Physics Flashcards
Thermocouples
A pair of bimetallic strips which create a small voltage at their junction proportional to temperature (Seebeck effect)
CVC
14G = 1.63mm diameter
16G = 1.3mm diameter
Transducer placed at 4th ICS, mid-axillary line
Critical pressure
Is the pressure required to liquefy a substance at its critical temperature
Dalton’s law
The pressure exerted by a gas in a minute of gases is the same as it would be if that gas alone were present
The pressure of a mixture of gases is the sum of the individual partial pressures
Henry’s law
The amount of gas dissolved in a liquid is proportional to the partial pressure of the gas
Density of volatiles
Sevo 6.12 Halothane 6.48 Desflurane 5.44 Isoflurane 5.19 Oxygen 1.23 N2 1.02 Air 1.22 CO2 1.98 Helium 0.17
Pressure units
1kPa = 0.01 atm = 1000Nm2 = 10.2cmH2O = 7.5mmHg
Mercury
Manometer measures gauge pressure so has the top open to atmospheric pressure
Barometer measures atmospheric pressure so the top is sealed and creates a torricelian vacuum
Absolute pressure = gauge pressure + atmospheric pressure
Newton
Force required to move 1kg 1metre per second per second
SI units: kg.m.s^-2
Pascal
Force of 1 newton over 1 m2
Kg.m.s^-2 / m2
= kg.m^-1.s^-2
Joule
Measures energy/work done when a force of 1N moves an object by 1m in the direction of its motion
= Force x distance
= Kg.m.s^-2 x m
= kg.m2.s^-2
Power
Rate of energy transfer, measured in Watts
Joules / time
Kg.m^2.s^-2 / s
=kg.m^2.s^-3
Volts
Units of electrical potential difference
Potential difference when a current of 1A dissipates 1W of power
1W/1A
Kg.m2.s^-3 / A
Kg.m2.s^-2.A^-1
Micro shock
Right ventricle is more sensitive than left
Current cannot induce asystole
electric shock
Frequency should be >100KHz to avoid arrhythmias
Extensor muscles are affected more
Electricity supply to theatre
Single phase supply 3 wires - line, neutral and earth 240V Local earth connection (not substation) Mains frequency is 50Hz
Electrostatic interference
Can be reduced by screening the cables with a conductive barrier (not insulating)
A form of capacitative leakage so increased at higher frequencies
Doubling the separation halves the field strength between the 2 wires
Electromagnetic interference
Requires the lead to be drawing a current (need AC to induce a magnetic field)
Cannot be prevented by screening
Magnetic field strength is inversely proportional to distance^2
Requirements of an earth connection
Resistance <0.1ohm
Leakage current <0.5mA for Type 1
Local earth connection not connected to the substation
Must be able to carry 25A for 5s (earth surge test) - this must not require a voltage of more than 2.5V if R <0.1ohm
the star point
Connects one side of the 240V supply to the earth at the substation, creating the neutral wire
Protects from lightning strike
Not used in a three phase electricity supply
Electrical transformer
Efficiency is 95%
Electrical burns
Depend on size of current passed (and therefore voltage and resistance)
Depend on current density
Usually focussed at areas of high current density - entry and exit points
Scavenging
Tubing should be low resistance and low compliance to prevent collapse under negative pressure
Float controls protect the system
Should be able to generate flows of >25L/min
Should be able to generate pressure of -500mmHg is 10s
Soda lime
Can produce CO with desflurane, halothane and isoflurane
1kg can absorb 250L CO2
T-piece
FGF must be delivered at the patient end, therefore Mapleson D, E and F can all function as T-pieces
Mains voltage
Root mean square = 240V = the constant value which would result in the same power output as the alternating voltage
Peak value of alternating voltage is 339V
Diathermy
Uses frequencies of 1-5MHz
High current density at the active electrode and low current density at the pad
Coagulation mode uses short bursts of sine waves, cutting mode uses continuous sine waves
AC not DC is used
Isolation transformers
Create an unearthed circuit with the patient
Inductors
Electrical impedance is directly proportional to frequency
Z = 2 x Pi x f x L
Where L = inductance
Voltage is proportional to rate of change of current so more voltage in AC which slows the current down more, so inductors filter high frequency currents (“low pass filter”)
Frequency-dependent resistance is called reactance
Capacitors
Electrical impedance is inversely proportional to frequency
Z = 1/2 x Pi x f x C
Where C = capacitance
Current flow is proportional to the rate of change of voltage in AC
Current flow is negatively exponential in DC
In a capacitor, more current flows at a higher frequency
RC circuit is a mathematical integrator - low pass filter
CR circuit is a mathematical integrator - high pass filter
Bain Circuit (Mapleson D)
FGF in inner tube
FGF of 3x MV is needed in spont vent pts
Henry’s Law
Solubility of a gas in a liquid is proportional to the partial pressure of the gas at the liquid’s surface
Also, solubility of a gas DECREASES as temperature INCREASES
