La Physique Thermique

Question 1

When are two bodies in thermal equilibrium ?

Answer 1

If no net flow of heat btw them when in thermal contact/ if both at same temp

Question 2

How to convert temperature from degrees Celsius to Kelvin? What to take note?

Answer 2

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!

Question 3

What is absolute scale of temperature? What about absolute zero?

Answer 3

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

Question 4

Explain how empirical evidence leads to gas laws and to an absolute scale of temperature

Answer 4

• 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

Question 5

What is ideal gas equation?

Answer 5

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

Question 6

What is an ideal gas?

Answer 6

Gas obeying eqn pV=nRT at all pressure p, vol V & temp T

Question 7

What is one mole?

Answer 7

Amt of substance containing 6.02E23 particles

Question 8

What are the basic assumptions of kinetic theory of gases?

Answer 8

1. A gas consist of large no of molecule
2. Molecules constantly in random motion
3. Molecules collide elastically w container, w each other
4. Duration of collis n negligible compared to time interval btw collis n
5. No imf except during collis n
6. Total vol molecules negligible compared to container vol

Question 9

Give specific heat capacity and heat capacity formula

Answer 9

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)

Question 10

Define specific heat capacity

Answer 10

of substance is heat needed per unit mass per unit temp change to raise temp of substance w/o change in phase of substance

Question 11

Give specific latent heat formula

Answer 11

Q=mL

where
m is mass that changes phase,
L is specific latent heat

Question 12

Define specific latent heat

Answer 12

of substance is heat needed per unit mass to change phase of substance w/o change in temp

Question 13

Define specific latent heat of fusion

Answer 13

of substance is heat needed per unit mass to change phase of substance btw solid & liquid phase w/o change temp

Question 14

Define specific latent heat of vaporisation

Answer 14

of substance is heat needed per unit mass to change phase of substance btw liquid & gas phase w/o change temp

Question 15

Define internal energy of a system

Answer 15

sum of RANDOM distribut n of kinetic & potential energies associated w molecules of system

Question 16

Define First Law of Thermodynamics. Give equation

Answer 16

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

Question 17

Give work done at constant pressure formula. What if not constant pressure?

Answer 17

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

Question 18

Give equation of pressure of gas

Answer 18

p=(Nm<c²>)/3V

where
<c²> is mean square speed of molecules,
N is no. of molecules,
m is mass of one molecule

Question 19

What do you call <c²> and √(<c²>)

Answer 19

<c²> is mean square speed

√(<c²>) is root mean square speed

Question 20

Give mean kinetic energy of molecule formula

Answer 20

Ek=0.5m<c²>=(3/2)kT

where
k is Boltzmann const,
<c²> is mean square speed

Question 21

(Thermal Physics) When is work done on system negative, positive and zero?

Answer 21

• negative if vol increases (expansion)
• positive if vol decrease (compression)
• zero if vol const

Question 22

How does kinetic theory of gases explain pressure exerted by a gas?

Answer 22

• 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

Question 23

Derive pressure exerted by gas formula

Answer 23

• 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>