MAE91 Midterm I

First law of thermodynamics

Energy is conserved, energy cannot be created or destroyed, only changed in form.

Second law of thermodynamics

There is a natural flow of energy, from high to low, you can go the other way but never for free.

Define: System

The object of interest defined by some boundary.

A system can be described by _____.

properties

A ____ is defined by a specific set of properties.

state

A ____ describes a system changing from one state to another.

process

Answer the following for a Closed system (control mass). Mass crosses? Energy Crosses? Give an example.

Mass Crosses: No.

Energy Crosses: Yes

Example: Gas sealed in a piston-cylinder

Answer the following for a Open System(control volume). Mass crosses? Energy Crosses? Give an example.

Mass Crosses: Yes

Energy Crosses: No

Example: Pipe section; turbine pump

Answer the following for a Open System. Mass crosses? Energy Crosses? Give an example.

Mass Crosses: No

Energy Crosses: No

Example: Sealed perfectly insulated tank

Define: Property

A macroscopic characteristic of a system, whose value can be read off at a given instant without knowing the systems history.

Define: Microscopic

Tracks individual constituents - molecules, their positions, and velocities.

Define: Macroscopic

Considers the collective behaviour of all constituent members at once, described by a handful of measurable qualities.

What is the feature and an example of : extensive properties

Feature: Depend on the extent of the system, additive.

Example: mass, volume

What is the feature and an example of : intensive properties

Feature: independant of the extent of the system

Examples: Density, temperature, pressure

What happens when you divide an extensive property with another.

You get an intensive property, density = M/V

Define: State

condition of a system as described by a specific set of its properties

Define: equilibirium

When the properties ofa system cease to change, given the constraints imposed on the system.

Define: Process

Describes a system changing from one state to another (most likely b/t equilibrium states).

Define: Quasi-equilibirium (quasi-static)

A process where a system departs from equilibrium only negligibly at every instant, so that each intermediate state itself is an equilibrium state.

Define: Isochoric

constant-volume process

Define: isobaric

Constant-pressure process

Define: Isothermal

constant-temp process

Define: adiabatic

process with no heat transfer

How many Pa in 1 bar

1×10^5 Pa = 1 bar

What is the guage pressure equation?

P_gage = P_abs - P_atm

What is the guage vaccum equation?

P_vac = P_atm - P_abs

Define: Temperature

its the quantity that describes thermal equilibrium: when two objects in contact have no heat transfer between them, they have the same temperature.

Define: Thermometric property

a property that visually changes when tempreture does

Define: Saturation temp(T_sat)

the tempreture at which a pure substance changes phase (liquid←→vapor) at a given pressure.

Define: Saturation Pressure (P_sat)

Pressure at which a pure substance changes phase at a given tempreture.

Define: Isobaric expansion

a thermodynamic process in which a system's volume increases while its pressure remains constant.

Define: Isothermal Expansion

a thermodynamic process in which a gas increases in volume while its temperature stays completely constant

what does v<v_f mean?

compressed liquid

what does v_f<v<v_g mean

two phase mixture

what does v > v_g mean?

superheated vapor

Explain the compressed liquid approximation

A compressed liquid at temperature T can be approximated by saturated liquid at the same temp. v(T,P) = v_f(T)

Most substances contract while freezing, but water ____.

Expands

Explain the triple line of a T-v diagram.

Where all 3 phases, liquid, solid and gas coexist.

Explain the problem and solution pertaining to corresponding states.

Problem: ideal gas law assumes gas molecules have zero volume and expert no intermolecular forces, when in reality this is not true.

Solution: by normalizing actual pressure(P) and temp(T) against a substances critical point values we get dimensionless ratios.

Define: State postulate

the state of a simple compressible system in completely specified by two independent, intensive properties.

“Single compressible system“ is one where the only relevant work is a change in volume.

Energy can only cross the boundary of a closed system in two way…?

Work and heat.

Define: heat(Q)

Energy that is transferred as a result of a difference in temperature between the system and its surroundings.

What does it mean for work to be positive (w>0)

Work is being put out by a system

What does it mean for work to be negative?

Work is done to the system

Equation for work during constant pressure

W = P (V_2 - V_1)

Equation for boundary work during a linear P-V relationship.

W = ½ (P_1 + P_2) (V_2 - V_1)

Define polytropic

Polytropic means a quasi-equilibrium process that follows PV^n equals a constant.

What does it mean for heat to be positive (Q>0)

Heat is being transferred into the system

What does it mean for heat to be negative(Q<0)

Heat is taken away from the system

Define conduction

Heat transferred through a solid or still fluid

Define convection

Heat transferred between a surface and a moving fluid.

Define radiation

Heat transferred by electromagnetic waves

KE, PE and U are _____. Which means?

State functions; their changes only depend on the end states.

W and Q are ___. Which means?

Path functions; their values depend on the process.

What are the characteristics of internal energy during constant volume.

No volume change means no work. So all the heat goes into internal energy.

W = integral (Cv dT)

What are the characteristics of internal energy during constant volume.

The piston can rise, so the gas does the work.

W = P deltaV, Q = deltaU + PdeltaV

Some heat leaves as work, so more heat is needed per kelvin

Define Enthalpy

a thermodynamic property defined as the sum of a systems internal energy plus the product of its pressure and volume.

H = U + PV or h = u + Pv