .50 – kg block of metal with an initial temperature of 54.5 ° C is dropped into a calorimeter holding 1.1kg of water at 20.0
° C. If the final temperature of the block water is 21.4
° C, what is the specific heat of the metal? Assume that the calorimeter can be ignored and that no thermal energy is exchanged with surroundings.
• Q = (1.0 g)(0.385 g/J ° C)(6.0
° C)
• Initially • T b = 54.5 ° C and T w = 20.00 ° C Thermal equilibrium is established T = 21.4
° C Energy flows from the block to the water For the block ∆T b For water ∆T = T W = T f –T i -T b Q block +Q water = 0
• • Q Q block water = m b c b =m w C b ( T (T f f – T b ) –T b ) • m w C w (T f –T wi ) + m B C b (T f –T i B ) = 0 • C b = m w c w ( T f – T b ) / ( m b (T b –T f ) =390J/kg ° C