Physics: Systems, Thermal Physics, and Thermo Flashcards

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1
Q

**What is heat vs temperature?
What is an intensive property vs extensive property (which is which)?
What is internal energy and what is is directly proportional to?

How does ΔE related to T?

A

Heat: is the transfer of non-mechanical energy b/w system and its environment

  • related to the total thermal energy of an object (measured in joules)
  • Extensive property: depends on the mass of the material ex. if cut 1000J object in half, each part will have 500J
    ex. fire adds heat to the system (E transfer)

Temperature: Macroscopic measure of the average internal (thermal) energy of a system (KE of molecules)
- Related to KEavg of molecules:
(1/2)mv^2 = (3/2)KbT
the v is v average
Intensive property: like density or color, it doesn’t depend upon the amount of the material

Internal energy is the same as thermal energy (heat) and temperature measures internal energy
Ideal gases are often used in First Law problems because the only internal energy is KE of the particles, which is measurable by temperature

***ΔE is directly proportional to T

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2
Q

What does Q mean?

An idea gas is described in terms of state variable:

A

Q is the transfer of thermal energy between a system and its environment Q>0 when heat transfers into the system, Q<0 heat is transferred out of system

A state variable is one of the variables used to describe the state of a dynamical system:
pressure, volume, temp, quantity (moles), entropy
and two gases, or the same gas at different times are said to be identical, in the same state. This is important for graphical problems, e.g. P-V diagrams

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3
Q

The Zeroth Law of Thermodynamics

**The first Law of Thermodynamics and equation that relates to it,
how does this relate to heat engine vs heat pump?

A

defines temperature as a fundamental property of any substance. When two substances are in contact, heat transfers between them until they achieve the same temperature, i.e., thermal equilibrium

The first Law of Thermodynamics: (2 ways in which energy can transfer b/w environment and system: heat and work)
Total E of universe is constant, E may be transformed to one form or another, but cannot be created or destroyed)
ΔE = Q - W
Q is pos when heat moving into system, and neg when moving out
W is pos when work being done by the system on the environment and neg when W done by environment on the system
MAKE SURE TO KNOW THESE VARIABLES BC THE OPPOSITE SIGNS FOR W ARE USED FOR CHEM (see pg 200 bottom if want)

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4
Q

Conduction

A

one of the three modes of heat transfer

Heat transfer through solids in contact

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5
Q

The rate at which objects conduct electricity varies but what variables does it depend on?

A

depends on rate of thermal energy transfer (P conductor)
Material itself (metal good conductor)
Increasing length b/w objects decreases P
Increasing change in temperature increases P

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6
Q

Convection

A

Heat transfer through fluid circulation (air can be a fluid)
one of the three modes of heat transfer
Think convection oven -> the forced movement of fluid (air) in the oven due to a fan results in faster heat transfer than in a normal oven that relies on natural convection and conduction
Pumping blood is forced convection

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7
Q

Radiation

A

Heat transfer by emission/absorption of electromagnetic energy
ex. sunlight

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8
Q

When they say energy, think about….

Work done by gas equation:

A

Temperature

W=PΔV

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9
Q

If you add heat to something, what are the possible effects?

A

Increase temperature
Phase change
Increase pressure
isothermal expansion

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10
Q

Thermal expansion

A

Another response to temperature difference is to change their physical dimension -> length and volume
when wear ring in summer ring gets stuck bc finger expand in heat, bridge expands in heat

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11
Q

**When would you think about (1/2)mv^2 = (3/2)KbT

(the v is v average) and concepts related to it?

A

Ideal gases often used in First Law problems because the only internal energy is KE of the particles, which is measurable by temperature

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12
Q

We heat a gas and put it in cylinder with a piston locked in position -> is work done? Is energy transferred? What is ΔE?

How do we describe ΔE internal and Q when we leave it to sit on the table (no flame)?

A

Hot gases tend to expand, and since the piston does not move, no work is done , but energy is transferred at heat (remember 2 ways to transfer E, work and heat)
so ΔE = Q - W but W = 0 so
ΔE internal = Q, Q > 0 (bc adding heat to system) and we know ΔE internal has to increase bc we feel the gas and cylinder are getting hotter, and the additional energy is added to our system in the form of heat Q.

