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Physics > Thermodynamics and Engines (need some editing) > Flashcards

Thermodynamics and Engines (need some editing) Flashcards

1
Q

What is the first law of thermodynamics?

A

Q=ΔU+W
- the energy transferred to a system through heating is equal to the sum of the increase in internal energy and the work done by the system

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2
Q

When is Q positive?

A

when energy is transferred to the system by heating

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3
Q

when is ΔU positive?

A

when there is an increase in temperature

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4
Q

when is W positive?

A

when work is done by the system

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5
Q

What is an Open system?
-e.g?

A

matter can go flow in and out the system
-aerosol can

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6
Q

what is a closed system?
-e.g?

A

no matter can enter or leave the system
-air inside a balloon

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7
Q

what is an ideal gas?

A

-perfectly elastic collisions between molecules
-no intermolecular forces
-no potential energy = internal energy is equal to the sum of the kinetic energies of all its particles

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8
Q

what is the ideal gas equation?

A

pV= nRT

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9
Q

what is the equation between pressure, volume and temperature?

A

p₁V₁/T₁ = p ₂V ₂/T ₂

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10
Q

What is an adiabatic process?

A

no heat leaves or enter the system
- Q=0
- ΔU=-W

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11
Q

what is the equation for an adiabatic process?

A

pVˠ=constant

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12
Q

what is an isothermal process?

A

the temperature of the system remains constant
-ΔU=0
- Q=W

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13
Q

what is the equation for an isothermal change?

A

pV=constant

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14
Q

what is a constant pressure change?
(isobaric)

A

the pressure of the system remains constant

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15
Q

what is the equation for a constant pressure change?

A

W=pΔV

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16
Q

what is a constant volume change?
(Isochoric)

A

the volume of the system remains the same
- W=0
- Q=ΔU

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17
Q

what is the area beneath the pV curve?

A

the work done during the process

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18
Q

what does isothermal mean?

A

constant temperature

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19
Q

How do you find the work done per cycle on a pV graph?

A

the area of the loop

20
Q

What are the Four steps in a four stroke petrol engine?

A
  1. Induction
  2. Compression
  3. Expansion
  4. Exhaust
21
Q

Explain what happens during Induction?
(four stroke petrol engine cycle)

A

the piston moves down, gas mixture drawn in through inlet valve
-volume of gas increases
-pressure is constant

22
Q

Explain what happens during Compression?
(four stroke petrol engine cycle)

A

inlet valve is closed and cylinder moves up, doing work on the gas
-volume of gas decreases
-pressure increases
mixture is then ignited and temperature and pressure rapidly increases at almost constant volume

23
Q

Explain what happens during Expansion?
(four stroke petrol engine cycle)

A

gas mixture expands, doing work on the piston causing it to move down the cylinder and exhaust valve opens.
- pressure drops to slightly above atmospheric pressure

24
Q

Explain what happens during Exhaust?
(four stroke petrol engine cycle)

A

piston moves up the cylinder, forcing the burnt gas out through the open exhaust valve

25
Q

How is a diesel engine different to a petrol engine?

A

Diesel only intakes air, not a gas mixture, the diesel is directly inputed into the cylinder as a fine spray using a fuel injector, causing it to ignite

26
Q

Why is the efficiency of a real engine far lower than in theory?

A

-time taken to open and close valves
-heating does not take place at constant volume
-fuel not completely burned

27
Q

How are the indicator graphs for theoretical and actual engine cycles different?

A

-actual has rounded edges
-theoretical has higher peaks
-actual has a smaller loop area

28
Q

What is indicated power?

A

the product of the area pf the main pV loop and the number of cycles it undergoes per second and the number of cylinders

29
Q

what is brake power?
-equation?

A

the power output of the engine
- P=T⍵

30
Q

what is the input power?

A

the product of the calorific value of the fuel and its flow rate

31
Q

what is the calorific value?

A

the measure of how much energy the fuel stores per unit volume or mass

32
Q

what are 3 types of engine efficiencies?

A

-Overall efficiency
-Thermal efficiency
-Mechanical efficiency

33
Q

what is the equation for Overall efficiency?

A

Overall efficiency = Brake power/input power

34
Q

what is the equation for thermal efficiency?

A

thermal efficiency=indicated power/input power

35
Q

what is the equation for mechanical efficiency?

A

mechanical efficiency =brake power/indicated power

36
Q

What is the second law of thermodynamics?

A

a heat engine must have both a source and a sink to operate

37
Q

why would an engine not abide by the first law of thermodynamics?

A

if the engine reached the temperature of the source (thermal equilibrium), no more heat would flow and so no work would be done. i.e not 100% efficient

38
Q

what is the equation for the efficiency of an engine?

A

W/Q(hot)

39
Q

what is regenerative braking?

A

allows the power caused by braking in a vehicle to be stored (in a flywheel) and be used later on to accelerate when needed

40
Q

what are reverse heat engines?

A

engines that have work on them in order to transfer energy form a colder region to a warmer one

41
Q

what are the two forms of heat engines and briefly explain function?

A

-Refrigerators: extract energy from cold region
-Heat pumps: transfer energy to hot region

42
Q

What is COP, coefficient of performance?

A

a measure of how effective a reversed heat engine is at transferring heat per unit of work done

43
Q

what is the equation of COP(ref)?

A

COP(ref)= Q(cold)/W

44
Q

what is the equation of COP(hp)?

A

Q(hot)/W

45
Q

What are CHP schemes, Combined heat and power?
-problems with CHP?

A

use heat transferred from power plants to surroundings to heat homes and businesses close by.
-homes and businesses tend to be far away from power plants

Physics (9 decks)

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