electricity Flashcards
what is charge?
the property of a molecule/ substance that causes it to experience a force when near another charged particle.
can be positive or negative
due to imbalance of electrons and protons.
in electricity it is measured in coloumbs.
define a coloumb
SI derived unit for charge
the amount of charge that passes a point when 1 current flows for 1 second.
6.24 x 10^18 electrons
what is current
the amount of charge passing a point over time.
measured in amperes - SI base unit
where 1 amp = 1 coloumb/ second
can be direct or alternating
what is electrical voltage?
the energy required to move one coloumb of charge between 2 points. also known as potential difference.
1 volt = electromotive force that can give 1 joule of work for each coloumb of charge passing a point
VOLT = joules / coloumb
(W=AV - can convert coloumb to amps x s)
draw a graph for AC and DC current
DC - always in same direction. doesnt have to be of constant magnitude e.g. can get a wave within positive volts but not below 0
AC - oscilates back and forth in both directions.
what is the frequency and voltage of UK mains?
50Hz
240 V - root mean square
(+340V to -340V)
define resistance…
the opposition to flow in a circuit
usually a property of direct current
measured in ohms
where 1 ohm is the resistance when 1 amp flows when 1 volt is applied
what is reactance?
reactance is the opposition of AC current - it is a sum of inductive and capacitive reactance
also measured in ohms and is dependant on frequency of AC current
what is impedence
the total of reactance and resistance in AC cirucuit
measured in ohms.
how does capacitance and inductance alter with frequency?
increased inductance with increased frequency
reduced capacitance with increased frequency
what are resistors made of?
thin wire often coiled to make them more compact
the thinner and longer the wire, the more resistance.
what is the difference between a conductor, semi conductor and a insulator?
conductor - allows flow of electrons and hence conducts electricity. measured in siemen
insulator - do not allow flow of electrons.
semi-conductor - low conductance e.g. metal oxide
what is power?
power is measured in watts
it is the amount of joules used per second
i.e. rate of energy usage
W = J/s
what is a transducer?
device that converts one form of energy into another e.g. microphone - sound to electric
draw a circuit in series and in parallel
in series - resistance is addititive and current is the same the whole way round
in parallel - current is additive and resistance is a sum of recipricols
resistance in series = voltage divider
in parallel = current divider
explain what 240V main current means?
this is the root mean squared
and gives an equivalent of the DC current that AC current is producing
actually +340V to -340V = average of this would be 0
instead
take square of voltage and then the mean and then square root it.
what are the advantages of AC current?
- easier to generate and therefore cheaper
- easier to switch off compared to DC
- easier to transform - i.e. change current and volts i.e. easier to convert AC to DC but not back
what is magnetism
a physical property associated with motion of charge within a substance.
results in attractive and repulsive forces between objects
what is a magnetic field
the region around a magnetic object that can produce force that acts on other magnets.
what is a ferromagnetic material?
a material exhibiting a strong magnetism due to allignment of its magnetic moments with a magnetic field. e.g. iron, nickle
may produce permanet or temporary magnets
where does magnetism originate from within a material?
all materials have electrons that spin. this creates a magnet. however these spins can cancel one another.
in ferromagnetic materials these spins allign with magnetic field.
initial magnetism relies on external magnetic field for this allignment.
the material may remain magnetised forever or it could be temporary
what is an electromagnet?
when charge flows through a conductive wire a magnetic field is created around this.
how does earth have a magnetic field?
molton core with current flowing
how is strength of a magnetic field defined?
magnetic flux = Φ = weber
amount of magnetic field passing through a given surface
define weber
the magnetic flux that would generate a potential difference of 1 volts in a coil of 1 turn if it were allowed to decay over 1 second
Wb = V.s
what is magnetic flux density?
the amount of weber (magnetic flux) per m2
e..g. Tesla = Wb/ m2 = SI unit
what is earths magnetic field?
0.5G
draw a magnet and magnetic field around it
the denser the lines, the higher the magnetic flux.
how is the strength of an electromagnet altered?
number of coils i.e. 2 coils, doubles it
wrapping around an iron core - this is because iron has high magnetic permeability compared to air.
what are the uses of electromagnets
can be used to induce currents in other circuits e.g. transformers
magnetism of materials - with a simple switch on and off
what happens if a wire is placed in a magentic field?
induces current in that wire.
proportional to the flux densitiy
what is faradays law
V = N. (dΦ/dt)
voltage created in a wire that is placed in a magnetic field is proportional to the number of turns in the coil and the rate of change in magnetic flux.
what is the difference between a magnet and charge?
magnets are dipolar - one north, one south. can be induced.
charge - either positive or negative
what is meant by dimagnetic and paramagnetic
ferromagnetic - attracted to magnet
paramagnetic - weak attraction
diamagnetic - repulsion
what is meant by magnetic hysteresis. draw a graph
hysteresis describes the phenomena whereby a state of a material/ system and hence its response depends on its history
magnets show this
initially magnetised to B
when 0 magnetic field, will still remain magnetised C
reversing magnetic field can bring its its magnetism back down to 0 but will never reach a state where it remains unmagnetised without field e.g. 0 , 0
H = magnetic field applied
B = magnetic flux of the object
what is a capacitor?