If we leave it on the table, the hot cylinder and gas will cool down as they lose heat to the room, until at room temp again so…

ΔE internal = Q, Q<0
we know that E internal has to decrease bc feel that the gas and cylinder are cooling down, and energy is lost from our system in the form of heat Q

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13
Q

If no heat is exchanged b/w gas and the environment, in general, expanding gases _____ cool/warm, and compressed gases_____ cool/warm.

What happens to
***We heat the gas in the insulated cylinder again but now the piston is allowed to move
What can we say about Q, W, and E internal?

A

*If no heat is exchanged b/w gas and the environment, in general, expanding gases cool, and compressed gases warm.
relates to am I getting heat or am I giving heat
(the equation doesn’t make much sense with this, but logically i can imagine particles crashing like crash when compress so more heat)

**When determining the sign of W in the ΔE internal equation and the sign of the number you plug into W need to consider 1) if the system is doing work or not (determines the sign of number you plug in) and 2) need to consider if gained or lost E internal bc if increase PE of piston you are moving up, that means you are losing E and transferring it to the piston (sign outside of W term)
The hot gas pushes the piston up, so gas doing work (+W) bc applying force over distance and when the piston moves the volume expands, therefore W can be defined as W = PΔV
Since there is an increase in volume, the work is described as a positive value and a pos W is the work done BY system
When the weight on our piston moves up, it gains potential energy and conservation of energy states that energy cannot be created or destroyed, therefore E gained by the weight must come from something else (the hot gas).
As long as the piston is insulated, such that no heat (Q) can go in or out, we have:
ΔE internal = –W, W>0

Now, if after the gas has expanded and cooled as far as it’s going to, we add more weight on the piston so that the piston and weights move down and compress the gas so:
ΔE internal = +W, W<0
E internal has to increase bc energy gained by our system, I think bc piston loses PE
Here, we see gas warms as it is compressed

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14
Q

**What is the sign of work related to system with a heat pump and a heat engine?

Expanding gases warm/cool and compressed gases expand/warm

A

W<0 Heat pump when work is done on the system
***Heat Pump takes heat from somewhere else (doesn’t make its own heat), but heat engine generates its own heat

W>0 Heat engine when the system does positive work

expanding gases cool, and compressed gases warm.

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15
Q

negative work being done by the gas is that same as ___ pos/neg work being done___on/by the gas

A

negative work being done by the gas is that same as positive work being done on the gas

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16
Q

If an ideal gas transitions from one state to another:
Q ≠ 0 or W ≠ 0 or both
How does Q - W compare b/w the two states?

A

Q - W (in other words, ΔE) is the same no matter how you get from state 1 to state 2

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17
Q

Isobaric

Is work done? How do you know from graph? Equation?

A

Process occurs at constant pressure ΔP=0
Imagine PV graph, we can take are under graph as W bc
W =pΔV
ΔE=Q - W
**ΔE=Q - PΔV
make sure to take into account which direction arrow is pointing in for final - initial
This also makes sense with PV = nRT

Heat cylinder such that the volume of gas expands, pushing the piston upward, but the pressure remains constant bc the weight on the piston is held constant
One of four reversible pathways, alone or in combo, used to transition from one state to another:

18
Q

Isochoric Process

Is work done? How do you know from graph?

A

Constant volume ΔV=0
Heating a gas with a locked piston would result in increasing pressure
Makes sense with PV = nRT
No work done bc area under graph is zero and know that since volume didn’t change, then piston didn’t move so no displacement so no work
Bc W=0, we know that
ΔE = Q

19
Q

Isothermal Process

Is work done? How do you know from graph?

