Electricity Flashcards
Electrical current definition and units
Flow of electrical charge
1 ampere = 1 coulomb / second
Electrical voltage definition
Voltage units
Potential difference between two points to drive electrons around a circuit
Volts = Joules/ Coulomb
What is a coulomb
Unit of charge (Q)
What is conductance what is its units
The ability of the material to allow the flow of electrical charge
Measured in sieman
What is resistance
The ability of a material to resist the flow of charge
Measured in Ohms
Ohms law
Resistance = voltage / current
Equation for power (work per second)
Power (watts) = voltage x current
What is a Wheatstone bridge?
Two voltage dividers in parallel.
Allows signal common to both arms to be undetected and AMPLIFIES biological signal.
Measuring resistor attached to a strain gauge transducer which changes resistance with change in pressure.
Voltmeter detects difference between sum total of resistance between the two ciruits.
Fourier analysis then amplifies and plots waveform.
What is a transformer
Device which transfers electrical energy from one circuit to another using electromagnetic induction.
Can be step up or step down depending on number of coils in each circuit
What is a diode
Allow current to flow in one direction only
What is a transistor
Components which open and close to allow movement of electrical current in response to a control voltage
What is common mode rejection ratio
An amplifier which neutralises signals common to both inverting and non inverting inputs. Only amplifies input voltage difference. High ratio means good quality op amp.
What is a transducer, how do they work
Change one type of energy into another.
What is a capacitor
A device which stores electrical charge.
Made of two conducting plates sandwiching a non conducting dielectric
What is one farad
The ability for a capacitor to store one coulomb of charge when one volt is applied across it.
What is inductance
Opposes the flow of current by creating an electromotive force.
This repels electrons and slows flow of current.
Factors affecting thoracic impedence
Stroke volume (increases)
Ventilation (increases)
Pulmonary oedema (decreases)
Voltage equation
V = IR
The energy required to move one coulomb of charge between two points in a cicuit.
Electromotive force
Why is an op amp helpful
Op-amps are incorporated into electrical circuits with many other components to achieve a desired signal output from a recorded biological potential
What elements are required to process biological signals to electrical ones
Transducer, amplifier, filter, convert analogue to digital, store and display
What happens if the analogue sampling rate is too low
How does analogue conversion work
Digitized waveform is a poor representation of the frequency and magnitude of the biological signal.
Sampling the analogue sine wave at the chosen frequency gives a digital staircase waveform which is smoothed out by low pass filtering.
Energy stored by capacitor equation
Capacitance of defib
E = 1/2 CV²
32 microfarads
Capacitance equation
C = Q/ V
Work equation for electricity
W = VQ
Describe how a defib works
Charging circuit - voltage applied to circuit through step up transformer. Current delivered to a capacitor creating a potential difference. Since W = VQ, as voltage increases work must increase to store charge. This slows down exponentially as it reaches a maximum.
Patient circuit - switch moves to allow capacitor to discharge until voltage falls to zero exponentially. An inductor increases the time over which the energy is discharged.
What is fourier analysis
Analyses sine wave via its component parts
Force equation for magents
F = QUB (where B is magnetic flux density, measured in tesla)
How do transformers work
Transfer electrical energy either step up or step down from one circuit to another by means of a magnetic field linking both circuits. Coils step copper around a steel core to minimise hysteresis loss. Only in AC.
Current 1mA effect
tingling
Current 5mA effect
pain
Current 15-30mA effect
muscle contraction
Current 50mA effect
Asphyxia
Current 100mA effect
VF
Features of class 1 electrical device
Mains powered
Earthed metal case
Fuse wire
Features of class 2 electrical device
2x insulated
Unearthed
Features of class 3 electrical device
Battery operated
