9. Electrical Components Flashcards
> Resistance:
What is it
How is it represented
How are they varied
what exploited in
What law
- Is the opposition to flow of direct current.
- It is represented by the symbols R and its unit is the ohm (Ω).
- 1 Ω is the resistance that will allow 1 A of current to flow when a potential of 1 V is applied across it.
• The resistance of different electrical components can vary with physical stresses such as temperature and stretch.
These changes are exploited in electrical thermometers and transducers.
• Resistance is key to Ohm’s law (V = I × R), which is a fundamental equation in electronics.
> Reactance:
• Is the opposition to the flow of alternating current caused by the inductance and capacitance
in a circuit rather than by resistance.
- Capacitative reactance decreases with increasing frequency.
- Inductive reactance increases with increasing frequency.
- It is represented by the symbol X and its unit is the Ω.
> Impedance:
• Total opposition to current flow
in an alternating current circuit, made
up of two components,
resistance and reactance.
• It is represented by the symbol Z and its unit is the Ω.
> Capacitor:
What is it
Whats composed of
What determine its ‘capacitance’
How does it handle currents
What’s device is it used in
What’s it measured
• A device that can store charge.
• It consists of two conducting plates
separated by an insulator (dielectric).
• The amount of charge that it can store depends on the size of the plates, separation gap and the dielectric material.
• It blocks DC (high resistance)
but passes AC (low reactance) as the
plates get alternately charged and discharged.
• As capacitative reactance decreases
with increasing current frequency,
diathermy devices with high frequencies
of 1.5 M Hz will have low reactance
and hence will be conducted.
Mains electricity at 50 Hz has
a high reactance and therefore will not be conducted.
This property makes capacitors useful filters.
.
The capacitor’s stored energy can also be discharged rapidly, making it the central component of the defibrillator.
• Charge (Q) is measured in coulombs (C), 1 C being the number of electrons passing a point when a current of 1 A flows for 1 s (6.24 × 1018 electrons).
• Capacitance (C) is measured in farads (F),
1 F being the capacity to hold
1 C of charge when a potential difference of
1 V is applied.
• The energy stored by a capacitor is given by the formula
E = ½QV or
E = ½CV2.
> Inductor:
What does it do
What does it consist of
How does it work
How do they handle current
What is a problem with them
What are they used in
• A device that resists a change in electric current.
.
• It consists of a wire coiled around a ferrous core (former).
- As current flows through the coiled wire a magnetic field is generated.
- These block AC (high reactance) but pass DC (low resistance).
• These are a source of interference
in electrical equipment where
electromotive activity in one circuic
can induce unwanted signals in another.
• They are used in transformers
and to isolate equipment from earth
and are also used in defibrillators
to smooth and lengthen the current pulse.
> Transformer:
What’s the use
What do they do
What it consist of
How does it work
Where is it utilised
• A device which transfers electrical energy
between two or more circuits
through electro-magnetic induction.
• They are used to increase (step up) or decrease (step down)
voltages of alternative current in electrical applications.
• They consist of two inductors
wound around the same former (core).
This close relationship means that
current changes in one circuit can
induce current in the second circuit
due to the coupling effects of the magnetic field.
• They are used to step up
the voltage of a current
to allow efficient transmission
over large distances
and to step the voltage
down to levels suitable
for household use.
• They are also used as isolating
transformers (see Chapter 60,
Electrical Safety’)
where they isolate appliances from earth.
• The change in voltage from the primary circuit
to the secondary circuit is calculated
from Faraday’s law of induction,
where the ratio of the number of coils of
each circuit around the transformer core
determines whether there is an
increase or decrease in the voltage
from one to the other
> Earth:
• A system of electrical safety where there is an electrical connection to ground.
• This protects people from the effects of faulty insulation in electrically powered equipment, as there is significantly less resistance through the earth circuit than there is through the person, and so electricity will
flow preferentially through the former.
• Class 1 equipment is earthed (see Chapter 60, ‘Electrical Safety’).
> Diode (or rectifier):
• Allows current to flow in one direction only.
> Battery (galvanic or voltaic cell):
• A collection of galvanic cells
that convert stored chemical energy
into electrical energy
when part of an electrical circuit.
• They consist of two half-cells
(positive anode and negative cathode)
connected by a conductive electrolyte.
• Oxidation occurs at the anode and
reduction at the cathode,
allowing a flow of electrons between the two.
> Transducer:
• A device which changes one form of energy into another, normally into an electronic signal for interpretation and recording.
• Examples include the microphone, which converts sound energy into an electrical signal, and the pressure transducer, which converts
pressure changes into electrical resistance.
> Amplifier:
• This differs from a transducer in that
it makes the input signal larger for
easier interpretation rather than changing it from one form to another.
• They are used because biological signals are often very small (EEG signals in the order of micro-volts) and need to be made bigger (amplified) for display.
• Amplifiers do not need to be electrical. Levers can produce a large movement at one end of a needle from a small movement at the other
(Bourdon gauge) and microscopes convert a small light field into a large one.
- For electrical signals, amplifiers increase the amplitude of the signal.
- The difference in the size of the input signal and the amplified signal is called the gain and is measured in bels (or decibels).
• In the amplification process there will
inevitably be an amplification of unwanted signal.
This is called the noise.
The amount of noise introduced compared to the signal is called the signal to noise ratio
and is a measure of the performance of the system.
• To reduce the amount of noise, amplifiers can also act as filters.
They can achieve this in a number of ways.
First, amplifiers are often differential amplifiers (also called operational amplifiers),
that is they look for signals that vary from one source to another
(e.g. different leads on an ECG) and amplify them, rejecting signals that are common to both as interference.
This is called common mode rejection.
If we consider the ECG signal again, the R wave will differ from lead to lead and so will be amplified for the ECG trace.
However, 50 Hz mains interference will be the same at all leads and so will be rejected.
amplifying only a certain frequency range (
bandwidth filter),
signals above or below a certain frequency
(high or low pass filters),
Rejecting particular frequencies (notch filter, e.g. 50 Hz mains signal) and by
amplifying signals of a particular amplitude.
