Title: Basic facts about electrical gadgets
1Basic facts about electrical gadgets
- Dr. Don Franceschetti
- Physics 1010
- Last Lecture !!!!!
2Remember 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).
3Electric 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.
4Attraction and Repulsion
5Tricks 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)
6The 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.
7A Capacitor
8And 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.
9The 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.
10Induced Polarization
11Water has a permanent dipole
12Electric Potential
- Moving electrically charged objects subject to
electrical forces changes their potential energy. - Electric potential
- Electric potential energy/charge
- volt joule/coulomb
13Electric 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
14Amps, 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
15Field 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.
16Electric Current
- An overall slow drift of electrons caused by an
electric field. Speed may be km/s but drift is
cm/sec.
17Voltage 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.
18This battery is a lemon!
19Something fishy about this battery..
20You can think of a battery as a pump
21Simple circuit
22Pump analogy again
23Normal 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
24Electric Shock
- Skin resistance is
- High when dry
- Low when wet
- Current can kill you
- Especially through the heart
25More 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.
26More 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.
27Electroconvulsive 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).
28Electric 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
29Direct 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).
30More 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!
31Electromagnetic 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.
32Converting AC to DC is Easy
- Use a diode, changes ac to pulsating dc.
- Capacitors and coils smooth out the current.
33What do you get from the electric company?
- Electrical Energy measured in Kilowatt hours.
- You supply the electrons.
34Nikola Tesla 1856-1943
35Simplest 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.
36A parallel circuit in the making
37House wiring is usually in parallel. But fuse
or circuit breaker is in series.
38Edisons bright idea
- Success has many fathers.
39Magnetic 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
40Permanent 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 -
41Magnetic 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
42Earth has a magnetic field.
43Magnetic compass
44Magnetic 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
45Magnetic 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 .
46Current carrying wired have a magnetic field.
47On 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.
48Ferromagnetic 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.
49Permanent magnets
50Electromagnets
- 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.
51Magnetic 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.
52Magnetism 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.
53biomagnetism
- Magnetite (a magnetic iron oxide) granules found
in - Bacteria
- Pigeon brains
- Bees
- Wasps
- Monarch butterflies
- Sea turtles and fish
- 1992, discovered in human brains!
54Electromagnetic 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.
55Magnetic levitation
56Maxwells 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!