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Primary SOE/ VIVA > Physics Definitions > Flashcards

Physics Definitions Flashcards

1
Q

What is absolute humidity?

A

Absolute humidity is the:

MASS of water vapour present in a given volume of gas
at a given temperature and pressure.

It is expressed as g/m3 or mg/L.

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2
Q

What is relative humidity?

A

Relative humidity describes the:

ratio of actual mass of water vapour in a gas compared to the maximum amount of water vapour that the gas could contain, at the same temperature and pressure, expressed as a percentage.

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3
Q

How does temperature affect humidity?

A

Increasing temperature does not affect absolute humidity since the mass of water vapour is the same.

However, the maximum possible absolute humidity to cause saturation increases with increasing temperature and, therefore,

relative humidity decreases.

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4
Q

What is the value of absolute humidity?

A

The absolute humidity is 17 g/m3 at any temperature.

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5
Q

If 2 m3 air, fully saturated with water vapour at 20°C and at sea level, is then heated to 37°C, what is the absolute humidity?

What is the relative humidity?

A

Absolute humidity is 17 g/m3 at any temperature.

At body temperature (37°C) the mass of water vapour in fully saturated air at sea level is approximately 44 g/m3.

Therefore —-> relative humidity becomes 17/44 (39%).

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6
Q

What is the mass of water vapour at body temperature?

A

At body temperature (37°C) the mass of water vapour in fully saturated air at sea level is approximately 44 g/m3.

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7
Q

How is saturated vapour pressure affected by pressure?

A

At full saturation the partial pressure of water vapour is the same as SVP.

SVP does not change with pressure.

When full saturation is not present, ideal gas laws come into effect.

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8
Q

What ideal gas law applies to the relationship between volume and pressure?

A

As a set volume of air is compressed, the water vapour pressure within increases according to Dalton’s law, leading to an increase in relative humidity since the SVP is unchanged.

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9
Q

What is dew point?

A

Dew point is the temperature to which a given volume of gas must be cooled in order for water vapour to condense out into water. If this point falls below freezing, it is called the frost point.

At the dew point, relative humidity is 100% and the air is fully saturated.

As air temperature increases, the mass of water vapour that can be contained within that air increases too.

If air has low relative humidity then a greater temperature drop is required to reach dew point.

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10
Q

How does change in humidity affect the dew point?

A

Increasing humidity increases the dew point.

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11
Q

What is Regnaults hygrometer?

A

A Regnault’s hygrometer consists of a silver tube containing ether through which air is bubbled.

This causes cooling,

which leads to condensation forming on the outside of the tube.

The temperature at which this occurs is the dew point, from which can be derived relative humidity.

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12
Q

What is the ideal size of droplet for the alveoli?

A

Droplets of 1 micron are deposited in the alveoli, which is ideal.

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13
Q

What size of droplet is adequate for droplets in the trachea?

A

Larger droplets of 5 microns may be deposited in the trachea, which does not humidify the distal airways.

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14
Q

What size of droplet is too large for entering and settling in the airway?

A

Droplets of 20 microns and above may simply condense out in the tubing of the equipment and provide no benefit whatsoever.

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15
Q

How does HME filter work? (Heat and Moisture Exchanger)

A

Internal paper, sponge or foam is impregnated with a hygroscopic substance.

When water passes through, it condenses, and therefore provides latent heat to the HME.

The next breath warms the cooler inspired gas which also absorbs the moisture collected.

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16
Q

What are the advantages, disadvantages and efficiency of HME filters?

A

Advantages: Disposable, cheap and can incorporate a bacterial filter

Disadvantages: Secretions collected on mesh can increase resistance in the breathing circuit

Efficiency: ~70%

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17
Q

What is the standard humidification device used on the wards?

A

The cold water bath/bubble humidifier is the most commonly used humidification device on a standard ward

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18
Q

How does a cold water bath/ bubble humidifier work?

Advantages, disadvantages/ efficiency?

A

Mechanism: Dry gas, usually oxygen, is bubbled through water at room temperature

Advantages: Cheap and easy to run. No power source is required as it is gas driven

Disadvantages: Loss of heat through latent heat of vaporization reduces maximum obtainable humidity

Efficiency: ~30% due to large bubbles and loss of heat

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19
Q

What form of humidifcation is used commonly in ITU and how does it work?

A

The hot water bath is commonly used in intensive care.

Mechanism:

As with the cold water bath, but with the addition of a heated element.

Requires thermostatic control at both the humidifier site and at the patient end to prevent scalding.

Temperature of the water bath is usually 40-45°C

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20
Q

What are the advantages/ disadvantages and efficiency of the hot water bath humidification?

A

Advantages: Greatly improved efficiency. No loss of heat via respiratory system

Disadvantages: Scalding possible. Hyperthermia possible in young children. Water vapour can condense in the ventilation tubing increasing resistance to breathing

Efficiency: ~90%

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21
Q

How does a nebuliser work?

A

There are two types: Gas driven and Ultrasonic

GAS DRIVEN: High-flow gas is ejected close to a tube filled with water. The drop in pressure created by the gas, i.e. the Bernoulli effect, causes entrainment of water. The stream then hits an anvil and the droplets divide.

ULTRA-SONIC: Water is dropped onto a vibrating plate at ultrasonic frequency, producing tiny droplets

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22
Q

What are the advantages and disadvantages of nebulisers?

A

Advantages: Full humidification

Disadvantages: Some droplets vaporize using latent heat so a heater should be added. Efficiency >100 % can cause water overload, especially in children. Tiny droplets are more stable, carrying over longer distances and therefore more prone to carrying infection

Efficiency: >100 %. This is highest with ultrasonic

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23
Q

Define adiabatic?

