Thermodynamics

Question 1

State the First Law of Thermodynamics

Answer 1

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

Question 2

Work done on the system is shown by

Answer 2

-W

Question 3

Work done by the system is shown by

Answer 3

+W

Question 4

What is meant by an isothermal process in an system?

Answer 4

A change at a constant temperature. Where Q = W

Question 5

What is meant by an adiabatic process in a system?

Answer 5

A change where no heat energy is supplied. U = -W

Question 6

What equation suits a closed system in terms of pressure, volume, and temperature?

Answer 6

pV/T (1) = pV/T (2)

Question 7

How much work is done by a system at constant volume?

Answer 7

Zero

Question 8

On a cyclic process graph, the work done per cycle is

Answer 8

The area of the loop

Question 9

Why don’t the real indicator diagrams have a sharp peak?

Answer 9

Because not all the fuel is burned in real life, so the pressure doesn’t go as high on the diagram.

Question 10

What is friction power?

Answer 10

The power that is needed to overcome the friction.

Question 11

What are the four strokes of an engine?

Answer 11

Induction
Compression
Expansion
Exhaust

Question 12

What is the difference in the compression stroke for diesel and petrol engines?

Answer 12

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

Question 13

When explaining the four strokes, what key things must you include with each step?

Answer 13

• The piston movement
• Volume and pressure
• The inlet and exhaust valves
• Work done on/by the fuel-air/air

Question 14

Define input power of an engine

Answer 14

The amount of heat energy per unit time it can gain from burning fuel

Question 15

How do you calculate Mechanical Efficiency?

Answer 15

Brake power/indicated power

This shows the amount of energy lost through moving parts (ie friction)

Question 16

How do you calculate Thermal Efficiency?

Answer 16

Indicated power/input power

This describes how well heat energy is transferred into work

Question 17

How do you calculate overall efficiency?

Answer 17

Brake power/input power

Question 18

Why are real heat engines’ efficiency lower than their theoretical maximum?

Answer 18

• Frictional forces inside the engine
• Fuel doesn’t burn entirely
• Energy is needed to move internal components of the engine

Question 19

What is a CHP plant?

Answer 19

A Combined Heat and Power Plant tried to limit energy lost through heat

Question 20

How does a CHP Plant maximise its energy usage?

Answer 20

They provide heat energy to nearby businesses, houses etc so less is wasted

Question 21

What is a reverse heat engine?

Answer 21

An engine that transfers heat energy from a cold space into a hot space by doing work on it

Question 22

What is a heat engine?

Answer 22

An engine that uses heat energy to do work, transferring some of that energy to a heat sink

Question 23

Explain the process of refrigerators doing work

Answer 23

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

Question 24

Explain the process of heat pumps doing work

Answer 24

Heat pumps transfer heat energy from the cold space (outside) to the hot space (indoors) to heat the room/water.

Question 25

What is the COP?

Answer 25

The Coefficient of Performance describes how well work is converted into heat transfer

Question 26

What is the Maximum Theoretical Efficiency?

Answer 26

The efficiency of an engine in perfect conditions

Question 27

What is the brake power?

Answer 27

This is the output power from the engine

Question 27

What is the indicated power?

Answer 27

This is the total power (brake power + friction power)

Question 28

What is the friction power?

Answer 28

This is the power used to run the engine. Ie overcoming friction

Question 29

Why can the COP of a reverse heat engine be more than 1?

Answer 29

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.

Question 30

Why is leaving the fridge door open not an effective way of cooling down a room?

Answer 30

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

Question 31

If in doubt, look back at what equation?

Answer 31

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.

Question 32

Constant volume in a engine system at different points means..

Answer 32

That the engine would have to stop for constant volume processes. Mention this as a negative point when we it comes up!