Electricity Flashcards
What is static electricity. Why is this relevant in anaesthetic practice
Rubbing substances together can result in the transfer of electrons from one substance to the other, so that one of the substances has an excess of electrons and the other a deficit. This is called static electricity.
In anaesthesia- can be the source of ignition in anaesthetic explosions.
What does electric potential difference mean
Excess electrons in on object versus another object create a electric potential difference between these objects.
How do electrons move
Under the influence of a potential difference (or changing magnetic field)
What are conductors, insulators and semiconductors
Conductors: Materials (metals and ionic fluids) whose outer shell of electrons is loosely bound
Insulators: Firmly bound electrons in outer shell not normally able to move and form and electric current
Semi-conductors: Less firmly bound outer shell of electrons than insulators. Intermediate condiuctivity. E.g. Thermistors, transistors and diodes
Describe how a thermistor works
By adding energy (heat) to a thermistor material, energy is provided to the substance and electrons are able to move more freely.
How does a photodetector work
Light falls on the detector. The electrons in the material absorb some of this energy of radiation and are able to move through the material more freely. The photodetector is connected in an electric circuit in which the electrons that have absorbed the light energy cause a change in current as the light falls on the device.
Define magnetism
A conductor with a current flowing through it can exert a force on another conductor carrying a current –> magnetism
Some substances (Iron alloys), can exhibit magnetism although it appears that no current is flowing through them (Iron alloys). In this case, the magnetism is due to the sum of the many minute currents formed by the flow of the electrons orbiting their nuclei
How do some substances without a current flowing through them exhibit the property of magnetism? Give examples
Magnetism is due to the sum of many minute currents formed by the motion of electrons orbiting their nuclei.
How is a electric current produced by a magnetic field
A changing magnetic field induces a flow of electrons in a conductor to produce and electric current
How can the strength of a magnetic field be increased
By placing a ferromagnetic material in the core of a coiled conductor.
E.g. Iron
What is a ferromagnetic material
A ferromagnetic material has two main properties
- It increases the strength of a magnetic field when placed into the core of a coiled conductor.
- They are strongly attracted into a magnetic field
Differentiate magnetic flux from magnetic field strength
Magnetic field strength:
The power of the magnetic field in a vacuum
Magnetic flux:
Describes the field which results when a magnetic field is present in any material and may be greater or less than the original magnetic field strength.
Magnetic flux increases with the addition of a ferromagnetic substance within the core of a coiled conductor.
Magnetic flux decreases with the addition of diamagnetic materials
Magnetic flux increases slightly with paramagnetic materials
Describe the terminology used to describe the magnetic properties of materials
Ferromagnetic materials - Increases magnetic flux
Paramagnetic materials -Slightly increased magnetic flux
Diamagnetic materials - Decreases magnetic flux
Define the unit of magnetic flux
Weber (Wb). The Weber is the flux that when linked with a single turn, generates an electromotive force (emf) of 1 volt in the turn, as it decreases uniformly to 0 in 1 second
Define the unit of magnetic flux density
Tesla (T) which is Wb.m^-2.
What is the magnetic flux density produced in air by the earth’s magnetic field vs that of the MRI machine
Earth in air: ± 60 microTEsla
MRI: 0.2 - 4 T
Define direct current and sources of direct current
Direct current is the steady flow of electrons in one direction only.
E.g.
1. Thermocouples
2. Batteries
Define a battery
Differentiate non-rechargeable batteries from rechargeable batteries
In a battery, chemical energy is converted into electrical energy through a chemical reaction
Non-rechargeable batteries = primary cells
- chemical reaction not reversible
Rechargeable batteries = secondary cells
- chemical reaction reversible by passage of the current in a direction opposite to that in which it is supplied by the battery
Why should primary cells (non-rechargeable batteries) never be recharged
Generation of gas and the destruction of the cell, an explosion may occur
Define the term alternating current
The flow of electrons first in one direction and then in the opposite direction along a wire
Define an ampere
Ampere (A) is the SI unit of current.
1 ampere represents 6.24 x 10^18 electrons per second past one point and is defined by means of the electromagnetic force which is associated with an electric current
How does a galvanometer work?
When a conductor carrying a current is placed within a magnetic field, there is a force that tends to move the conductor in a direction perpendicular to both the magnetic filed and the electric current. A coil of wire is suspended on jewelled bearings in a magnetic field and balanced by a hair string. The interaction of the forces lead to the rotation of the wire which is connected to a pointer over a gauge calibrated to the current present.
How does an electromagnetic flowmeter work
If a conductor is moved through a magnetic field, an electric potential develops proportional to the rate at which the conductor is moved through the magnetic field.
As blood is a conductor, the electromagnetic flow meter introduces a magnetic field and two electrodes which measure the the electric potential and current generated which is proportional to the blood flow.
What are the limitations of a electromagnetic flowmeter?
- Velocity of blood flow varies across the diameter of the vessel, an average velocity must be used.
