Option C: Engineering Physics (Thermodynamics) Flashcards
In thermodynamics, what is a system?
A volume of space filled with gas.
What are the two types of system?
- Open
* Closed
What are open systems?
Those that allow gas to flow in, out or through them.
What are closed systems?
Those that don’t allow gas to enter or escape.
What does the first law of thermodynamics describe?
How energy is conserved in a system through heating, cooling and doing work.
State the first law of thermodynamics.
The heat energy supplied to the system either increase the internal energy or enables it to do work.
Give the equation for the first law of thermodynamics.
Q = ΔU + W
Where:
• Q = Heat energy transferred to the system
• ΔU = Change in internal energy
• W = Work some by the gas
In thermodynamics, what is Q?
Heat energy transferred to the system
In thermodynamics, what is ΔU?
Change in internal energy
In thermodynamics, what is W?
The work done by the system (i.e. by the gas)
What is the internal energy of a gas?
The sum of the potential and kinetic energies of all of the particles.
When heat is transferred to the system, what is the sign of Q?
Positive
When heat is transferred away from the system, what is the sign of Q?
Negative
When a gas expands, what is the sign of W?
Positive
When a gas is compressed, what is the sign of W?
Negative
A cylinder is sealed by a moveable piston. The gas in the cylinder is heated with 60J of heat to move the piston. The internal energy of the gas increases by 5J.
a) Calculate the work done by the gas to move the piston.
b) Now the piston does 60J of work on the gas to compress it. No heat is lost. Calculate the change in the internal energy of the gas.
a) W = Q - ΔU = 60 - 5 = 55J
b) ΔU = -W = 60J
What is another name for a closed system?
Non-flow process
What must be assumed when doing calculations about a gas in a closed system?
That the gas is an ideal gas.
What are some of the assumptions when working with an ideal gas?
- Internal energy ONLY depends on the temperature (i.e. potential energy is negligible)
- A change in volume means work is done
- pV = nRT applies
For an ideal gas, what does the internal energy depend on?
- Temperature (and therefore kinetic energy)
* NOT potential energy
Does the potential energy of an ideal gas in a closed system change?
No
What does a change in volume of a gas in a closed system indicate?
That work is being done.
What is the ideal gas law equation?
pV = nRT
Where: • p = Pressure (Pa) • V = Volume (m³) • n = Moles of gas (mol) • R = Gas constant (8.31J/K/mol) • T = Temperature (K)
What is the equation used in working out pressures, volumes and temperatures in a closed system undergoing a change?
p₁V₁ / T₁ = p₂V₂ / T₂
NOTE: Not given in exam