Thermodynamics Flashcards
State the First Law of Thermodynamics
The heat energy (Q) supplied to a system is equal to the increase in the internal energy (U) of the system plus the work done (W) by the system on its surroundings
Work done on the system is shown by
-W
Work done by the system is shown by
+W
What is meant by an isothermal process in an system?
A change at a constant temperature. Where Q = W
What is meant by an adiabatic process in a system?
A change where no heat energy is supplied. U = -W
What equation suits a closed system in terms of pressure, volume, and temperature?
pV/T (1) = pV/T (2)
How much work is done by a system at constant volume?
Zero
On a cyclic process graph, the work done per cycle is
The area of the loop
Why don’t the real indicator diagrams have a sharp peak?
Because not all the fuel is burned in real life, so the pressure doesn’t go as high on the diagram.
What is friction power?
The power that is needed to overcome the friction.
What are the four strokes of an engine?
Induction
Compression
Expansion
Exhaust
What is the difference in the compression stroke for diesel and petrol engines?
In petrol, it is a fuel/air mixture, and it is ignited by a spark plug.
In Diesel engines, it is just air that is compressed (which heats it up) and diesel is sprayed to ignite it
When explaining the four strokes, what key things must you include with each step?
- The piston movement
- Volume and pressure
- The inlet and exhaust valves
- Work done on/by the fuel-air/air
Define input power of an engine
The amount of heat energy per unit time it can gain from burning fuel
How do you calculate Mechanical Efficiency?
Brake power/indicated power
This shows the amount of energy lost through moving parts (ie friction)
How do you calculate Thermal Efficiency?
Indicated power/input power
This describes how well heat energy is transferred into work
How do you calculate overall efficiency?
Brake power/input power
Why are real heat engines’ efficiency lower than their theoretical maximum?
- Frictional forces inside the engine
- Fuel doesn’t burn entirely
- Energy is needed to move internal components of the engine
What is a CHP plant?
A Combined Heat and Power Plant tried to limit energy lost through heat
How does a CHP Plant maximise its energy usage?
They provide heat energy to nearby businesses, houses etc so less is wasted
What is a reverse heat engine?
An engine that transfers heat energy from a cold space into a hot space by doing work on it
What is a heat engine?
An engine that uses heat energy to do work, transferring some of that energy to a heat sink
Explain the process of refrigerators doing work
They take heat energy from the cold space (inside the fridge) and transfer it to the hot space (the room) via the pipes on the back.
This is to store perishable food for longer
Explain the process of heat pumps doing work
Heat pumps transfer heat energy from the cold space (outside) to the hot space (indoors) to heat the room/water.
What is the COP?
The Coefficient of Performance describes how well work is converted into heat transfer
What is the Maximum Theoretical Efficiency?
The efficiency of an engine in perfect conditions
What is the brake power?
This is the output power from the engine
What is the indicated power?
This is the total power (brake power + friction power)
What is the friction power?
This is the power used to run the engine. Ie overcoming friction
Why can the COP of a reverse heat engine be more than 1?
For a heat engine to have a COP more than one, it would have to output more work than the energy taken from the hot reservoir.
It is different for a reverse heat engine as the heat energy it transfers can be greater than the work put in, as the energy goes from cold place to hot place.
Why is leaving the fridge door open not an effective way of cooling down a room?
A fridge does work to transfer heat energy from cold place to hot place.
Opening the door basically combines the hot and cold places, so there is no hot place to transfer heat to, so the system is no longer a reverse heat pump
If in doubt, look back at what equation?
The gas law equation!! In a closed system, n would be constant (as is R always) so you can use this to calculate T, P, or V at a particular point.
Constant volume in a engine system at different points means..
That the engine would have to stop for constant volume processes. Mention this as a negative point when we it comes up!