A device that stores electrical charge
what is capacitance
the ability of a material to store electrical charge
measured in farads.
define a Farad
1 farad = ability to store 1 coloumb of charge when 1 volt applied
F = C/V
describe how a capacitor works
2 conductive plates, separated by dielectric material (insulation - air, glass, ceramic).
electrons flow on one, repelled by another, charge flows until fully saturated.
AC current - charges and discharges - favours this and the higher the frewuency.
Describe the current flow in DC circuit with a capacitor..
initially high flow of current as capacitor is charging up.
then dissipitates - exponential decay
describe the current flow in an AC circuti with a capacitor..
current flows in both directions and can continue
the faster the charge in current - i.e. higher frequency, the higher the root mean square current.
how does an inductor work
consists of coil of wire +/- ferromagetic core to strengthen it
the EMF created to change in current
so initially very high and then dissipitates exponentially
therefore the current flow is slowed as iniitally EMF is high and as it reduces the current builds up.
explain how a defibrilator works
2 adhesive pads placed on patients chest with good contact
5000V batery - transformer and AC –DC conversion
parrallel circuit
capacitor
inductor
switch
what is the difference between monophasic and biphasic defibrilation?
monophasic - used originally. current in one direction. needed higher peak current and higher energy for same outcome (360 joules , 45amps)
biphasic - current goes into myocardium and back out. lower enerfy and lower current needed (150joules and 20amps ). less burns and damage to myocardium
what are fuses and circuit breakers?
fuse - piece of electrical equiptment that breaks the circuit when current gets to certain amplitude. usually a wire that heats up and melts with current.
circuit breaker - use an electomagnet that will pull on a switch when the magnetic force reaches a certain point.
advantage of a circuit breaker is the switch can easily be put back.
what is a transformer?
piece of electrical equiptment that uses electromagnetic induction to transfer electrical energy from one circuit to another.
allows stepping up and down voltages
made from 2 coils of wire - one from each circuit
sometimes wrapped around iron/steel core to improve strength.
the magnetic field created in one induces current in the next circuit.
the voltage is proportional to the number of coils and thus can be worked out via the ratio of number of coils in each circuit. - faradays law
this only works with AC current as inductance relies on changing current
what is a transistor?
piece of electrical equiptment that will allow flow of current depending on flow in another limb of the circuit.
when there is current through the base it will allow flow via the other limbs
what is class I, II and III electrical equiptment ?
class 1 - earth casing - all conducting surfaces are earthed so that they have 0V.
class II - double insulated casing - may be earth free but outer casing is not exposed to conducting surface.
Class III - battery operated with safety extra low voltage (SELV) e..g 24V of power - not enough to cause damage.
what is the difference between type B, BF and CF electrical equiptment ?
classed based on leakage current
usually for equiptment inserted into the body and risk of microshocks
B = max AC 500uA , DC 50uA
multiple circuits AC 100uA, DC 10uA
BF = same as above but floating circuit i.e. isolated
CF = cardiac floating
max AC / DC 50uA, multiple 10uA
how is AC current produced?
wire placed into a magnet and magnet or wire rotated
alternating magnetic field induces current e.g. as wire approaches N pole - e in one direction and then south pole - e in other direction. AC current produced.
at power station a 3 phase AC current produced
out of phase because exposed to magnetic field at different times as the magnet spins.
describe how mains electricity is generated and travels to a house..
275KV at power station
step down transformers and again at local substation.
240V provided to each house
what occurs at the local electrical substation
step down transformer to give 240V
one of the limbs is earthed and other provide Line wire to supply house - 3 of each.
the substation earthing is to protect houses from lightening - the current from lightening will go down earth rather than house.
what is the role of earthing an appliance draw a diagram.
the substation needs to be earthed to protect from lightening.
however now if patient touches circuit, current can flow through earth and down patient.
by adding a local earth, the circuit can be completed with a path of lower resistance than person to protect person
some current will flow through patient however depends on ratio of resistance e.g. if patient has resistance 10x more than earthing then 10x less current through patient.
draw class 2 equiptment and explain mechanism
insulated to prevent leakage currents getting to person
however if they do , also earthed casing
what is meant by earth free supply
isolating transformers are used to isolate electrical equiptment from mains.
ideally all equiptment would be like this but expensive
only safe if there is no accidental earth connection
how does frequency of current effect likelihood of shock?
current amplitude needed to give muscle spasm / VF
because biological cells have natural freq of 50Hz so will resonate at this freq - most damaging
what is meant by equipotential bonding?
different pieces of equiptment may be at different potentials and current can flow between them if connected. patient may be a conduct for this current.
terminals of each piece of equiptment are connected together to bring them to the same potential - earthed
what is a galvanometer and symbol
measures small currents.