A

‘relating to or denoting a process or condition in which heat does not enter or leave the system concerned.’

24
Q

What is a cryoprobe?

A

A surgical probe, a long slender pointed surgical instrument, used to apply extreme cold to tissues.

25
Q

What type of gases absorb infra-red?

A

Infrared radiation is only absorbed by gases with two or more different atoms in the molecule.

26
Q

What is Entonox a mixture of?

A

50:50 Oxygen and Nitrous by VOLUME.

27
Q

Define absolute zero.

A

The lowest possible temperature where nothing could be colder and all thermal motion stops, precisely zero kelvin or -273. 15 degrees C.

28
Q

Define boiling point.

A

The temperature at which the vapour pressure of a liquid equals the surrounding ambient pressure and the liquid changes into vapour.

29
Q

Define freezing point

A

The temperature at which the liquid and solid phases of a substance of a specified composition are in equilibrium.

A liquid turns to a solid when its temperature is lowered below its freezing point.

30
Q

What is heat capacity?

A

The amount of heat required to raise the temperature of an object by 1 degree C, in the units Joules per kelvin.

31
Q

Latent heat

A

The energy released or absorbed by a substance when it changes phase at a given temperature, ie ice melting.

‘Latent’ refers to the hidden change in energy state as there is no change in temperature during the phase shift.

32
Q

What is a gas?

A

It is a substance above its critical temperature.

It is distinguished from liquids or solids by the vast separation between individual gas particles.

A gas will expand to fit any space available.

33
Q

What is a joule?

A

It is the derived SI Unit of Energy where:

1 J is the work done to an object when applying of a force of one newton through a distance of 1 meter.

34
Q

What is SVP?

A

The pressure exerted by a vapour when in contact with and in equilibrium with its liquid phase within a closed system at a given temperature.

35
Q

What is a vapour?

A

A substance in its gas phase and below its critical temperature.

36
Q

What is pH?

A

The negative logarithm to the -10 of the hydrogen ion concentration in solution.

37
Q

What is accuracy?

A

This determines how closely the output reflects the true value being measured .

38
Q

What is sensitivity in a measurement system?

A

Sensitivity: the ability of a test to correctly identify patients with a disease.

39
Q

In a measurement system what is drift?

A

This is the movement of the output away from the true input value, and is usually linear and unidirectional.

It is usually caused by changing properties of the component of equipment eg. ageing of thermistors.

40
Q

What is gain in a measurement system?

A

This refers to the degree of amplification of the measurement system.

41
Q

What is hysteresis in a measurement system?

A

In a system with hysteresis, the output of the system alters depending on whether the input is rising or falling. Without hysteresis, input alone would accurately predict output.

An example of this is hysteresis seen in lung compliance due to the elastic energy that is stored within the system.

42
Q

What is damping?

A

Damping refers to the resistance of a system to oscillation resulting from a change in input. It is the result of frictional forces working in that system.

43
Q

In a measurement system, whats is:

  1. Response time?
  2. Rise time?
A

In a clinical system it takes time for a measured input to be reflected by a change in output.

RESPONSE TIME: time taken for the output to reach 90% of its final reading.

RISE TIME: The time taken for the output to rise from 10 to 90% of the final reading.

44
Q

What is under-damped?

A

The output changes quickly in response to a step up in input and overshoots, then oscillates around the true value before coming to rest at it. This takes longer to reach the true value and may continue to be inaccurate.

45
Q

What is critically damped?

A

The resonse time and rise time of the system are longer than an underdamped response, but there is no significant overshoot and oscillations are minimal.

D = 1.

46
Q

Describe over-damped.

A

Defined as damping greater than critical damping. The output here could potentially change so slowly it never reaches the true value, and not useful in clinical practice.

47
Q

Define optimally damped

A

Optimally damped where 64% of the energy is removed from the system and D = 0.64, which means there will be some overshoot and oscillations but a faster response time. 7% overshoot in this case.

48
Q

Define Ampere

A

SI unit for electric current.

The current that produces a force of 2 x 10(-7) Newtons per metre between two parallel wires, of infinite length, 1 m apart in vacuum.

49
Q

Define calorie

A

The amount of energy required to increase the temperature of 1g water by 1 degree C.

1 calorie = 4.16 J

50
Q

What is Candela

A

The SI unit of luminosity intensity.

51
Q

What is coulomb?

A

Unit of charge

I cd is the amount of charge passing at a given point per second, when 1A of current is flowing.

52
Q

Define force

A

Is defined as that which changes a body’s state of rest or motion, derived SI unit of newton. It has both magnitude and direction, making it a vector. Can be calculated by mass x acceleration.

53
Q

Define Kelvin

A

SI unit of temperature

1 K is equal to 1/ 273.16 of the thermodynamic scale temperature of the triple point of water. Its an absolute thermodynamic scale, using absolute zero as its null point.

54
Q

Define the latent heat of fusion and latent heat of vapourisation.

A

Latent heat of fusion - the amount of energy required to convert a unit mass of a SOLID at its MELTING POINT to a liquid, without an increase in temperature.

Latent heat of vaporisation - the amount of energy required to convert a unit mass of LIQUID at its BOILING POINT to a gas, without an increase in temperature.

55
Q

Define mass

A

The amount of matter contained in a body, with the SI unit of kilograms.

56
Q

Define Mole

A

SI unit of the amount of a substance

Quantity containing the same number of particles as there are atoms in 12g of carbon-12. This number is avogadro’s constant and is 6.022 x 10^23

57
Q

What is specific heat capacity?

A

The amount of energy required to raise the temperature of 1 kg of a substance by 1 degrees C.

Primary SOE/ VIVA (13 decks)

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