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Is it correct to say that a system contains work? why?
2020 oct
It is incorrect to say so because work, like heat, is not a property. It only exists during the transfer from one system to another
If a system is at steady state, does this mean intensive properties are uniform with position troughout the system or constant with time? Both uniform with position and constant with time?
2020 Oct
Steady state means that no properties change with time. During a steady state there can still be intensive properties which can vary from place to place within the system at any moment. It is therefor wrong to say that it the properties are uniform with position during steady state.
Which method of heating is better in terms of operating cost:
Electric resistance baseboard heating
or a Heat pump?
EXPLAIN!
2020 Oct
Heat pump!
Baseboard heaters are 100% efficient whereas heat pumps are 200-300% efficient.
What factors influence the actual coefficient of performance achieved by heat pumps in family residences?
2020 Dec
The COP is determined by the ratio between energy usage of the compressor and the amount of useful cooling at the evaporator (for a refrigeration instalation) or useful heat extracted from the condensor (for a heat pump). A high COP value represents a high efficiency.
The unit J (Joule) is a derived quantity, please give its expression by basic quantities and show the derivation
Dec 2020
From Ep = mgh we get
[J] = [kg] [m/s^2] [m] =[kgm^2/s^2]
A well insulated room with a refrigerator running and door open, the air temperature in the room keeps increasing, why?
Dec 2020
The average room temperature in the room will rise. Of course there will be hotter spots and colder spots but we are talking about the average. The reason is that with the door open the temperature will start to rise inside the refrigerator. The thermostat will kick in and try to cool it back down.
What forms of energy and energy transfer are present in the life cycle of a heat
pump?
Oct 2021
The heat pump system is designed around the concept of latent heat of vaporisation. Thermal energy from the outside atmosphere is moved through a working fluid to the air inside the house. The working fluid (refrigerant) used for most heat pumps nowadays is tetrafluoroethane.
How does the control volume energy rate balance account for work where mass
flows across the boundary?
Oct 2021
Why is it incorrect to say that a system contains heat?
Oct 2021
It is incorrect to say so because heat, like work, is not a property. It only exists during the transfer from one system to another
A well insulated room with a running refrigerator door open, does room temper-
ature increase, decrease, or remain the same? Why? Assumptions: the temperature in the
room is uniform.
Dec 2021
The average room temperature in the room will rise. Of course there will be hotter spots and colder spots but we are talking about the average. The reason is that with the door open the temperature will start to rise inside the refrigerator. The thermostat will kick in and try to cool it back down.
For a heat pump running in winter in a house, is the indoor component a condenser or an evaporator?
Dec 2021
In the heating mode, the indoor coil functions as the condenser and the outdoor coil functions as the evaporator. Air from outside the structure passes over the outdoor coil (the evaporator) and heat is transferred from the air to the coil.
What is an isolated system? Give an example of isolated system.
Dec 2021
A system which can exchange neither matter nor energy with its surroundings is called an isolated system. For example a thermos flask.
Describe the characteristics of a system that is adiabatic. Describe the characteristics
of an isolated system.
Oct 2019
An adiabatic process is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment.
A system which can exchange neither matter nor energy with its surroundings is called an isolated system. For example a thermos flask.
Explain why the specific heat capacity cp and cv for a gas have different values.
Oct 2019
Cp is always more than Cv because when heat is added at constant pressure, the substance expands and works. P ∆V = n R ∆T. This signifies as said above Cp always exceeds Cv by an amount n R [ n is moles of gas and R is the universal gas constant.
List two factors that lead to irreversible processes.
Oct 2019
Some factors that cause a process to become irreversible are following: Friction, Unrestrained expansion, Compression, and Mixing.
