ORT21BMI

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ORT21BMI

• Topic 2--Review of electricity electrical
  circuits

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Clinical context

• Remember the patient from Scenario 1?
• You want to carry out a common test of retinal
  function--the electroretinogram (ERG)
• Where does the ERG come from in the eye?
• What conducts it to the outside of the eye?
• What does conduct mean?
• How big a signal is it?
• How is bigness of an electrical signal
  measured?
• What would you do to measure it?
• How would you connect the patient to the
  instrument?
• What are some necessary instrument
  characteristics?

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Learning Goals

• Whats electrical charge?
• Whats electrical current?
• What are the two key carriers of current?
• What is voltage?
• What is resistance?
• What is power?
• How do you determine the currents and voltages in
  a simple circuit?

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A reminder...
Patients eyes brain
Instrument
You
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History

• Ancient Greeks rubbing fur on amber caused
  attraction between the two substances
• Static electricity
• 18th century - Luigi Galvani and Alessandro Volta
  used frogs to understand electricity
• Dead frogs leg twitched when touched with
  dissimilar metal

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History

• Galvani and Volta agreed that frog bits touched
  by dissimilar metals twitched. Why?
• Galvani Intrinsic animal electricity causes
  the muscles to contract
• Volta The bimetallic arc somehow gives rise to
  the electricity and this causes the muscular
  contraction

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Notice what begins here

• Neurophysiology
• Nerves and muscles are electrical
• Electrical science
• Bimetallic junctions give rise to an electrical
  potential

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Electricity Basics

• All matter consists of atoms
• Atoms are made of protons, neutrons and electrons

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Electric Charge

• Protons positive () charge
• Electrons negative (-) charge
• Neutrons no charge
• Electrons can move from one atom to another

http//www.sciencemadesimple.com/static.html
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Static Electricity

• Electrons can move by rubbing two objects
  together
• Static electricicty the imbalance of positive
  and negative charges
• Opposite charges attract

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Electrical basics

• Electricity is related to magnetism
• Recall the electromagnetic waves from optics?
• Magnetism has north and south poles, electricity
  has positive and negative charges
• Like magnets, opposite charges attract, like
  charges repel
• Benjamin Franklin first identified these 2 types
  of charge as positive and negative

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Why is electricity clinically relevant?

• Retinal neurones communicate electrically
• The ERG consists of summed activities of many
  retinal cells
• Measuring devices used to record the ERG are all
  electrical
• The above movements of charge are called
  currents. So
• What is electrical current?
• How do you describe it?
• How do you measure it?

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Current

• Current is the flow of electric charge
• Metallic conduction electrons flow through
  conductor eg. Metals
• Ionic conduction charged atoms flow through
  liquids eg. Saline solution
• A material which doesnt conduct is an insulator
• What carries current is clinically important

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Metallic Conduction

• Electrical example--a copper wire.
• Metals are crystalline lattices where the atoms
  are arranged in nice, regular structures.
• Some of their electrons, when in this
  configuration, are not bound to their particular
  atoms
• These electrons are free to move under the
  influence of an applied voltage (well define
  that in a minute)
• Thus, in metals, it is the movement of electrons
  that carries current

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Ionic Conduction

• We arent metal.
• The frogs werent metal.
• Both they and us conduct electricity (remember
  nerve conduction and electrical synapses?)
• How do organisms carry current?
• Ions, both positive and negative, carry current
  in solutions
• Which ions carry current in organisms?

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Voltage

• Electric potential difference between two points
• Current is analogous to water flowing through a
  pipe
• Voltage is analogous to pressure difference that
  makes the water flow
• By convention, current flows from a higher
  voltage to a lower that is, from positive to
  negative

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AC and DC sources

• Direct current (DC) is a unidirectional flow
• eg. battery
• Alternating current (AC) reverses direction
  repeatedly
• eg. 50 Hz, 240 V mains supply
• Next week well discuss the safety implications
  of each type

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What determines the current caused by a given
voltage?

• The resistance does
• Think of water flow again--for a given pressure,
  more water will flow through a fire hose than a
  garden hose

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Units of Measurement
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How are they related?

• The relationship between these three quantities
  is V IR
• A sometimes useful mnemonic is the following
• To get one of the 3 quantities, cover it
• What remains is how to get it
• This relationship is Ohms Law

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Analogy for Ohms Law
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Analogy for Ohms Law
If flow rate stays the same and resistance
decreases, required pump pressure also decreases
http//www.allaboutcircuits.com/vol_1/chpt_2/2.htm
l
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How to connect things

• Resistors can be connected end-to-end this is a
  series connection
• They can also have all their ends connected
  together this is a parallel connection
• Complicated assemblies of resistors can be
  reduced to a single equivalent resistance, whose
  current or voltage (as needed) may be easily
  calculated

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Series combination
Rtotal R1 R2 R3, so i1 i2 i3 iTotal
iTotal V / Rtotal V / (R1 R2 R3)
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Parallel combination
iT i1 i2 i3. iT V/RT, using the
relationship 1/RT 1/R1 1/R2 1/R3
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What about more complicated configurations?

• Use the two combination rules youve just seen
  and gradually simplify the circuit until you can
  calculate the quantity you want.
• You may not have to completely break it down to
  do so.

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Exercise
For the three resistors connected to the 9 V
battery, calculate the source current
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Exercise
Calculate the total resistance and the power
supply current for the circuit shown below
Ans R 2000 ohms I 0.003 amps
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Power

• You can do work with electricity. Work expended
  over time is called power.
• A common way electrical power is demonstrated is
  via heating, either intentionally or by accident.
• The latter is an extremely common way to start
  fires.
• Electrical power may be described as P VI or
  P I2R
• Notice that square of the current above
• If something happens to increase current, things
  can get hot quickly!

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Power, contd

• Think about a defective extension lead, with
  corroded contacts, and a lot of things plugged
  into it.
• What would the current be, compared to a good
  cord?
• What would the resistance be?
• Which cord would have more power dissipated in
  it?
• How is this power dissipated?
• Whats the risk?
• Next week, electrical safety.

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