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Basic facts about electrical gadgets

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Title: Basic facts about electrical gadgets


1
Basic facts about electrical gadgets
  • Dr. Don Franceschetti
  • Physics 1010
  • Last Lecture !!!!!

2
Remember basic facts about atoms
  • There is a positive nucleus of protons and
    neutron.
  • The orbital electrons balance the charge and
    provide chemical bonding.
  • Rubbing can transfer electrons from an insulator.
  • Electrons can run around inside a conductor
    (metal).

3
Electric Charge
  • Electric charges can be positive or negative.
  • Bulk matter is positively charged if it has fewer
    electrons than protons.
  • Bulk matter is negatively charged if it has more
    electrons than protons.
  • Electrons get transferred when dissimilar
    materials are rubbed against each other. This is
    static electricity.

4
Attraction and Repulsion

5
Tricks for storing charge
  • Use parallel metal plates (capacitor or Leyden
    jar) separated by a thin insulator.
  • Excess charge arranges itself to screen out
    electric field inside conductors (van de Graff)

6
The Leyden Jar
  • Was the first device to store static charge
    today.
  • Basis for the capacitor today.
  • Funny factoid. Ben Franklin tried to electrocute
    a turkey with a Leyden Jar. Nearly killed himself
    instead.

7
A Capacitor

8
And the battery
  • Acts like a pump to set up a certain potential
    energy difference per electron.
  • Changing the plate spacing changes the amount of
    charge how your computer keyboard works.

9
The Coulomb
  • 1 coulomb (C) is a lot of charge.
  • Elementary charge is 1.6x10-19C.
  • Charge on protons in 1 g. of hydrogen is 96,500 C.

10
Induced Polarization

11
Water has a permanent dipole

12
Electric Potential
  • Moving electrically charged objects subject to
    electrical forces changes their potential energy.
  • Electric potential
  • Electric potential energy/charge
  • volt joule/coulomb

13
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

14
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

15
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.

16
Electric Current
  • An overall slow drift of electrons caused by an
    electric field. Speed may be km/s but drift is
    cm/sec.

17
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.

18
This battery is a lemon!

19
Something fishy about this battery..

20
You can think of a battery as a pump

21
Simple circuit

22
Pump analogy again

23
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

24
Electric Shock
  • Skin resistance is
  • High when dry
  • Low when wet
  • Current can kill you
  • Especially through the heart

25
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.

26
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.

27
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).

28
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

29
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).

30
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!

31
Electromagnetic induction
  • Faraday found that changing the magnetic field
    inside a coil created (induced) a voltage.
  • Rotating a coil in a magnetic field accomplishes
    the same thing. Basis of electric generator and
    most of 20th Century.

32
Converting AC to DC is Easy
  • Use a diode, changes ac to pulsating dc.
  • Capacitors and coils smooth out the current.

33
What do you get from the electric company?
  • Electrical Energy measured in Kilowatt hours.
  • You supply the electrons.

34
Nikola Tesla 1856-1943

35
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.

36
A parallel circuit in the making

37
House wiring is usually in parallel. But fuse
or circuit breaker is in series.

38
Edisons bright idea
  • Success has many fathers.

39
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

40
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 -

41
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

42
Earth has a magnetic field.

43
Magnetic compass
  • Also shoes dip.

44
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

45
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 .

46
Current carrying wired have a magnetic field.

47
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.

48
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.

49
Permanent magnets

50
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.

51
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.

52
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.

53
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!

54
Electromagnetic induction
  • Faraday found that changing the magnetic field
    inside a coil created (induced) a voltage.
  • Rotating a coil in a magnetic field accomplishes
    the same thing. Basis of electric generator and
    most of 20th Century.

55
Magnetic levitation

56
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!
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