Lecture notes for 26 June 2007
Electric circuits and electrical safety
DC
or direct current, from batteries. AC is
alternating current, produced by generators.
The equations in this chapter, Ohm’s law and the power equation, apply
to DC and AC cases. Electronic devices
are inherently DC, have to have a “power supply” to convert AC to DC. Power is wasted in the process, and heat is
generated.
Electrical
devices are often represented as resistances, because they do have some amount
of resistance, large or small, and therefore use power according to P =
I2R. The power in watts how the device
is rated, but because of this equation,
it could be rated by its resistance instead. Thus this squiggly symbol
is important because it is a way of model real devices in a simple circuit.
Series and parallel resistances: Two basic ways to connect two or more resistances or
electrical devices: series or
parallel. What y’all need to know. SERIES:
Resistors in series each have the same current in them, and connecting
more resistances in series increases the total resistance. PARALLEL: Resistors in parallel each have the
same voltage across them, and connecting more resistances in parallel decreases
the total resistance.
What
they look like: Series: draw it with 3 R’s. Parallel: Draw it with 3, too. Show open circuit in
each. Same voltage across parallel. Thus parallel is used in electric utility
wiring, for homes and all users of the electric grid. Series would make no sense, as in the old Christmas
tree lights. New lights have a parallel
(shunt) resistor/insulator combination.
Circuit
breakers are needed because as more appliances or higher wattage ones are
connected, the total resistance is decreasing—this is one of the two
characteristics of a parallel circuit. That means current is increasing in the
circuit, because the voltage isn’t changing.
Different parts of the house have different circuit breakers to protect
them.
A
bimetallic strip is used to trip the circuit breaker and open the circuit so
that no current will flow. Fig 8.12.
SAFETY:
Discuss fig 8.9. Stereo unlikely to be
220v. What is the neutral wire? It’s at zero voltage, literally the potential
of the earth or ground. Hotwire varies
from 0 to 120. Other hotwire varies from
zero to –120 v. Ground wires and polarized plugs are the ways
that electrical devices are made safer. If the hot wire comes loose and touches the
metallic case then current flows to ground through the “dedicated” ground wire,
not through you. What then is advantage
of polarized plugs?
Electrical
safety highlight---your wet body has much less resistance than your dry body, possibly
allowing a lethal amount of current in the wet body case whereas for the same
voltage the dry body would get a slight shock.
Magnetism!
What is a magnet? Any material that’s capable of being
magnetized, meaning it can exhibit external N and S magnetic poles. Poles come in pairs also! External magnetization is really the result
of the lining up of the magnetic fields of many spinning electrons in a
material. That wasn’t fully know until the 1920s, about one hundred years after
the connection between electricity and magnetism was discovered in 1829, when
Hans Oersted was giving a lecture in and noticed that an electric current in a
wire caused a nearby compass needle to deflect.
All
magnetism is the result of electric charge in motion. In the ferromagnetic elements, iron, nickel, &
cobalt for example, the magnetic fields of many atoms (the outer electrons in
the atoms actually) line up with the magnetic poles in the same direction and
produce tiny magnets within the material.
For an external field to be created, these “magnetic domains” must be forcibly
lined up in the same direction by an external magnetic field.
Demagnetizing
by heat: Rapid and random motion of
atoms of a magnetized material above the Curie temperature causes magnetic
domains to become unaligned. 770 degrees F for iron. Also hitting a permanent
magnet hard enough can cause domains to be shaken out of alignment.
Current
in a wire always is accompanied by an external magnetic field. When a wire is coiled in a helix formation,
the magnetic field external to the wire is just like a bar magnet’s field. If the coil is wrapped around an iron bar
then the bar becomes magnetized and greatly increases the strength of the
coil’s magnetic field. See fig
8.18. This is an electromagnet.
Earth’s
magnetic field. Resembles a bar magnet
inside the earth with its south pole pointing approximately to geographic
north. Thought to be caused by earth’s rotation causing currents of
electrically charged particles deep below surface. Magnetic declination is the angle between
geographic north (“true north”) and magnetic north. This must be corrected for when using a
compass in navigation.
Electromagnetism
Two main points: 1)
Moving electric charges create magnetic fields. Already discussed. 2) A
magnetic field exerts a force on a moving electric charge. This is how motors, generators and most
speakers work. Motors convert electrical
energy into mechanical energy by the magnetic force on a wire carrying a
current (electric charge in motion). Generators convert mechanical work
(energy) into electrical energy. Same
set up as motor but the wire is rotated by mechanical force, so that a current
is created in the wire and in the external circuit.
Transformers make it possible to step up or step
down an AC voltage but not DC. The changing magnetic field created by the AC
current in the primary coil creates an AC current in the secondary coil, as
predicted by Faraday’s Law. This is one big reason that early in the history of
power transmission, AC was chosen instead of DC. Edison versus Tesla (immigrated from
Russia). Edison won. See figure 8.30. High voltage used to transmit electricity
because then current is lower and heat losses, P = I2R, are less.