A
Constant temperature 
ΔT=0
Isothermal Processes: No temp fluctuation 
Since no temp change then no E change
Inverse relationship b/w P and V 

When heat is allowed to pass freely b/w system and environment, an isothermal process can occur, where temp of the system remains constant
For ex, for gas to expand at constant temp, the pressure must decrease (Boyle’s law) PV=nRT
The work done by the gas on the piston will be equal to the area under the curve
since ΔE is directly proportional to T, we can say that in an isothermal process, ΔE=0 and Q=W

20
Q

Adiabatic Process

What magic word should trigger this?

A

No heat transfer between system and the environment so all energy is transferred as work
“a perfectly insulated system”
This system will not raise the temperature of the gas at all by definition

the previous example with the insulated container is one instance
Q=0 so E = –W

21
Q

How do Cp and Cv related?

A
Cp = Cv + R
Cp constant pressure molar specific heat
Cv is constant volume molar specific heat
not mentioned in class but could be good to know

A constant pressure process requires more heat for an equal change in temp than a constant volume process

22
Q

What are thermodynamic cycles and what can you assume about them when see them?

A

On a PV graph, you see a cycle, starts at one point and goes in circle back to same spot
Sequences of processes that lead a gas back to its state function
This means ΔE=0 for the overall cycle, and thus Q=W
During a cycle, the system either converts heat to work (for a clockwise cycle) or work to heat (for a counterclockwise cycle)

23
Q

*Think about ΔE and T together bc directly proportional

The second law of thermodynamics

If the entropy stays the same during a thermodynamic cycle…

A

The second law of thermodynamics: The entropy of an *isolated system (exchanges neither E nor matter with an outside system) either stays the same or increases during any thermodynamic process

  • If the system is closed, its entropy can decrease but not without a corresponding increase in entropy of its surroundings (environment increases by a greater amount)
  • In a thermodynamic cycle, it is impossible to convert all input heat (disordered energy) into work (ordered energy): it must always output some heat as well

If the entropy stays the same during a thermodynamic cycle, the cycle is said to be reversible, meaning you could return from each state to the previous state by the same path in reverse, in the real world all macroscopic processes are irreversible due to friction and other “loss” effects

Khan academy: every energy transfer that takes place will increase the entropy of the universe and reduce the amount of usable energy available to do work (or, in the most extreme case, leave the overall entropy unchanged). In other words, any process, such as a chemical reaction or set of connected reactions, will proceed in a direction that increases the overall entropy of the universe.

24
Q

What is a closed system?
Open system?
Isolated system?

A

Closed: exchanges energy but not matter with the environment

Open system: An open system can exchange both energy and matter with its surroundings

Isolated system: exchanges exchanges neither E nor matter with an outside system

Human open system: we eat, excrete

25
Q

Equation for Pressure and units (2 modes to write)
1atm = ____kPa
Vector or scalar?

A

P = Fperpendicular/A or F=PA
units [P] = N/m^2 = Pa
Think of submarine in water, feels pressure from all sides
Pressure on one side submarine adds to pressure on other side of submarine, pressure scalar so add it up bc its scalar

26
Q

Pressure can have units of Pa and also…

A

J/m^3

use W=PΔV to help you find units (work is on cheat sheet)

27
Q

Density of water?

A

1000kg/m^3 or 1g/cm^3

28
Q
  • Density of water?

* *Density equation and units -both forms?

A

1000kg/m^3 or 1g/cm^3
p = m/V
In SI: kg/m^3 or g/cm^3

29
Q

**What is specific gravity?

A

Unitless number tells us how dense something is compared to water
specific gravity = density of substance/density of water
sp. gr. = p/pH2O

30
Q

Weight of a fluid equation and what are the units for g?

A

W = mg
Weight/Ffluid = pVsubg bc p=m/V so m = pV
p is density
Use g = N/kg

31
Q

Pressure equation and gauge pressure

Total pressure of something under water?
How does it relate to depth
What is Patm?

A

P=Fperpendicular/A ->
Gauge pressure is the pressure of fluid
Pgauge=pgD
D= depth! Measured surface down!

