Topic 1: Systems And Energy

Question 1

What is an adiabatic process?

Answer 1

An adiabatic process is one that occurs without transfer of heat or matter between a system and its surroundings; energy is transferred only as work.

Question 2

What is a closed system, and when is it ‘fully defined’?

Answer 2

A closed system is a system in which none of the ‘matter’ within the system crosses the boundary of the system, only work and heat.

A closed system is fully defined when the following details are known:

The fluid in the system e.g. air, hydrogen or water

The boundary between the fluid and its surroundings

The mass of the fluid within the boundary

Question 3

What defines the ‘state’ of a closed system?

Answer 3

A state is determined by the properties of the thermodynamic fluid (in a closed system)

Question 4

What defines a ‘Property’ of a closed system?

Answer 4

A ‘property’is a‘characteristic’that can be measured. The characteristic must have a‘unique value’ when the fluid is in a particular state.

The value of a property must be independent of the process (change in value only dependent on initial and final state).

e.g.

Pressure

Volume (or specific volume)

Temperature

Internal Energy

Enthalpy

Entropy

Question 5

What is the basic definition of the state postulate?

Answer 5

When the thermodynamic state of the fluid has been determined by a knowledge of two independent and intensive properties any third thermodynamic property can be found from a relation expressing it as a function of the two know properties (in a simple system).

An extension of this is that if any two independent properties are know, all of the other four can be determined, and thus the state of the system.

Question 6

What does it mean to say that the pressure in a fluid is Isotropic?

Answer 6

To say that pressure in a fluid is isotropic means that the pressure is uniform at all points and boundary interfaces within the fluid.

Question 7

What is the Zeroth Law of Thermodynamics?

Answer 7

Consider systems A, B and C, where A is a special system called a “thermometer”. The zeroth law states that if A is in equilibrium with both B and C, it follows that B and C are in equilibrium. One concludes that if the state of A indicates the (equal) temperatures of B and C, two objects at the same temperature are at thermal equilibrium with each other.

Question 8

What is the basic definition of an Extensive Property?

Answer 8

An extensive property is one that depends upon the size of the system (volume, V, and mass, m, and total energy, E, etc.)

Question 9

What is the definition of an Extensive Property?

Answer 9

Extensive properties are additive. Thus, if the system is divided into a number of sub-systems, the value of the property for the whole system is equal to the sum of the values for the parts. Volume is an extensive property.

Question 10

What is the basic definition of an Intensive Property?

Answer 10

An Intensive Property does not depend upon the size of the system. Intensive properties refer to unit mass.

(T, p, ¯v¯, u, h, s)

Question 11

What is the definition of an Intensive Property?

Answer 11

Intensive properties do not depend on the quantity of matter present. Temperature and pressure are intensive properties.

Specific properties are extensive properties per unit mass and are denoted by lower case letters. For example:

specific volume = V/m = ¯v¯

Specific properties are intensive because they do not depend on the mass of the system.

Question 12

Define equilibrium in a thermodynamic system.

Answer 12

A system, in which the properties have definite and unchanged values, as long as the external conditions remain unchanged, is said to be in a state of equilibrium.

A system in thermodynamic equilibrium satisfies:

mechanical equilibrium (no unbalanced forces)

thermal equilibrium (no temperature differences)

chemical equilibrium.

Question 13

What is 1 bar in N/m² (Pa) and atm?

Answer 13

1 bar = 10⁵ N/m²

= 10⁵ Pa

= 1 / 1.01325 atm

Question 14

Define Heat and explain what role it plays in Thermodynamics.

Answer 14

Simply put:

Heat is not a thing. Heat is a process.

More specifically:

Heat is the transfer of energy between two objects due to temperature differences.

Ultimately, energy is expressed in the motion of substances. If it is moving, it has energy. If it has the capacity to move, there is some potential energy stored away.

We can therefore see, that given that heat is not dependent upon , or a characteristic of, state that is is not a property of the system.

Question 15

What is Temperature?

Answer 15

Simply put:

Temperature is a property which is directly proportional to the kinetic energy of the substance under examination.

It is a measure of the average kinetic energy of the molecules in a system.

Question 16

What is the First Law of Thermodynamics?

Answer 16

When a closed system is taken through a cycle, the net work delivered to the surroundings is proportional to the net heat taken from the surroundings, and vice versa.

ºΣ_{net in} deltaQ =c ºΣ_{net out} deltaW

and

ºΣ_{net out} deltaQ =c ºΣ_{net in} deltaW

Question 17

What is the Non-Flow Energy Equation?

Answer 17

Q + W = /\U

= U₂ - U₁

Question 18

What is the 1^st Corollary of the 1^st Law of Thermodynamics?

Hint: NFEE

Answer 18

There exist a property of a closed system such that a change in its value is equal to the sum of net heat and work transfers during any change of state.

Q + W = U₂ - U₁

Question 19

What is the 2^nd Corollary of the 1^st Law of Thermodynamics?

Answer 19

The internal energy of a closed system remains unchanged if the system isolated from its surroundings.

Often called the Law of Conservation of Energy

“Energy can be neither created nor destroyed, it can only be converted from one form into another.”

Question 20

What is the 3^rd Corollary of the 1^st Law of Thermodynamics?

Answer 20

A perpetual motion machine of the first kind is impossible.

Question 21

What is a process in terms of thermodynamics?

Answer 21

A process transforms a system from one equilibrium state to another - e.g. from T₁, p₁ to T₂, p₂. The transition between start and end states follows a path.

Question 22

What is a Quasi-equilibrium Process in Thermo?

Answer 22

A quasi-equilibrium process is such that the system is always very close to equilibrium as it moves between start and end states.

Question 23

There are six system properties. What are they?

Answer 23

Firstly, a property is a characteristic of a system that can be measured and have a definite value for a given state. They are independent of path.

The six properties are:

pressure (p), temperature (T), volume (V) or specific volume (¯v¯), internal energy (U) or specific internal energy (u), enthalpy (H) or specific enthalpy (h), entropy (S) or specific entropy (s).

Question 24

Define a simple system.

Answer 24

A simple system is one that is pure, i.e. there is only one chemical component. We treat air as pure because there is no significant chemical interaction between its constituents. In addition, a simple system only comprises one phase.

Question 25

Briefly discuss Work and Heat.

Answer 25

Work and heat are not properties of a system.

They are energy in transit from one place to another, from the surroundings to the system.

In other words, work and heat cross the system boundary and change the total energy of the system.

Work and heat are processes.