Current Electricity and Electromagnetism

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Current Electricity and Electromagnetism

• Dr. Don Franceschetti
• Physics 1010
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Electric Current is

• The flow of electric charge
• In a metal, the flow of conduction electrons
• Measured in Amperes (amps, A)
• Remember
• Coulomb in the unit of charge
• Elementary charge is 1.6x10-19 C, this is the
  quantum of charge

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Amps, Watts, Ohms, Volts (AVOW)

• Amps measure current flowing past a point in a
  circuit. AmpCoulomb/sec
• Watts measure electrical power supplied by a
  source. WattJoule/sec
• Ohms measure resistance to current flow.
  O Volt/Amp. MhoAmp/Volt, now
  called Siemens,
• Volt measures potential energy difference.
  VoltJoule/Coulomb

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Field Concept

• We imagine each charge as surrounded by an
  electric field. Field points away from positive
  charges and towards negative.
• Batteries set up an electric field in the
  conductors they are connected to.

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Electric Current

• An overall slow drift of electrons caused by an
  electric field. Speed may be km/s but drift is
  cm/sec.

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Voltage sources

• Create a difference in electrical potential
  across the ends of a conductor. Electrical
  potential (voltage) is potential energy/charge
• 1 volt 1 joule/coulomb
• Battery converts chemical energy in to
  electricala source of d.c.
• Photocell converts light energy into electrical
  a source of d.c.
• Thermocouple converts heat energy into
  electricala source of d.c.
• Generator converts mechanical energy into
  electricity a source of a.c.

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This battery is a lemon!

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Something fishy about this battery..

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You can think of a battery as a pump
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Simple circuit

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Pump analogy again

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Normal conductors

• Offer a resistance to electric currentturn
  electrical energy into heat.
• Obey Ohms Law VIR.
• But superconductors offer no resistance at all!
• R measured in ohms
• 1 ohm 1 volt/ampere

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Electric Shock

• Skin resistance is
• High when dry
• Low when wet
• Current can kill you
• Especially through the heart

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More about bioelectricity

• Nerves carry electrical impulses across their
  membranes allowing message speeds of 100m/s.
• Some fish use electrical fields to detect prey.
  A few use electric discharges to kill them.
• Galvanic Skin Response used in cheap lie
  detectors.

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More about bioelectricity

• Electrocardiogram (ECG, EKG) records small
  voltages due to heart activity.
• Current through heart may be needed to
  synchronize muscle fiber contractions
  (fibrillation).
• Electroencephalogram (EEG) records small voltages
  due to brain activity.

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Electroconvulsive therapy

• A. k. a. electroshock
• Based on observations that febrile seizures were
  often followed by improvement in schizophrenic
  patients,
• Mechanism not fully understood.
• Still in use!
• May be replaced by TCM (transcranial magnetic
  stimulation).

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Electric Power

• PIV, wattsamps x volts
• Using VIR, power loss in a resistance is
• PI2R
• PV2/R
• The higher the voltage and the smaller the
  current, the less energy is lost getting from the
  source to the destination

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Direct vs. Alternating

• Direct is simple to understand, but hard to
  transform from low voltage to high to reduce
  power loss.
• Alternating is easy to transform, but hard to
  describe with equations.
• Edison loved DC, Westinghouse and Tesla loved AC
  and Westinghouse won (fortunately).

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More on AC

• Frequency 60 Hertz is most common in US.
• In some places 25 hertz, 30 hertz or 50 hertz is
  used.
• Root-mean square voltage is 120 volts in US, made
  lightbulbs glow as brightly as a gas lamp.
• Europe uses mostly 220 volts. Buy a converter!

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Converting AC to DC is Easy

• Use a diode, changes ac to pulsating dc.
• Capacitors and coils smooth out the current.

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What do you get from the electric company?

• Electrical Energy measured in Killowatt hours.
• You supply the electrons.

