Physics Flashcards
What is the First Gas Law?
Boyle’s Law (1662) stating pressure is inversely proportional to volume for a fixed mass of gas in a closed system at constant temperature
What does a substance’s ‘critical temperature’ refer to?
The temperature above which the substance exists in the gaseous phase
What is the term that refers to the temperature above which a fluid exists as a gas?
The ‘critical temperature’
Define a vapour
A vapour is a gaseous phase in which a state of equilibrium is present with the same substance’s liquid form. This is only possible below a substance’s critical temperature.
Give an example of a gas which exists as a vapour at room temperature
Nitrous oxide (critical temperature 36.6°C)
What is the Second Gas Law?
Charles’ Law (1787) which states that volume is directly proportional to temperature for a given mass of gas in a closed system at constant pressure
What is the Third Gas Law?
Gay-Lussac’s Law (1809) which states that temperature is directly proportional to pressure for a given mass of gas in a closed system of fixed volume
What is the Combined Gas Law?
PV/T = k
Where k is a constant
What is the equation relating the volume and pressure of a gas in two settings?
P1 x V1 = P2 x V2
What is the Ideal Gas Law?
PV = nRT
Where n = no of moles of gas and R = universal gas constant
Explain Dalton’s Law of Partial Pressures
Dalton considered the pressures exerted by each gas in a mixture of gases.
He found that the total pressure is the sum of the individual pressures of the gases in the mixture, as if they were alone in the volume.
Explain saturated vapour pressure (SVP)
As a liquid evaporates into the space above it, the pressure of its vapour increases.
A point of equilibrium is reached where the rate of evaporation equals the rate of condensation.
The pressure at this point is called the ‘saturated vapour pressure’
What happens when a liquid’s SVP is equal to atmospheric pressure
It boils
Describe and explain the law which governs gases moving in and out of solution
Henry’s Law states that as molecules of gas pass into solution, they exert an increasing partial pressure within the liquid.
This will continue until an equilibrium is reached at which the partial pressure of the gas is equal in the solution and in the gaseous phase above.
At a constant temperature a higher pressure of gas will force more molecules into solution.
What is a partition coefficient?
A partition coefficient describes the relative concentration of a substance in two phases when equilibrium is reached
How does temperature relate to gas solubility?
Gas solubility is inversely proportional to temperature
What factors affect danger of electrical injury
Strength of current
Type and frequency of current
Local current density
What are the features of mains electricity?
Alternating current 50Hz 16kV from power station to substation 240V from substation to hospital 2 wires - live and neutral Current flows out through live and back through neutral
What is Ohm’s law?
Ohm’s law describes the relationship between direct electrical current, voltage and resistance and states that:
V = IR
For alternating current, impedance (Z) is used instead of resistance and this varies with frequency.
What are some relevant current values in terms of effect when applied via skin?
Up to 5mA: Tingling
10 - 20mA: Pain, sustained muscular contraction
50mA: Risk of arrhythmias eg vent ectopics
100 - 300mA: risk of VF
Explain the concept of microshock
Microshock is the conduction of an aberrant current through a device connected or local to the myocardium which can occur at low voltages and currents as low as 0.1mA
This can be caused by ‘leakage current’ which is a current generated by incomplete insulation of equipment circuits. The earth becomes a return pathway for the current to flow.
Describe the British Standards classification for isolation for medical equipment
Type B: Least stringent, for devices with non-conducting patient connections. Earthed with no floating circuit.
Type BF: For equipment with conducting patient connections but no direct cardiac connection. Has a floating circuit.
Type CF: Most stringent, for equipment with direct conducting cardiac connection. Has a floating circuit.
What is a ‘floating circuit?’
A floating circuit or isolated patient circuit is achieved using a transformer which generates an electrical source in the equipment through EM induction but breaks the continuous circuit from mains to patient.
What mechanisms are employed to prevent accidental shock?
How do they work?
- Insulation
Prevents electrical contact between body and live source
Uses high impedance to minimise current in the event of aberrant conduction.
Three British Standards classifications of insulation:
Class I: Earthed exposed metal parts. Low risk equipment. Functions in combination with cut-out device.
Class II: Exposed parts are double-insulated. No earth.
Class III: Battery powered with voltage <40V DC. If AC used then isolating transformer used.
- Cut-out devices
Fuses: fault causing casing to become live causes surge in current which melts the fuse, disconnecting the mains supply.
Circuit breakers: Current-operated Earth Leakage Circuit Breaker (COELCB) monitors difference between live and neutral currents. A current difference operates a relay switching off power.
What is Avogadro’s constant?
6.022x10^23
How much volume does 1 mole of gas at STP occupy?
22.4 litres
What is 1 kilogram weight in terms of force?
9.81 Newtons
What mass exerts a gravitational force (weight) of 1 Newton?
102g
How are pressure, force and area related?
P=f/a
What is the conversion between kPa, mmHg and cmH2O?
1 kPa = 10.2cmH2O = 7.5mmHg
What conditions comprise STP?
Temperature 273.15K (0°C)
Pressure 101.325kPa (760mmHg / 1 bar)
What is the value of the universal gas constant R?
8.31 J / mol / K
