iThoughts

Physics 2

Gas

laws

Henry’s

the amount of gas dissolved in a liquid (solubility) is directly proportional to the partial pressure of that gas in equilibrium with the liquid

nitrogen narcosis + the bends
fizzy drinks
Colligative properties (FOBS)

properties that are dependent on the amount of solute in a solution
depression of freezing point
increase in osmolar pressure
elevation of boiling point
increase in SVP

Dalton’s

total pressure of a gas is equal to the sum of the partial pressures

Boyle’s
Charles'
Gay-Lussac’s
Avogadro's

1 mole of an ideal gas at STP occupies 24.45L = 6.022 x 1023 particles
(Equal volumes of gases at the same temperature and pressure contain the same number of molecules)

Ideal gas law

PV=nRT

Fick’s

V = (P1-P2)AD / T
V = rate of particle movement, A = surface area, D = diffusion constant (solubility / square root of MW), T = thickness
can be rearranged to flux = gradient x diffusion constant

Graham's

rate of diffusion is inversely proportional to the square root of the molecular weight of the molecule OR DENSITY!

Rault’s

The freezing point is reduced and boiling point and SVP are increased in proportion to the osmolality of a solution
Colligative properties: ones that depend solely on the number of molecules of solute in a solvent. As solute increases there is an:

increase in osmolality
increase in BP
decrease in FP
decrease in SVP

Electricity

diathermy

numbers

0.5 - 3 MHz
200 - 300mA through patient
30 - 100 W

monopolar
bipolar
cutting

high frequency

coagulation

bursts of lower frequency sine waves

Wheatstone bridge

R1/R2 = R3/Rx

harm

electrocution with AC 50Hz

macroshock

current (at 50Hz)

1-5mA

tingling

5-10mA

pain

15mA

no-let-go

50mA

respiratory arrest

100mA

VF

5A

asystole

frequency

impedance of the body to AC current increases at low AND high frequencies (> 100KHz does not cause spasms/arrythmias, but still may cause burning… diathermy)

microshock

44microA

harm

100microA

arrythmias

do not cause asystole
e.g. CVC’s, pacing wires, TOE
Right ventricle is most sensitive

burns

direct heating
fire
explosions

equipment interference

electrostatic

when lamp is plugged in but off
potential difference, but no current is flowing
can act as a CAPACITOR plate

doubling the separation halves the field strength
increases at high frequencies

easily prevented by using a screened cable

electromagnetic

when lamp is on
current is flowing
electromagnetic field will transfer energy to nearby conductors, acting like an inefficient TRANSFORMER

varies as the reciprocal of the separation squared!

cannot be prevented by screened cable

circuit safety

wires

‘egy brown loaf’
earth - green/yellow
line - brown
neutral - blue

electricity substation

a local transformer, where the 3 phases are reduced from 11kV to 250V
power can be taken from any two, so a transformer is connected to P1&P2, P2&P3 and P1&P3

hospital or street only gets ONE phase

each supply has a line (brown) wire going to the hospital and a neutral (blue) wire returning to substation which then splits at the star point to the substation earth and other side of the transformer to complete the circuit
star point provides a path for lightening to reach the ground

local earth

must be able to carry more current than the equipments fuse, so the fuse blows before the earth connection blows out

Surge test: >25A for 5 seconds (requires PD of 2.5v - as V=IR —> 25A x 0.1ohms = 2.5v

impedance must be <0.1ohm so most of the current passes through it and not you!

isolating transformer

no need for earth
need a line isolation monitor to ensure not accidentally earthed!

must be below 2mA

fuse

cuts out the circuit above a certain current
determined by how much current a device requires to operate

ELCB’s

voltage operated
current operated

detects difference in current between line and neutral wires and disconnects

(earth leakage circuit breaker)
limitations

does not replace a fuse

(high current could flow through circuit but would still be equal in both wires so would not be stopped by ELCB

if someone touches both wires, there would be no difference in current flowing between the wires

MOST IMPORTANT THING IS AVOIDANCE OF EARTH LEAKAGE CURRENTS!!!

electricity contd

electrical safety

classes of equipment

I

earthed
permissable leakage

B & BF

NC

0.1mA

SFC

0.5mA

CF

NC

0.01mA

SFC

0.05mA

II

extra layer of protection
permissable leakage

B & BF

NC

0.1mA

SFC

0.1mA

CF

NC

0.01mA

SFC

0.05mA

III

battery powered
Safety Extra Low Voltage (SELV)
<60V DC or <25 AC

degrees of protection

B
BF
CF
defib-safe BF
defib-safe CF

Theatre shoes

Should have impedance between 75 kilo-ohms and 10 mega-ohms
low enough to dissipate electrostatic charges
high enough to protect against mains shock
(normal skin impedance is 50 - 150 ohms)

terms

RMS = the average voltage for AC

RMS = 338/√2 = 240v

impedance

resistance in AC
impedance = resistance + reactance
reactance

measures the oppostion to a CHANGE in current
(resistance measures the opposition to current)
can be capacitive reactance or inductive reactance

unlike resistance, impedance varies with frequency

capacitors

basics

store enery in the form of an electric field
DC: current starts high and falls to zero exponentially
AC: the current leads the EMF
no current passes through the plates

frequency

high frequency passes more easily through capacitors
impedance proportional to 1/f
‘CLIFF’

capacitors, low frequency filter

inductors

basics

store energy in the form of a magnetic field
DC: current starts small and increases with time
AC: the current lags behind the EMF
current passes through the inductor

frequency

‘IPL’

impedance proportional to f
low frequency passes more easily through inductors

resistor

wire with properties of inductance and capacitance???
impedance is independent of frequency for resistors???

RCL circuits

time constant = RC
time for a capacitor to charge to 99.7% of its max value = 5 time constants
time constant = L/R
CR = high pass filter

‘Jamie’s CRap was high in the loo’

RC = low pass filter

resistivity

degree to which a material opposes the flow of electric current
constant for a given material at a constant temperature
resistance = resistivity x length / cross sectional area
ohm meters
as temperature increases, resistivity:

decreases w. semiconductors
increases w. metals

e.g. wire strain gauge transducer in arterial line - as pressure increases, length decreases and cross sectional area increases —> resistance decreases

defibrilators

equipment

capacitor

stores charge
equations

1 farad = capacity to store 1 coulomb of charge when 1 volt is applied
Q = CV
E = 0.5 x QV
(can rearrange to energy stored = 0.5 x CV²)
C = KEA / D
(Q = charge, C = capacitance, V = potential difference, E = energy, K = dielectric constant of the material, E = permittivity of dielectric constant, A = area of plate, D = distance between plates)
Impedance = D / AF
(D = distance between plates, f = current frequency, A = plate area)

power source

240V AC, transformed to 5000 V AC then rectified to 5000V DC
battery powered ones are inverted to AC, transformed to 5000V then rectifed back to DC

diode

switches to DC

inductor

slows current
measured in Henrys (H)

2 switches
pads

numbers

current reaching the heart: 35 A
normal chest impedance: 70 ohms
starting energies

VF/VT

monophasic

360 J

biphasic

150 - 200 J (up to 360 J)

unstable AF

monophasic

200 J

biphasic

120 - 150 J

NB

if stable AF <48hrs duration, do TOE, start anticoagulation and cardiovert pharm/DC
if >48hrs duration, do TOE, ESTABLISH on anticoagulation and cardiovert pharm/DC

unstable flutter/narrow complex regular

monophasic

100 J

biphasic

70 - 120 J