Basics of electricity for anaesthetists Flashcards
In electronics, what are the solids classified into?
Conductors Insulators Semi-conductors Based on their ability to move/”conduct” electrons
What is Ohm’s law?
Describes the relationship between the three basic quantities of electricity. V = IR
What is the RMS?
It’s the root mean square voltage = mains voltage
What is the peak voltage?
Peak voltage = 1.413 x root mean square voltage
Where is the neutral wire attached to earth?
At the substation
What are the 4 common classes of components in electrical circuits?
Resistors Capacitors Inductors Transformers
What is a conductor?
Has outer electrons of their atoms loosely bound and are free to move through the material under the influence of an electrical potential - they conduct electricity
What material makes a good conductor?
Metals Non-metals like carbon Solutions containing ions, such as saline
What is an insulator?
Has outer electrons which are firmly bound, and application of electrical potential has no effect on them
What materials are insulators?
Rubber Mica Glass
What is a semi-conductor?
Group of materials that normally behave as non-conductors but under certain circumstances (eg temp change) can perform like conductors
What is an example of a semi-conductor?
Thermistors Transistors Diodes
What materials are semiconductors made of?
Silicon Germanium Lead sulphide Selenium Gallium arsenide
What is direct current?
Charge flows in one direction: the current (or voltage) does not change with time
What is alternating current?
It varies sinusoidally, driving an alternating current around the circuit which reverses it’s direction in a sinusoidal manner from positive to negative and back, repeating the pattern 50 x a second (frequancy = 50Hz)
Why are high frequencies not used in AC?
Because they can’t be economically transmitted over long distance
What is the disadvantage of using lower frequency to transmit AC power?
At low frequencies, excitable tissues such as muscle and nerve are at greater risk of excitation and damage, so increased risk of electrical shock
What is a magnetic field?
The region throughout which a magnet or current-carrying conductor exerts it’s effects
How are magnets used to generate power?
When you move a magnet in and out of a conductor coil, or move a coil in and out of a magnetic field this produces movement of electrons in the conductor
What happens at a power station to generate power?
Water/steam drives turbine Turbine connected to generator Generator made of magnet surrounded by coils Spinning magnet generates movement of electrons in coils Produces electricty
What is the peak voltage in the UK?
1.413 x 240V = 340V
What is transmission voltage in the UK?
16 kV
What is responsible for wasted energy in transmission cables?
Resistive heating due to current
How is wasted energy minimized in transmission cables?
Resistive power loss = I (squared) x R Therefore need to keep current low - transfer at high voltage
How is the voltage brought down from high voltage transmission cables?
Step down transformers at the substations
What does a normal electrical supply consist of?
3 conductors - live - neutral - earth
Where is the earth normally connected to?
The casing of the equipment so that under faulty conditions, the chassis and enclosure of the equipment won’t be rendered live
What is current?
Any movement or flow of electrical charge and is a measure of the number of moving electrons flowing past a point along a wire during any time interval, divided by that time interval
What is the unit of current?
Ampere = 1 coulomb of charge each second past a point in a conductor
What is a coulomb?
Unit of electrical charge equivalent to 6.24 x 10^18 electrons
What is voltage?
Voltage = potential difference The electrical force which drives electric current.
What is 1 Volt?
One volt is the potential difference between 2 points on a conducting wire carrying 1 amp of current between which 1 watt of power is dissipated. It’s also 1 joule of energy per coulomb of charge (1 volt = 1 joule/coulomb)
What is resistance?
The amount of current that can flow when a given potential (voltage) is present Low resistance = higher current High resistance = lower current
What does resistance depend on?
Material used and shape eg long wire has higher resistance than a short one, thick wire has lower resistance
What is the unit for resistance?
Ohms (= volts per amp)
What is power?
Measured in watts. Equal to the voltage x current, and since V = IR = V(I x R) x I = I (squared) x R
What is a fuse?
Over-current protection device Consists of a metal wire which melts when heated by an electric current -> opens circuit -> protects it from overload
What material are fuses made of?
