Engineering Physics- PART 4

Question 1

What is a heat engine?

Answer 1

A heat engine is a system that converts heat to usable energy which is then used to do mechanical work

Question 2

What is the second law of thermodynamics?

Answer 2

A heat engine requires a source and a sink to operate

Question 3

What is a source?

Answer 3

A source is a high-temperature reservoir

It has a high temperature TsmallH and the heat energy from it is QsmallH

Question 4

What is a sink?

Answer 4

A sink is a low-temperature reservoir

It has a low temperature TsmallC and the heat energy going into it is QsmallC

Question 5

What is another ways of saying the second law of thermodynamics?

Answer 5

Thermal energy cannot spontaneously transfer from a region of lower temperature to a region of higher temperature

Or

When extracting energy from a heat reservoir, it is impossible to convert it all into work

Question 6

What happens when the engine reaches the temperature of the source?

Answer 6

If the engine reached the temperature of the source, no heat would flow as they would have reached thermal equilibrium

Therefore, no work would be done

This means it is impossible for a heat engine to work solely on the First Law of Thermodynamics

Question 7

If a heat engine only obeyed the First Law (there is no friction), what would the source-sink diagram look like?

Answer 7

It is assumed that TH remains at a constant temperature

This engine is 100% efficient however, it is not possible to make this type of engine, due to frictional losses in real life

Therefore, all engines obey the Second Law of

Thermodynamics, and a source-sink diagram for an actual heat engine is

Question 8

Draw a diagram of the actual source-sink diagram for a heat engine that obeys the First and Second Law of Thermodynamics

Answer 8

Heat energy (QH) is transferred from the source at temperature TH

Some of this energy is transferred into work, W

The remaining energy (QC), is transferred to the sink at temperature TC

Question 9

TIPPPPP!!!!!

Answer 9

Remember that heat can only flow from a hot place to cold.

Therefore, if you’re sketching source-sink diagrams, take care with the arrows and make sure you have all the components included (especially the work done, otherwise, the engine wouldn’t work!).

Take care of your terminology:

• Heat engines convert thermal energy into mechanical work (as above)
• Heat pumps transfer heat energy from low temperature to high temperature (this is explored later)

Question 10

What is the goal of a heat engine?

Answer 10

to transfer thermal energy into useful mechanical work as efficiently as possible

Question 11

How can the efficiency of a heat engine be calculated?

Answer 11

[ONLY SECOND GIVEN]

Question 11

What happens as the efficiency of a thermodynamic system increases?

Answer 11

As the efficiency of a thermodynamic system increases, the difference between the temperatures of the source and sink increases

Question 11

Draw a source-sink diagram for a heat engine

Answer 11

Since the efficiency of a heat engine can never be 0 (otherwise there would no work!) this means no heat engine can completely convert heat into work

Question 11

TIPPPP!

Answer 11

This equation is given on your data sheet. Make sure all the variables are in the same units i.e. J or kJ. You must convert these into the same units before you do your calculation.

If the efficiency is asked for as a percentage instead of a decimal, remember to × 100

Question 12

When can the maximum theoretical efficiency of a heat engine equation be used?

Answer 12

if an ideal gas is used as a substance for the engine

Therefore, to make an engine as efficient as possible, the source temperature must be as high as possible, and the sink temperature as low as possible

The maximum theoretical efficiency is 100% only if the sink temperature is at 0 K

Question 13

What is the maximum theoretical efficiency of a heat engine?

Answer 13

Question 13

Do practical or theoretical engines have better efficiencies?

Answer 13

Practical engines have a much lower efficiency than their theoretical equivalent

Question 14

How is the work done to overcome frictional forces within the engine a limitation?

Answer 14

An engine is made up of multiple parts (such as crankshafts and pistons) all in contact with each other which will naturally cause friction

There is also a transfer of energy out of the system by the heating of the cylinder walls that make up the engine

Question 14

TIPPP!

Answer 14

This equation is valid for all idealised reversible engines, irrespective of the particular cycle and the particular working substance. You may be given unfamiliar cycles in the exam (such as the Carnot cycle) to apply this to.

Question 15

What are the limitations of real heat engines?

Answer 15

1. Work done to overcome frictional forces within the engine
2. The fuel is not completely burnt in the process, so the temperature rise isn’t as high as expected
3. The power is used to drive internal components, such as pumps and motors
4. The petrol-air mixture is not an ideal gas
5. Imperfect combustion
6. The processes that form the engine cycle are irreversible

Question 16

How is the fuel not completely burning in the process, so the temperature rise isn’t as high as expected a limitation?

Answer 16

The higher the difference in the temperature between the source and sink, the higher the efficiency

Question 16

How is the power is used to drive internal components, such as pumps and motors a limitation?

Answer 16

This power is therefore not used for useful work

Question 17

How is the petrol-air mixture is not an ideal gas a limitation?

Answer 17

It is actually a mixture of polyatomic molecules, which will sometimes be under high temperatures and pressures

Question 18

How is imperfect combustion a limitation?

Answer 18

The heat energy in the compression stroke is taken not entirely at the single temperature TH and not entirely rejected at the single temperature TC

In reality, the heat is usually taken in over a range of temperatures and rejected also over a range of temperatures

The maximum temperature is therefore not always obtained

Question 19

How is the processes that form the engine cycle are irreversible a limitation?