- Must be calibrated with the same type of flow as that which will be present in the vessel on which the measurements are made.
Define a volt
The volt is the unit of potential difference which drives current and is the power per unit current
1 volt is defined as that potential difference which promotes a current of 1 ampere in a substance when the rate of energy dissipation is 1 watt
Potential difference = Power (W)
________
Current (A)
V = P
__
I
What does a root mean square voltage of 240 V mean?
An alternating current with a maximum amplitude of one ampere has a smaller heating effect than a constant direct current of 1 ampere because in AC, the peak flow occurs for only a fraction of the time. When electricity is used commercially it is the heating and energy production that are of interest. Therefore, some units are required to relate the current and voltage values of AC electricity to the DC units so that the heating effects are the same. –> these are the rms (root mean square values).
An rms voltage of 240 V has an identical heating effect to that of DC voltage of 240V when applied across an identical resistor, but its peak voltage is 340 V
All the values of the sine wave are squared to make them positive. The mean of these is taken and then the square root of this is taken.
What is a fuse and why is it necessary
If current exceeds the rating of a fuse, excessive heat melts the fuse and stops the flow of current.
Safety of personal and equipment
What is current density
The current flow per unit area. The greater the current density the greater the heating effect.
Why doesn’t electrosurgical cautery (diathermy) cause ventricular fibrillation
The passage of DC or LOW frequency AC may cause physical sensation, muscular contraction and ventricular fibrillation.
These effects become less as the frequency of the current increases being small above 1 kHz and negligible above 1 MHz, but the heating and burning effect can occur at all frequencies.
Electrosurgical cautery passes a current with frequency of 1 MHz through the body in order to cause cutting and coagulation by local heating of body tissues. The degree of heating produced depends on the CURRENT DENSITY
What is electrosurgical cautery
Electrosurgical cautery passes a current with frequency of 1 MHz through the body in order to cause cutting and coagulation by local heating of body tissues. The degree of heating produced depends on the CURRENT DENSITY.
Describe the electrosurgical cautery apparatus and describe the current that flows through each component
Active/cutting electrode
- small mall area means max current density and pronounced heating
Patient / Neutral plate (electrode)
- Large area, lower current density, minimal heating or tissue damage.
What are the risks and complications when using monopolar electrosurgical cauterisation apparatus
- Incomplete contact of neutral plate with patient
- small area increases current density here –> high heat and tissue damage. - No contact of neutral plate with patient
- Current may return to current generator through any point that patient is touching an earthed metal object (e.g. patients hand touching metallic point on arm rest)
- Current flowing through ECG lead electrodes –> burns here (high current density
If bipolar diathermy is safer, why is it not always used?
When is bipolar diathermy preferred
The variety of available bipolar active electrodes is limited, especially cutting electrodes. Cutting and multiple large regions of haemorrhage in a large surgical filed is more suited to monopolar diathermy.
Bipolar diathermy is indicated in:
- Patient’s with pacemakers
- Neurosurgery (minimize tissue damage)
Draw the symbols for the 9 major electrical components
Capacitor
Inductor
Resistor
Battery
Diode (rectifier)
Earth
Amplifier
Switch
Transformer
Page 156 Kenny and Davis
Define capacitance.
The measure of an objects ability to hold electric charge, charge being the measure of amount of electricity.
SI unit of charge is the Coulomb (C)
What is the unit of charge
The Coulomb
Define the coulomb
The Coulomb is quantity of electric charge which passes some point when a current of 1 ampere flows for a period of 1 second.
Coulomb = Ampere (A) x Seconds (s)
The Coulomb is the amount of electricity equivalent to 6.24 x 10^18 electrons
What is the unit of capacitance
The farad (F)
Define a Farad
The fared is the capacitance of an object for which the electrical potential increases by 1 Volt when 1 coulomb of charge is added to it.
Define a defibrillator
An instrument used for the treatment of ventricular fibrillation in which electric charge is stored in a capacitor and then released in a controlled fashion.
Describe how charge is stored within a defibrillator
In a capacitor. The capacitor comprises two plates separated by an insulator. A potential difference is introduced across the plates of 5000 V. This is equivalent of a store of electrons equivalent to 160mC of charge
What does the amount of energy stored in a capacitor depend on
- Potential difference applied
- Capacitance
- Charge
What is the formula used for calculating stored Energy within the defibrillator (capacitor). Give an example if the
E = 1/2 x CV^2
or
E = 1/2 x QV
E - Energy
C - Capacitance
V - Voltage or potential difference
E.g.
E = 1/2 x 160 microCoulombs x 5000 V = 400J
Why is there an inductor placed within the defibrillator circuit
To ensure that the electric pulse that passes through the heart has an optimum shape and duration.
Which variable of the defibrillator function determines the clinical effect and how is this variable calculated. What does this mean in terms of calibration of modern (versus old) defibrillators?