Ptotal = Patm + Pgauge
Patm = 1*10^5 pascals
Ptotal NOT proportional to depth but Pgauge is
When you triple D you triple Pgauge but not Psurface

32
Q

**What does Fbuoyant equal?
For a floating object, we always have Wobject = ___
How can we use this fact to make AN EXTREMELY HELPFUL EQUATION ON MCAT?
If the object’s density is 3/4 the density of the fluid, then ____ of the object will be submerged (and vice versa)

A

The magnitude of the buoyant force is equal to the weight of the fluid displaced by the object
For a floating object, we always have Wobject = Fbouyant
so *Vsub/V = pobject/pfluid
**If the object’s density is 3/4 the density of the fluid, then 3/4 of the object will be submerged (and vice versa)

33
Q

if object is denser than fluid, object will… if object is less dense than fluid, object will…
If object has same density as fluid, it will…
If object is completely submerged, what equation can we write and simplify? What does this equation mean when the fluid is water?

A

if object is denser than fluid, object will sink (the object’s weight is greater than Fbouyant), if object is less dense than fluid, object will float
If object has same density as fluid, it will be hovering (in static equilibrium) underneath the fluid (submerged i think)

34
Q

pascal’s law about fluid pressure, fluid pushed through

A

*F1/A1 = F2/A2
Fd=Fd -> W1=W2
also Ad=Ad

35
Q

Flow rate equation (2)

What is difference between flow rate and flow speed?

A

f=Av my “fav” formula
Flow rate tells us how much fluid flows per unit time
flow speed tells us how fast the fluid moves

Av=Av

36
Q

What is Bernoulli’s equation and when can you use it? Is the flow speed or flow rate steady? What is the trigger word?

What is the value for Patm and units?

At any two points of equal height, faster fluid flow means ____ higher/lower pressure (venturi effect)
Any fluid exposed to the atm is at atm pressure

A

The equation is basically KE + PE = KE + PE but for fluids
P1 + (1/2)pv1^2 + pgy1 = P2 + (1/2)pv2^2 + pgy2
Use when 1) fluid incompressible (p constant), 2) the flow is smooth (laminar) 3) there’s no friction 4) the flow rate is steady - then total mechanical E will be conserved for the ideal fluid
Trigger word = ideal fluid flow

Patm = 1.01 × 10^5

At any two points of equal height, faster fluid flow means lower pressure (venturi effect)

37
Q

If we punctures a hole in the side of a bucket and wanted to know the speed of eflux out of that hole, what equation should we use?

A

Use Torricelli’s result -> vefflux = square root of 2gD
since Av=Av and A1 is so much larger than A2, that means V1 is SO much smaller than V2 that we can cancel out the (1/2)pv^2 from the first part of the equation
and both points set for this type of problem (one on surface of bucket and one on hole of bucket) are in contact with atm so P1=P2 and you can cross that out of the equation

38
Q

The pressure is ____ higher/lower where the flow speed is greater

A

lower

39
Q

What is stress? equation? What is it similar to? Units?

A

Stress = F/A very similar to pressure!! But not the same bc force in stress equation does not need to be perpendicular

Stress doesn’t have to be perpendicular to the area over which it acts
ex, shear force acts parallel to the areas at the ends
the unit of stress is N/m^2 or pascal (Pa)
IMPORTANT: Stress is inversely proportional to square of the cross-sectional radius

40
Q

Strain

A

As a result of these forces, the object’s shape will change
the ratio of appropriate change in length to the object’s original length is called strain

Tensile or Compressive
strain = ΔL/Lo
Shear strain = X/Lo
X -> distance of shear

Mnemonic = stress is pressure, strain is change

41
Q

Hooke’s law
Young’s modulus
Shear modulus

2 main formulas?

A

Hooke’s law -> stress and strain are proportional
stress = modulus * strain
Young’s modulus -> Y or E
Shear modulus -> S or G

“Flea” formula
Tension/compression ->
ΔL = FLo/EA

“Flag” formula
Shear -> ΔL = FLo/AG

42
Q

Does blood flow faster in capillaries or in aorta?

Pressure in arms or legs greater?

A

Blood flows faster in aorta bc area through capillary bed is much greater and AV=AV

Pressure in legs greater than arms bc legs at more depth and arms closer to heart
also blood in legs have to go against gravity