Silver Copper Tin
What are fuses supposed to do?
Allow passage of nromal current only
How do you work out what fuse is required for an appliance?
Divide the power in Watts by the voltage eg. for a 3000W appliance at 240V = 3000/240 = 12.4 amps = needs a 13 A fuse
What are the symbols for the common electrical components of a circuit?
What is a resistor?
Dissipates energy
What is a capacitor?
Stores energy in an electric field
What is an inductor?
Stores energy in a magnetic field
What is a transformer?
Adjusts levels of voltage and current
What is impedance?
Impedance (Z) = a measure of how much the circuit impedes the flow of current and like resistance is measured in Ohms
It takes into account the effects of capacitance and inductance opposing current flow
Varies with frequency
Depends on: resistance (R), capacitance (C), inductance (L) and frequency (f)
What is the resistance of dry skin vs wet skin?
Dry = 50000 ohms
Wet = 500 ohms
What is the current required to produce VF?
100 mA
What happens to the resistance of skin with repeated shocks?
It reduces impedance of skin and increases chance of cardioversion
What is a capacitor?
Electrical component capable of storing electrical energy.
Consists of 2 metal plates (conductors) insulated from each other by a dielectric( insulator).
When fully charged, the potential difference across the capacitor is the same as that supplied by the battery which charges is
What is the formula for capacitance?
Q ∝ VQ is the magnitude of charge on the plates measured in coulombs and V the voltage across the capacitor. Since Q is proportional to V, the ratio of Q/V is constant and is a measure of the amount of charge that a capacitor can store per volt.
C (farads) = Q/V
What is a farad?
One coulomb per volt
1 Farad is a very large amount of capacitance, most capacitors will have much smaller values (microfarads/nanofarads/picofarads)
What is the formula for stored energy of a capacitor?
E = 1/2 CV (squared)
How does reactance due to capacitors (Xc) vary with frequency?
It’s large at low frequencies, and high at small frequencies.
For steady DC (zero frequency) Xc is infinite, so capacitors pass AC but block DC
What is an inductor?
An electrical component consisting of a coil of conducting material wrapped around a core of ferromagnetic material or air.
Inductors can store energy in a magnetic field inside the coil, created when a current passes through them, changes in the current will change the strength of the magnetic field
This induces an electromotive force (EMF; voltage) in the coil which opposes the rising current - this smooths out rapid changes in current
What is inductance measured in?
Henries (H)
What is inductance?
Induction of voltage in a current carrying wire where the current in the wire itself is changing
What are inductors used in?
In defibrillators to create a heavily damped sinusoidal waveform (Lown waveform) of finite duration (~ 5ms)
How does inductive reactance (XL) vary with frequency?
Inductive reactance (XL) is small at low frequencies and large at high frequencies. For steady DC (frequency zero), XL is zero (no opposition), hence the rule that inductors pass DC but block high-frequency AC.
What is a transformer?
A device that utilizes the principle of induction to adjust the levels of voltage and current. It transfers energy from one circuit to another.
It consists of primary and secondary windings or coils, tightly coupled on a core of ferromagnetic material
The primary coil is connected to the source of alternating voltage or current which generates alternating magnetic flux, inducing EMF in the secondary coil.
What is an isolation transformer?
It’s designed to enhance the electrical isolation between the primary and secondary coils and between both coils and any other conductor.
They’re an important part of floating circuits.
What does a defibrillator circuit look like?
What is a floating circuit?
If the secondary coil of an isolation transformer is not earthed, it’s a floating circuit.
This helps reduce the amount of stray leakage of mains frequency current that can pass through a patient and cause electrical injury
How does a defibrillator work?
It has a step up transformer which increases the voltage from 240V to 9000V. This is then converted to DC using a rectifier (which uses diodes to allow the current to flow in one direction only). Once the capacitor is charged, the circuit switches to the patient circuit. The paddles are applied to the chest to complete the circuit. The inductor in the patient circuit allows the current to be delivered for several milliseconds.
