eLFH - Simple Circuits, Definitions and the Wheatstone Bridge Flashcards
Battery definition + circuit symbol
Power source that supplies electricity as direct current (DC)
Resistor definition + circuit symbol
A device that opposes electric current by producing a voltage drop in proportion to the current flow
Transducer definition
A device for converting one form of energy into another
Four examples of transducers with circuit symbols
Charge (Q) definition
Presence (negative) or absence (positive) of electrons on conducting or insulating surface
Battery drives electrons to a conducting plate - if they have nowhere to go then they remain on the plate as negative charge
Unit of charge (Q)
Coulomb (C)
Current (I) definition
Electron flow through a conductor or around a circuit
Can be considered as rate of change of charge
Unit of current (I)
Ampere (A)
Direction of current / electron flow
High potential to low potential
Electrons move from negative to positive, but convention dictates circuit diagrams show current flowing from positive to negative - in practice is does not affect circuit calculations so is irrelevant
Voltage definition
Aka Potential
‘Driving force’ to drive electrons around a circuit
A drop in voltage occurs as current flows through a circuit
Voltage units
Volt (V)
Electromotive force (E) definition
The voltage that drives current around the circuit
Supplied by battery or mains supply
Essentially describes the driving voltage from power source
Units of electromotive force (E)
Volt (V)
Resistance definition
Impedance to current flow, causing a voltage drop across the circuit
Units of resistance
Ohm (Ω)
Relationship between voltage drops, current and resistance across elements in a series circuit
Current flowing through each element is equal:
I1 = I2 = I3 = I
Total voltage drop is divided between all resistive elements:
V = V1 + V2 + V3
Therefore rearranging Ohm’s law gives total resistance in circuit:
R = R1 + R2 + R3
Relationship between voltage drops, current and resistance across elements in a parallel circuit
Current is divided between all elements:
I = I1 + I2 + I3
Voltage drop across all resistive elements is equal:
V = V1 = V2 = V3
Rearranging Ohm’s law gives total resistance:
1/R = 1/R1 + 1/R2 + 1/R3
Calculating output voltage in voltage divider
Bridge circuit
2 voltage divider circuits connected back to back in parallel
Values of R1, R2, R3 and R4 can be arranged so that the voltage at x and y are equal
Therefore if connected, no current would flow as there is no potential difference between x and y
Forms basis of Wheatstone bridge
Wheatstone bridge
Pair of voltage dividers in parallel which equally divides unwanted high voltage signals - i.e. rejects noise signals that are common to both arms of the bridge
Low voltage signal to be measured (e.g. arterial waveform from a transducer) appears in one arm of the bridge and thus produces a measurable voltage output across the voltmeter
R1, R2 and R4 are constant
R3 varies with the mechanical strain from a strain gauge diaphragm in an arterial transducer
