Simple Circuits and definitions Flashcards
Define the following terms: cell, resistor, transducer?
Cell: a power source that supplies electricity as a direct current.
Resistor: A device that opposes current by producing a voltage drop in proportion to the current flow (Ohms law V=IR)
Transducer: A device that converts 1 form of energy into another.
Give examples of different transducers?
Piezoelectric sensor: Converts pressure to electrical energy
Earphone: Converts electrical energy to sound
Microphone: Converts sound to electrical energy
Thermistor: Converts temperature to resistance
What is electrical charge?
CHARGE: (Q) is the presence (negative) or absence (positive) of a quantity of electrons on a conducting or insulating surface.
Unit is coulomb (C). 1 Coulomb = 6.24 x1018 electrons.
Batteries drive electrons onto conducting plates and if remain on it – generates a negative Q.
What is current (I)?
Current is the flow of electrons around a circuit or conductor. It can be considered the rate of change of charge.
Electrons flow from areas of -ve to +ve charge: an area of high potential to low potential. Note in diagrams flow is depicted from +ve to -ve.
The unit of current is amps. 1 amp = 6.24 x1018 electrons/s
What is voltage?
VOLTAGE: (V) Also known as potential and is the driving force for the current.
The potential of any point (V) is the voltage above zero (referenced to earth).
A drop in voltage occurs as current flows through the circuit.
What is electromotive force?
ELECTROMOTIVE FORCE (EMF): (E) Simply the voltage delivered by a battery or mains supply
What formula links energy, charge and voltage?
Energy = Voltage x Charge (Q)
J=VQ
What formula links current voltage and power?
Power (watts) = Voltage x Current
W=VI
Using Ohms law derive a formula for Power using Current and resistance.
Ohms law: V=IR
W=VxI
W= (IR) xI
W= I squared x R
If you have a circuit with resistors in series what is the overall resistance?
The resistance will be the sum of all the individual resistors in the circuit
If you have a circuit with resistors in parallel how do you calculate the overall resistance?
1/Rx = 1/R1 + 1/R2 + 1/R3 etc
*How to derive the above:
If you have a circuit in parallel the current is divided between the different elements.
I = I1 +I2 +I3
V=IR therefore I=V/R
V/R = V/R1 + V/R2 + V/R3
V/R = V(1/R1 +1/R2 +1/R3)
Divide both sides by V
1/R = 1/R1 +1/R2 +1/R3
What is a voltage divider (draw a diagram)?
A device for producing an output voltage that is a proportion of the input voltage.
A wire is placed between the 2 resistors in series so that:
Vin = I x (R1+R2)
Vout = I x R2
Therefore Vin/Vout is (R1+R2)/R2
Draw a diagram of a wheatstone bridge?
Page 98 of RCOA physics revision guide
Explain how a wheatstone bridge works?
A wheatstone bridge is 2 voltage dividers in parallel, with a connection containing a voltmeter.
The idea is that by having 2 voltage dividers in parallel with balanced resistors that there should be no voltage through the voltmeter. R1/R2 = R3/R4.
However one of the resistors is replaced by a transducer (e.g for an arterial line).
As the pair of voltage dividers are used to equally divide any unwanted high voltage signals common to a whole circuit to cancel each other out i.e. mains, diathermy and then to allow a possibly much smaller signal (such as an arterial waveform from a strain gauge transducer) to be detectable at a meter applied between the two arms of the bridge.
