Physics: Systems, Thermal Physics, and Thermo Flashcards
**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?
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
What does Q mean?
An idea gas is described in terms of state variable:
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
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?
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)
Conduction
one of the three modes of heat transfer
Heat transfer through solids in contact
The rate at which objects conduct electricity varies but what variables does it depend on?
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
Convection
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
Radiation
Heat transfer by emission/absorption of electromagnetic energy
ex. sunlight
When they say energy, think about….
Work done by gas equation:
Temperature
W=PΔV
If you add heat to something, what are the possible effects?
Increase temperature
Phase change
Increase pressure
isothermal expansion
Thermal expansion
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
**When would you think about (1/2)mv^2 = (3/2)KbT
(the v is v average) and concepts related to it?
Ideal gases often used in First Law problems because the only internal energy is KE of the particles, which is measurable by temperature
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)?
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
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?
*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
**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
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.
negative work being done by the gas is that same as ___ pos/neg work being done___on/by the gas
negative work being done by the gas is that same as positive work being done on the gas
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?
Q - W (in other words, ΔE) is the same no matter how you get from state 1 to state 2
