eLFH - Simple Circuits, Definitions and the Wheatstone Bridge

Question 1

Battery definition + circuit symbol

Answer 1

Power source that supplies electricity as direct current (DC)

Question 2

Resistor definition + circuit symbol

Answer 2

A device that opposes electric current by producing a voltage drop in proportion to the current flow

Question 3

Transducer definition

Answer 3

A device for converting one form of energy into another

Question 4

Four examples of transducers with circuit symbols

Answer 4

Question 5

Charge (Q) definition

Answer 5

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

Question 6

Unit of charge (Q)

Answer 6

Coulomb (C)

Question 7

Current (I) definition

Answer 7

Electron flow through a conductor or around a circuit
Can be considered as rate of change of charge

Question 8

Unit of current (I)

Answer 8

Ampere (A)

Question 9

Direction of current / electron flow

Answer 9

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

Question 10

Voltage definition

Answer 10

Aka Potential
‘Driving force’ to drive electrons around a circuit

A drop in voltage occurs as current flows through a circuit

Question 11

Voltage units

Answer 11

Volt (V)

Question 12

Electromotive force (E) definition

Answer 12

The voltage that drives current around the circuit
Supplied by battery or mains supply

Essentially describes the driving voltage from power source

Question 13

Units of electromotive force (E)

Answer 13

Volt (V)

Question 14

Resistance definition

Answer 14

Impedance to current flow, causing a voltage drop across the circuit

Question 15

Units of resistance

Answer 15

Ohm (Ω)

Question 16

Ohm’s Law

Answer 16

V = IR

Applies to whole circuit or individual elements within it

Question 17

Relationship between voltage drops, current and resistance across elements in a series circuit

Answer 17

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

Question 18

Relationship between voltage drops, current and resistance across elements in a parallel circuit

Answer 18

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

Question 19

Voltage divider definition

Answer 19

A device for producing an output voltage that is a modification / proportion of the input voltage

Question 20

Calculating output voltage in voltage divider

Answer 20

Question 21

Bridge circuit

Answer 21

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

Question 22

Wheatstone bridge

Answer 22

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