Electric Circuits and Capacitance

Capacitors

• When capacitors are connected in series, the total capacitance decreases
• The total capacitance (CT) of two capacitors (C1 and C2) connected in series is CT = 1 / (1/C1 + 1/C2)
• When capacitors are connected in parallel, the total capacitance increases
• The total capacitance (CT) of two capacitors (C1 and C2) connected in parallel is CT = C1 + C2

Electric Current

• Direct Current (DC) is a continuous unidirectional flow of charged particles
• Alternating Current (AC) is an intermittent (variable) bidirectional flow of charged particles
• Low Frequency Currents have a frequency range of 1-1000 Hz
• High Frequency Currents have a frequency range of 1000,000 Hz or greater
• High Frequency Currents have no effect on sensory and motor nerves

Electric Pulses

• Pulse Frequency measures the number of pulses in a unit time
• Pulse Duration is also known as Pulse Width
• Phase Duration is the time for one phase to complete its route
• Interpulse Interval refers to the time between pulses
• Burst modulation in electrical currents involves a series of groups of pulses delivered at a specified frequency over a specified time interval followed by a brief time interval without charged particle movement
• Duty Cycle is measured as the ratio of on-time to total time

Faradic and Galvanic Currents

• Faradic currents can stimulate denervated muscles
• Galvanic currents are not related to the frequency of a wave with a periodic time

Waves

• Peak-to-peak value measures the difference between the lowest negative amplitudes and the highest positive amplitude on both sides
• Root-Mean-Square (RMS) value relates the amount of AC to DC values for heating effect
• The RMS value is used to compare one wave to another in terms of maximum values

Ohm's Law

• The relationship between voltage (V), current (I), and resistance (R) is V=IxR

Electric Circuits

• The unit of electric current is the Ampere
• When resistors R1 and R2 are connected in parallel, the total resultant resistance R is less than the individual resistances
• When resistors R1 and R2 are connected in series, the total resultant resistance R is the sum of the individual resistances

Capacitance

• The unit of capacitance is the Farad
• Coulombic repulsion limits the addition of more charge on the plates of a capacitor
• The charge stored on the plates of a capacitor is Q = CV
• Increasing the surface area of the plates increases the capacitance of a capacitor

Waves and Ultrasound

• Mechanical waves require a medium, while electromagnetic waves do not
• Ultrasound waves are classified as longitudinal mechanical waves
• Frequency is defined as the number of waves produced in a unit time
• Ultrasound frequency range is above 20 kHz

LC Oscillators

• The main components of an LC oscillator are an inductive coil and a capacitor
• Energy storage is different between a capacitor and an inductive coil in an LC oscillator
• The quality or Q-factor of the circuit determines the amount of damping in an LC circuit
• Resonance in an LC tank circuit occurs when the capacitive and inductive reactance are equal

Magnetostriction Ultrasonic Generator

• The condition of resonance in a magnetostriction ultrasonic generator is achieved by matching the frequency of the vibrating rod with the circuit frequency
• The strength of the magnetic field affects the change in length of a ferromagnetic rod in the magnetostriction ultrasonic generator

Piezoelectric Crystals

• A piezoelectric crystal produces an alternating potential difference signal when receiving ultrasound
• Using a quartz crystal makes a piezo-electric oscillator more efficient due to increased stability in frequency