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Nikola Tesla 1856-1943

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Simplest circuits

• Series current flows through each element in
  sequence. Break circuit anywhere and all current
  stops
• Parallel current flows through several parallel
  paths.
• House wiring is in parallel.
• Fuses or circuit breakers used to prevent
  overload.

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A parallel circuit in the making

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House wiring is usually in parallel. But fuse
or circuit breaker is in series.

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Edisons bright idea

• Success has many fathers.

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Magnetic Forces

• Exist between moving charges in addition to
  electrostatic force.
• Permanent magnets exert magnetic forces because
  charge motion is synchronized on atomic level.
  Only a few elements (Fe, Co, Ni, Gd) and a few
  alloys, like Alnico can be made into permanent
  magnets

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Permanent magnets have poles

• North pole is really north-seeking pole
• South pole is really south-seeking pole
• So the Earths N pole is really a S pole and
  vice versa.
• Like poles repel each other.
• Unlike poles attract each other.
• Dont confuse N and S with and -

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Magnetic Fields

• Point from North Pole to South Pole.
• Exert force only on moving charges and currents.
  Force is perpendicular to both field direction
  and velocity.
• Exert torques on permanent magnets, making them
  line up with firld

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Earth has a magnetic field.

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Magnetic compass

• Also shoes dip.

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Magnetic Force acts on Currents or Moving Charges

• Parallel currents attract.
• Anti parallel currents repel.
• Torque on current loop makes it line up with
  magnetic field.
• Basis for galvanometer

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Magnetic field

• Directed from north pole to south pole of
  permanent magnet.
• Directed in circles around current carrying wire.
• Adds (like vectors) from different forces.
• B (maximum value of F)/qv on charge q moving at
  speed v .

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Current carrying wired have a magnetic field.

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On a moving charged particle

• Magnetic force is at right angles to both field
  and velocity.
• In uniform magnetic field, path is circular or
  helical.
• Used in mass spectrometer and cyclotron to
  separate particles by mass or speed.

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Ferromagnetic materials

• Force between neighboring atoms lines up electron
  currents
• (Currents due to electron spin and orbital
  angular momentum)
• Unmagnetized material is composed of microscopic
  domains, each a tiny magnet.
• Magnetizing occurs because domains grow or
  shrink. Basis of much recording/data storage
  technology
• Magnetism disappears above Curie temperature.

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Permanent magnets

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Electromagnets

• Current flowing through a solenoid (coil) creates
  stronger magnetic field.
• Inserting an iron core makes field much stronger
• Magnetism can be turned on or off.

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Magnetic Earth

• Earth is a big magnet.
• Cant be permanent magnet too hot inside. Must
  be a dynamo.
• Layers of some rocks record evidence for changes
  in magnetic field direction. More than 20
  reversals in past 5 million years.
• May be due to collisions with asteroids.

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Magnetism and the solar system

• Van Allen belts, charged particles trapped in
  Earths magnetic field.
• Sun has complex magnetic field which changes over
  a 22 year cycle. Tied to sunspot activity.
• Other planets have magnetic fields too. Useful
  in modeling their innards.

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biomagnetism

• Magnetite (a magnetic iron oxide) granules found
  in
• Bacteria
• Pigeon brains
• Bees
• Wasps
• Monarch butterflies
• Sea turtles and fish
• 1992, discovered in human brains!

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Electromagnetic induction

• Faraday found that changing the magnetic field
  inside a coil created (induced) a voltage.
• Changing a magnetic field produces an electric
  field.
• Maxwell realized that changing a magnetic field
  produced an electric field.

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Magnetic levitation

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Maxwells Equations

• Summarized all that had been discovered about
  electricity and magnetism.
• Showed that self-sustaining electric and magnetic
  fields would travel through space at exactly the
  speed of light.
• Light is an electromagnetic wave.
• Other forms of electromagnetic waves exist
  (radio, microwave, x-ray, etc).
• Led to Einsteins discovery of relativity to
  explain how speed of light always measured to be
  the same, even by observers in relative motion!