Answer 19

Energy is dissipated out of the system There is no equilibrium with the surroundings as the processes are too quick The inlet and exhaust values take a finite time to open and close (this gave the 'curved' edges in the actual p-V diagram for the petrol engine) The pistons are always moving, so the heating is not always at a constant volume The compression and expansion strokes are not truly adiabatic, as heat energy is lost from the system

Question 20

In heat engines, is the useful work output (W) is usually less or more than the heat energy transferred to the sink (QC)?

Answer 20

the useful work output (W) is usually less than the heat energy transferred to the sink (QC)

Question 21

What are combined heat and power (CHP) schemes used for?

Answer 21

Combined heat and power (CHP) schemes are used to maximise the useful work output (and hence, power output) and the energy transferred to the source (QH)

Question 22

What is the conventional power station efficiency and theoretical efficiency?

Answer 22

Conventional power stations that use heat engines are in reality, about 35% efficient Their maximum theoretical efficiency is around 61%

Question 23

Describe how combined Heat & Power (CHP) Schemes use energy

Answer 23

They transfer large amounts of energy to their surroundings through cooling towers or a local river or sea Instead of removing this heat through cooling, this heat could then be used to heat homes and businesses which are close by This is used in CHP power stations which are much more heat and energy-efficient

Question 24

In the UK, how are most power plants positioned?

Answer 24

most power plants are naturally positioned far away from homes and businesses so the heat would have cooled down by the time it has reached them, so they are not as popular

Question 25

Draw a diagram of a CHP system

Answer 25

A CHP system in a power plant can generate some electricity, but the huge amount of wasted heat energy is useful for other supplying heat to water or buildings Test yourself

Question 26

What is a reversed heat engine?

Answer 26

A reversed heat engine is one that transfers heat energy from a cold space (sink, at a low temperature) to a hot space (source, at a higher temperature) by inputting work Work needs to be done because heat energy naturally flows from a warmer to a colder space (like in heat engines)

Question 27

What are reversed heat engines used for?

Answer 27

- Refrigerators or air-conditioning - Heat pumps (an engine used to heat a building)

Question 28

What does the source-sink diagram for a reversed heat engine looks like?

Answer 28

Source-sink diagram for a reversed heat engine, notice the direction of all arrows are reversed from the normal source-sink diagram Where: QC = energy extracted from the cold space (J) QH = energy delivered to the hot space (J) W = work inputted (J)

Question 29

What is QH sometimes referred to as?

Answer 29

QH is sometimes referred to as Qout and QC as Qin QH will always be greater than QC for this to work

Question 30

Applying the First Law of Thermodynamics means that...

Answer 30

[NOT GIVEN]

Question 31

How is efficiency calculated for a reversed heat engine?

Answer 31

Efficiency is not calculated the same way as a heat engine, because its effectiveness depends on the device and its function

Question 32

Describe the temperature of a refrigerator?

Answer 32

A refrigerator extracts as much energy as possible from the cold space per joule of work done A refrigerator wants to stay cold, so it is taking the heat out of the system The inside of the fridge is the cold space and the outside is the hot space

Question 33

Describe the temperature of the heat pump?

Answer 33

A heat pump (e.g. to heat a house) provides as much energy as possible to the hot space per joule of work done A heat pump wants to make a room warm, so it puts heat into the system The outside of the house is the cold space and the inside is the hot space

Question 34

Are the heat pump and refrigerator similar?

Answer 34

They are both identical in principle, and it is possible to use one to fulfil the function of the other, but it wouldn't work as well This is because a domestic refrigerator keeps the contents inside it cool, but simultaneously acts as a heater, as it warms up the room it is placed in

Question 35

TIPS!

Answer 35

When answering qualitative questions, be careful to define terms precisely. Be wary of terms such as 'input energy' and 'output energy' without clarifying it further. 'Input energy' could refer to energy input to a room or hot space, or energy input to the device from the cold space, or even energy input in the form of work. Always say where the energy is coming from and going to, so this is clear. You only need to know the basic principles of heat pumps and refrigerators (that they're reversed heat engines). A knowledge of practical heat pumps or refrigerator cycles and devices is not required for your exam. The 'source' in the exam question may be referred to as the 'hot space' or 'hot reservoir' instead of 'source' - look out for this different terminology, they mean the same thing

Question 36

What does the efficiency of a reversed heat engine depend on?

Answer 36

on its purpose

Question 36

What is a coefficient of performance (COP)?

Answer 36

a coefficient of performance (COP) is used to measure the effectiveness of a reversed heat engine instead It is not a measure of efficiency, as it can be greater than 1 The COP is a measure of how effective a reversed heat engine is at transferring heat per unit of work done E.g. A COP of 7 is 7 J of heat energy is transferred per 1 J of work done

Question 37

What is the COP of a refrigerator?

Answer 37

[GIVEN]

Question 38

What is the COP of a heat pump?

Answer 38

[GIVEN]

Question 39

Since QH = QC + W, the COPs could be written in terms of each other...

Answer 39

[NOT GIVEN]

Question 40

What is the unit of the COP?

Answer 40

Because the COP is a ratio it has no units [NOT GIVEN]

Question 41

Why are heat pumps used instead of conventional electric or gas heaters for large-scale buildings?

Answer 41

because the energy transferred by a heat pump exceeds the work done on the pump An electric or gas heater will at most convert 1 J of energy per 1 J of work done, so would be far more expensive to run on large scales

Question 42

TIPPP!!!!!!!

Answer 42

The COP equations in terms of Q and W are included in your data sheet, but not the equation with TH and TC. For these values, you can assume the engine is running at maximum theoretical efficiency. When defining the COP, make sure not to use the words 'heat input' without specifying where the heat is being input to, as this is too vague and will not be accepted by the examiner.