Transcript Thermal Energy

Heat
Chapter 9 &10
Kinetic-molecular Theory
• Matter is made up of many tiny particles that are
always in motion
• In a hot body the particles move faster and thus
have greater kinetic energy than particles in a
cooler body
• The overall energy of motion of the particles that
make up an object is the thermal energy
Temperature
• The average energy of particles in a hot
body is higher than that of particles in a
cold body
• Temperature depends on the average
kinetic energy of the particles in the object
Equilibrium
• Conduction is the transfer of kinetic energy
when particles collide
– Hot hand touching a cold glass will heat the
glass
• Thermal equilibrium happens when the
rate of energy back and forth is equal
– Your hand and the glass are now the same
temp
Temperature Scale
• Kelvin scale is based on absolute zero
(-273° C)
– 0°C = 273 K
– 100°C = 373 K
• Tc + 273 = Tk
• TF = 9/5 Tc + 32
Heat
• Energy that flows between 2 objects
– Symbol for heat is Q
– If Q is negative heat has left
– If Q is positive heat has been absorbed
– Heat is measured in joules
• Conservation of Energy
– ∆Pe + ∆Ke + ∆U = 0 (U = internal energy)
Thermal Energy Transfer
• Conduction
• Convection- motion of fluid caused by
temperature differences (particles are not
in direct contact)
• Radiation- does not depend on the
presence of matter- transfer of energy by
electromagnetic waves
Specific Heat
• The amount of energy that must be added
to a material to raise the temp
• Symbol is C and the unit is J/kg * K
• The heat gained or lost by an object as its
temp changes depends on mass, change
in temp, and specific heat of the substance
– Q = mCΔT = mC(Tf – Ti)
Calorimetry
• A calorimeter is a device used to measure
changes in thermal energy
• The total energy of a calorimeter system is
constant
• Conservation of Energy in a calorimeter
– EA +EB = constant
Calorimetry- Heat Transfer
• In a calorimeter the change in thermal
energy is equal to the heat transferred
because no work is done
• To find the final temp of 2 objects
– Tf = (mACATAi + mBCBTBi)/(mACA + mBCB)
Assignment
• P.303 (1-5)
• P. 316 (1-4)
State of Matter
• Solid → Liquid → Gas
• Increase the thermal energy and the
motion of the particles increases which
increases the temperature
• Heat of fusion is the energy needed to
melt
• Heat of vaporization is the energy need to
turn to a gas
State of Matter
• Heat Required to Melt (heat of fusion)
– Q = mHf
• Heat Required to Vaporize (heat of
vaporization)
– Q = mHv
Latent Heat
• P. 317
• P. 318
Work done by a gas
• W = P∆V
• Work = pressure x change in volume
• Work is done only if the volume changes
• If the gas expands, ∆V is positive and the work
done by the gas on the piston is positive
• If the gas is compressed, ∆V is negative and the
work done by the gas on the piston is negative
Thermodynamics
• First Law of Thermodynamics- The total
increase in the thermal energy of a system
is the sum of the work done on it and the
heat added to it
• Law of conservation of energy- energy is
neither created nor destroyed but can be
changed into other forms
Thermodynamics
• The conversion of mechanical energy to
thermal energy is easy
– Rubbing hands together
• Conversion of thermal energy to
mechanical energy is more difficult
– A heat engine is used to convert thermal to
mechanical energy continuously
Thermodynamics
• Heat flows spontaneously from a warm
body to a cold body
• It is possible to remove thermal energy
from a colder body and add it to a warm
body
– Refrigerator does this with the use of a
mechanical energy
Assignment
• P. 338 (1-2)
• P. 346 (1-3)