Engineering Physics 2

Question 1

Equation showing the relationship between P,T and V

Answer 1

P₁V₁/T₁ = P₂V₂/T₂

PV/T = Const.

Question 2

What is a property of an ideal gas interms of its energy

Answer 2

Energy of the molecules is entirely kinetic and depends only on the temperature

No forces acting between the molecules, no work done against these forces

Question 3

Define a system

Answer 3

A fixed mass of some substance, enclosed by a boundary, outside of which are the surroundings

Question 4

What is the internal energy (U) of a gas

Answer 4

The sum of the total kinetic energy of its constituent particles and (unless it is an ideal gas) the total potential energy of the particles.

Question 5

What is the first law of thermodynamics

Answer 5

Q = ΔU + W

ΔU=Q-W

Q = Energy transferred to the system by heating

W = always negative

U = Internal Energy

Change in internal energy = Energy transferred to the system by heating - Work done by the system

Question 6

What is W in the first law of Thermodynamics

Answer 6

Work done by the system

eg. energy transferred out by compression

Value is negative if energy is transferred out

Question 7

Define a non flow process

Answer 7

A process during which the fluid does not move in or out of the system during the process

Question 8

What is an isothermal process

So what is its restriction

Answer 8

A process through which a system stays at the same temperature

So Change in U = 0

A non-flow process

Question 9

What is the first law of thermodynamics for an isothermal process

Why

Answer 9

Q=W

No change in temperature, so average kinetic energy of particles is constant

So total kinetic energy and therefore internal energy is constant, change in U = 0

Question 10

What is the ideal gas equation for an isothermal process

Answer 10

pV=Const.

Boyles Law

n and T are constant

Question 11

Use the first law of thermodynamics to explain how an isothermal change can happen

Answer 11

If a gas expands and does external work W, an amount of heat Q has to be supplied to the gas in order to maintain its temperature constant, and vice versa.

Question 12

How does pV change as the temperature that the isothermal process takes place at changes

Answer 12

Graph gets further from the origin

Question 13

Equation for work done on a gas during compression

Answer 13

W=pAΔs

Δs = distance moved by the piston

p = pressure

A = Area

Question 14

What is the first law of thermodynamics for a constant pressure process

How do you use the equation for a compression

Answer 14

Q = ΔU + pΔV

For a compression

ΔV is negative as volume is decreasing

pΔV =W is negative as work is being done on the gas

Question 15

What is the first law of thermodynamics for a constant volume process, explain it

Answer 15

ΔU = Q

When the volume is constant, no work has been done on/by the gas

So all thermal energy transferred to the gas will be transferred to the internal energy

Question 16

What is an adiabatic process

Answer 16

An isolated system, where no energy is able to transfer in or out by heating from or to the external environment.

Any work being done on the system increases the internal energy so its temperature increases

Question 17

What is the first law of thermodynamics for a adiabatic process

Explain why

Answer 17

−W = ΔU

As no heat is transfered to the system, Q=0

Question 18

What is The constant of gas atomicity and its symbol

Answer 18

A constant for the particular gas that is dependent on the degrees of freedom

About the no. of atoms in each molecule of the gas, this may add rotational KE to particles, affecting the value of average KE used

Gamma sign

Question 19

2 equations that can be used for an adiabatic process

Answer 19

pxV^{(Constant of atomicity)} = const.

TV^{(Constant of atomicity) - 1} = const.

Question 20

What is the restriction in an Adiabatic process

Answer 20

Q=0

Question 21

What is the restriction in a constant volume process

Answer 21

W=0

No work is done on the gas as the volume doesn’t change - no compressed etc.

Question 22

What is the restriction in

Question 23

What is the ideal gas equation for a constant volume process

Answer 23

p/T = Const.

Question 24

What is the ideal gas equation for a constant pressure process

Answer 24

V/T = Const.

Question 25

pV graph for an isothermal and Adiabatic process

Explain it

Answer 25

Question 26

How to calculate work done from a p-V graph

Answer 26

Area under graph

Question 27

How to calculate area under pV curve

Answer 27

W = Area under curve = ∫_V^V₂ dV

Question 28

What is an indicator diagram

How do you interpret one

Answer 28

Just a pV graph with an arrow to show the direction of the change

So if the arrow is pointing right, volume is increasing so work is being done by the system, hence W is positive

If the arrow is pointing left, volume is decreasung so work is being done on the system, hence W is negative

Area under graph is W

Question 29

What is a cyclic process

Answer 29

A process through which the system goes through 2 or more consecutive changes such that the final state is the same as the initial state.

Question 30

Example of an indicator diagram for a cyclic process

Question 31

Use the concept of cyclic processes to explain the basis on which an engine works by

Answer 31

An engine is meant to force the working substance into such a cycle, so that the work done by the gas exceeds the work done on the gas as it is returned to the starting point.

The net difference means that the engine is doing useful work