The Energy delivered through the heart.
E = 1/2 CV^2
or
E = 1/2 QV
E - Energy
C - Capacitance
V - Voltage (potential difference)
Modern: calibrated in terms of delivered energy not stored energy
Why is the delivered energy different to the stored energy in defibrillators
The inductor present within the circuit absorbs some of the stored energy during delivery
How is does the energy delivery differ between external defibrillation and defibrillation using internal cardiac electrodes (open chest)
Energy maximum for external paddles: 360 J
Internal electrodes: 100J
What is the difference between monphasic and biphasic defibrilation
Monophasic: Single pulse of current flows through the heart –> higher energy required
Biphasic: Two consecutive pulses in one direction and then the other –> lower Energy required for defibrillation
When are electrostatic charges formed
This can build up on any object insulated from its surroundings
E.g. bobbin in rotameter - electrons removed from rubbing against glass walls as it rotates
What type of current may flow through the insulating plate of a capacitor and give an example of this relevant to anaesthesia.
AC current can flow through (DC cannot)
Light (plate 1) - Air (Insulator)- Patient (Plate 2)
ECG leads on patient may pick up the 50Hz mains frequency from the theatre light as this is alternating current. The amplitude may be significant enough to obscure the ECG recording
this is called interference resulting from capacitance
What is the difference between an inductor and a capacitor
Both electrical components store energy supplied by a potential difference (voltage supply).
- -> Capacitors store energy in an electric field
- -> Inductors store energy in a magnetic field
Give an example of inductance interference in theatre
The transformer found in mains-operated apparatus is an inductor. The magnetic filed that results from this can cause interference for ECG monitoring or other biological signal displays.
What is an inductor
It is an electrical component that stores energy in the form of a magnetic field. It is coiled conductor. The coil concentrates the magnetic field lines forming the magnetic field which stores the energy.
What is screening with regard to interference
The interference from inductive coupling and capacitance effects can be reduced by a system known as screening.
The insulated patient monitoring leads are covered by a sheath of woven metal which is earthed, so that interference is induced in this metal screen and not in the signal leads. The screening layer is covered by a further layer of insulation.
How can the signal to noise ratio be improved
- Reduce noise (eliminate source of noise)
- Differential signal amplifiers
- -> e.g. these filter out high frequency diathermy noise on the ECG trace as these amplifiers amplify repetitive waveforms and eliminate unpredictable waveforms
Define the unit of electrical resistance
The ohm is that resistance which will allow one ampere of current to flow under the influence of a potential of one volt
R = V / I
What is a strain gauge and how does this relate to a electrical resistance
Pressure measured in a tank containing saline and a diaphragm. Diaphragm connect to a wire. As the diaphragm bulges or contracts (with pressure changes) the length and diameter of the wire changes and hence changes the resistance of this wire. The change in resistance is proportional to the change in pressure and can be measured indicating the pressure change from a change in resistance = strain gauge
How does temperature influence resistance
In a conductor: Increased temperature –> increased resistance
In a semiconductor: Increased temperature –> decreased resistance
Descibe the diagram and application of a Wheatstone bridge
Battery 4 resistors - R1 (known - R2 (known) - R3 (variable) - R4 (being measured) Galvanometer
It is a null deflection system meaning no current flows through the galvanometer when the bridge is balanced.
In practice, an amplifier and recorder replaces the galvanometer and the extent to which the bridge is out of balance is recorded.
What is the difference between resistance and impedence
The term impedance is used instead of resistance when there is a dependency on the frequency of the current flowing
AC and DC
Resistor: independent of frequency (resistance)
AC only
Capacitor: More readily permits high frequency current through (impedance)
Inductor: More readily permits low frequency current through (impedance)
What is a signal attenuator
V1 across 5 resistors in series (R1 - R5)
If a potential V2 is measured only over R1 –> this V2 will equal 20% of V.
If a potential is measured over R1 to R4 (and the resistors have equal resistance), then V3 will be 80% of V1
Sensitivity switches within apparatus may employ a pattern of resistors of this type forming an attenuator
Why is electrode gel present on ECG electrodes
To reduce impedance at the surface of the skin between the source of the biological signal and the electrode.
Apart from resistance, at the point of contact of an ECG electrode, what other factors may cause attenuation of the biological signal
Capacitance and Inductance (Impedance)
What does the degree of signal attenuation depend on. Which of these need to be relatively larger and why?
- Skin and electrode impedance
- Input impedance of the monitoring apparatus used to detect it
Input impedance needs to be larger as if the skin/electrode impedance is larger, then the signal is greatly attenuated as the potential over the amplifier apparatus will be greatly attenuated considering its relatively lower impendance (in series)
What is the galvanic skin response (GSR)
The degree of moisture depends on the SNS tone.
Skin impedance is lowered when the skin is moist.
Hence skin impedance has been used as a measure of ANS activity
This test is known as the galvanic skin response or GSR
