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Physics > Thermodynamics > Flashcards

Thermodynamics Flashcards

1
Q

Define Adiabatic Change

A

Energy change with no heat transfer

i.e. ΔU+W=0

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2
Q

How does an Adiabatic change usually occur?

A

When a change happens fast enough that there’s not enough time for heat transfer.

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3
Q

How can work be done while internal energy remains constant?

A

If the change happens slowly enough for heat transfer to occur at the same rate.

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4
Q

What causes internal energy in an ideal gas?

A

The kinetic energy of molecules

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5
Q

What assumption about an ideal gas means that the internal energy equals kinetic energy of molecules?

A

There are no forces of attraction between molecules therefore no molecular potential energy

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6
Q

What does temperature of a gas depend on?

A

Internal energy (KE of molecules)

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7
Q

What is the work done by an expanding gas?

A

PΔV

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8
Q

What is the adiabatic equation for an ideal gas?

A

PV^γ=constant

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9
Q

What does γ depend on in the adiabatic equation?

A

The number of atoms in each molecule of the gas

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10
Q

What is the derivation of the equation W=PΔV?

A

W=Force x distance=FΔx=PAΔx=PΔV

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11
Q

Define an ideal gas

A

A gas which obeys the law PV=constant (at constant temperature)

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12
Q

Define Isothermal

A

The internal energy of a gas stays the same

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13
Q

What does the area under any P-V curve represent?

A

Work done on or by the gas

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14
Q

Define the terms in: Q(in)=W+Q(out) for an engine

A

Q(in)=heat transfer to the engine
W=work done by the engine
Q(out)=heat losses to the surroundings

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15
Q

What would happen if there was no condenser in a heat engine?

A

The gas becomes low pressure after transferring energy to the turbines, so without a condenser it would rise and stop the flow of steam. The condenser turns it back to water before this can happen. Therefore energy has to be wasted for it to work.

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16
Q

What is the second law of thermodynamics?

A

It is impossible for heat transfer from a high temperature source to produce an equal amount of work.

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17
Q

What is the efficiency of an engine?

A

Work done / Energy supplied

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18
Q

What are the six stages of a four-stroke petrol engine cycle?

A 
Compression
Ignition
Power
Cooling
Exhaust
Induction
19
Q

Describe the compression stage of a four-stroke petrol engine cycle

A

The air in the cylinder is compressed adiabatically by the piston.

20
Q

Describe the ignition stage of a four stroke petrol engine cycle

A

A spark ignites the mixture, raising the pressure and temperature rapidly.

21
Q

Describe the power stage of a four stroke petrol engine cycle

A

High pressure forces the piston outwards. Work is done adiabatically by the expanding gas.

22
Q

Define each term: Q=ΔU+W

A

Heat transfer = Change in internal energy + Work done

23
Q

What is work done by one engine cylinder in one cycle?

A

W=Q(in)-Q(out)

Area of indicator loop

24
Q

What is the indicated power of an engine?

A

Rate of net work done by the engine/area of indicator loop x number of cycles per second x number of cylinders

25
Q

What is the input power to the engine?

A

Calorific value x fuel flow rate

26
Q

What is the calorific value of fuel?

A

Energy released per unit mass of fuel burned

27
Q

What is the thermal efficiency of an engine?

A

Work done per second/energy supplied per second OR indicated power/input power from fuel combustion OR work output per cycle/heat input at high temperature

28
Q

How is fuel ignited in a Diesel engine?

A

Fuel ignites on contact as it is injected into the cylinder

29
Q

What is brake power?

A

The output power of an engine

30
Q

What is the mechanical efficiency of an engine?

A

Brake power/indicated power

31
Q

What is the friction power?

A

Indicated power-brake power

32
Q

How is brake power calculated?

A

Engine torque x angular speed of drive shaft

33
Q

What is the overall efficiency of an engine?

A

Brake power/input power from fuel combustion OR mechanical efficiency x thermal efficiency

34
Q

In a reversible engine, what is the heat transfer to the high temperature reservoir?

A

Q(in)=W+Q(out)

35
Q

Define a reversible engine

A

A theoretical engine which can be operated in reverse so work done on it causes maximum heat transfer from cold to hot.

36
Q

What is the efficiency of a reversible engine?

A

(T(hot)-T(cold))/T(hot)

37
Q

What is the only way to make a heat engine 100% efficient?

A

T(cold)=0 (which is impossible)

38
Q

Why is thermal efficiency much lower in practice than in theory?

A
  • Indicator loop does not have sharp corners so less work is done (valves do not open and close instantly)
  • Imperfect combustion
  • The expansion and compression strokes are not perfectly adiabatic as assumed in calculations
39
Q

Why is mechanical efficiency much lower in practice than in theory?

A
  • Friction between the moving parts cannot be eliminated

- Oil used to lubricate the bearings in an engine are viscous which means they cause some resistance.

40
Q

Define a heat pump

A

A device that transfers energy from a cold space to a hot space

41
Q

What is the coefficient of performance of a heat pump?

A

Heat transferred, Q(in), into the hot space/the work done, W, to bring about the heat transfer

42
Q

What is the coefficient of performance of a refrigerator?

A

Q(out)/W

43
Q

What are the stages of refrigeration?

A

The refrigerant is in the vapour state and is compressed adiabatically to high pressure. The high pressure vapour condenses in the tubes outside the refrigerator, releasing internal energy to the surroundings. The condensed liquid passes through an expansion valve, causing its pressure and temperature to fall. The liquid evaporates in tubes inside the box, gaining internal energy from objects in the ice box.

44
Q

Define Q(out), Q(in) and W for a refrigerator

A

Q(out) is the energy gained from the ice box. Q(in) is the energy lost to the surroundings. W is the work done by the pump.

Physics (1 decks)

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