La Physique Thermique Flashcards
When are two bodies in thermal equilibrium ?
If no net flow of heat btw them when in thermal contact/ if both at same temp
How to convert temperature from degrees Celsius to Kelvin? What to take note?
K=C+273.15
where
K is temp in kelvin (K)
C is temp in degrees Celsius (°C)
*NOTE: change in temp is same value for both degrees Celsius and Kelvin!
What is absolute scale of temperature? What about absolute zero?
Absolute scale of temp: temp scale not dependent on property of any particular substance & hv absolute zero
Absolute zero: temp at which all substances hv min internal energy
Explain how empirical evidence leads to gas laws and to an absolute scale of temperature
- fr expts on gas, we discover linear r/s btw vol & temp at const Pa, btw Pa & temp at const vol (gas laws)
- linear r/s extrapolates to same lowest possible temp the absolute zero regardless of type, amt & property of gas
- this results in absolute scale of temp independent of property of any particular substance & hv absolute zero
What is ideal gas equation?
pV=nRT
OR
pV=NkT
where
n is amt gas in mol,
N is no of gas particles,
R is molar gas const,
k is Boltzmann const,
T is temp IN KELVIN (K),
V is vol in m³ ,
p is pressure in Pa
What is an ideal gas?
Gas obeying eqn pV=nRT at all pressure p, vol V & temp T
What is one mole?
Amt of substance containing 6.02E23 particles
What are the basic assumptions of kinetic theory of gases?
- A gas consist of large no of molecule
- Molecules constantly in random motion
- Molecules collide elastically w container, w each other
- Duration of collis n negligible compared to time interval btw collis n
- No imf except during collis n
- Total vol molecules negligible compared to container vol
Give specific heat capacity and heat capacity formula
specific: Q=mcΔθ
non-specific: Q=CΔθ
where
Q is heat supplied,
m is mass,
c is specific heat capacity/C is heat capacity,
Δθ is change in temp (same in Celsius or K)
Define specific heat capacity
of substance is heat needed per unit mass per unit temp change to raise temp of substance w/o change in phase of substance
Give specific latent heat formula
Q=mL
where
m is mass that changes phase,
L is specific latent heat
Define specific latent heat
of substance is heat needed per unit mass to change phase of substance w/o change in temp
Define specific latent heat of fusion
of substance is heat needed per unit mass to change phase of substance btw solid & liquid phase w/o change temp
Define specific latent heat of vaporisation
of substance is heat needed per unit mass to change phase of substance btw liquid & gas phase w/o change temp
Define internal energy of a system
sum of RANDOM distribut n of kinetic & potential energies associated w molecules of system
Define First Law of Thermodynamics. Give equation
state that INCREASE in internal energy of system equal to SUM of heat SUPPLIED to system & work done ON system
ΔU=q+w
where
ΔU is increase in internal energy of system,
q is heat supplied to system,
w is work done on system
Give work done at constant pressure formula. What if not constant pressure?
Work done, W=pV
where
p is pressure (const value),
V is change in vol
*if Pa not const, Pa-vol graph must be given, and area under graph is work done
Give equation of pressure of gas
p=(Nm<c²>)/3V
where
<c²> is mean square speed of molecules,
N is no. of molecules,
m is mass of one molecule
What do you call <c²> and √(<c²>)
<c²> is mean square speed
√(<c²>) is root mean square speed
Give mean kinetic energy of molecule formula
Ek=0.5m<c²>=(3/2)kT
where
k is Boltzmann const,
<c²> is mean square speed
(Thermal Physics) When is work done on system negative, positive and zero?
- negative if vol increases (expansion)
- positive if vol decrease (compression)
- zero if vol const
How does kinetic theory of gases explain pressure exerted by a gas?
- gas consist of large no molecules in constant random motion
- when molecules collide with container walls, they rebound fr wall
- direct n of (velocity +) momentum of molecule change during collis n w wall, so change in momentum of molecule during collis n
- By N2L, force exerted on molecules by wall
- By N3L, molecules exert force on wall, so pressure on wall
Derive pressure exerted by gas formula
- Consider cube vol V, side L containing N molecules of ideal gas. Each molecule hv mass m & component Cx velocity normal to one wall A
- When molecule hit A, rebound elastic with Cx in opposite direct n
Step 1:
mag of change in momentum in one collision is,
Δp = mCx - (-mCx) = 2mCx
- after collis n, molecule moves to opposite wall & back to wall A again over total dist 2L
Step 2: time btw successive collis n w same wall A is,
Δt = 2L/Cx
Step 3:
Force exerted on A by one molecule is
p/t = 2mCx/(2L/Cx) = mCx²/L
- There r N molecules, each diff Cx so avg of Cx² is <Cx² >
Step 4:
TOTAL force exerted on A by N molecule is,
F = (Nm <Cx² >)/L
Step 5: pressure P exerted on A
P = F/A = F/L² = Nm <Cx² >/L³ =
(Nm/V)<Cx² >
- Since motion is random, no preference in any direct n, so
<Cx² > = <Cy² > = <Cz² >
- Also, being components of velocity in 3 dimension, <Cx² > + <Cy² > + <Cz²> = <C²>
-Together, this gives <Cx²> = <C²>/3
Step 6:
Motion is random in 3 dimension, so
<Cx²> = <C²>/3
Thus,
P = Nm/(3V) <C²>
