Download ISNS3371_041907_bw

ISNS 3371 - Phenomena of Nature
Circuits in Series
•Resistance (light bulbs) on same path
•Current has one pathway - same in every part of the circuit
•Total resistance is sum of individual resistances along path
•Current in circuit equal to voltage supplied divided by total resistance
•Sum of voltages across each lamp equal to total voltage
•One bulb burns out - circuit broken - other lamps will not light (think of
string of old Christmas lights)
ISNS 3371 - Phenomena of Nature
Water Analogy for Series Circuits
ISNS 3371 - Phenomena of Nature
Parallel Circuits
•Bulbs connected to same two points of electrical
circuit
•Voltage same across each bulb
•Total current divides among the parallel branches equals sum of current in each branch - current in each
branch inversely proportional to resistance of branch
•Overall resistance of circuit lowered with each
additional branch
•Household wiring (and new Christmas light strings)
designed in parallel - too many electrical devices on too much current - trip fuse/breaker
ISNS 3371 - Phenomena of Nature
Water Analogy for Parallel Circuits
ISNS 3371 - Phenomena of Nature
AC vs DC Current
Direct Current (DC) - electricity that you get from batteries - current
(movement of electrons) flows in one direction - from positive (high
potential) to negative (low potential) - note: electrons actually flow from
negative to positive.
Alternating Current (AC) - electricity that comes from the wall plug in your
home - current alternates in direction flowing first one way then the other
- electrons move back and forth in wire
ISNS 3371 - Phenomena of Nature
System of AC electricity in the US cycles the current from positive to
negative and back to positive 60 times a second - 60 Hz.
Standard household wiring design has two 120 volt "hot" wires and a
neutral which is at ground potential
- two 120 volt wires are obtained by grounding the centertap
of the transformer supplying the house so that when one hot
wire is swinging positive with respect to ground, the other is
swinging negative
- allows the use of either hot wire to supply the standard 120
volt household circuits.
Major advantage of AC over DC - AC voltages can be transformed to
higher or lower voltages.
- high voltages used to send electricity over great distances
from the power station can be reduced to a safer voltage for
use in the house
- higher voltage in transmission means less loss of power
ISNS 3371 - Phenomena of Nature
Magnetism
ISNS 3371 - Phenomena of Nature
The situation with forces due to magnetism is more complicated that
electrical/Coulomb forces.
-despite numerous searches, no evidence of
magnetic charges exist. In other words, there are
no particles which create a radial magnetic field
(usually termed monopoles by physicists) in the
way an electric charge creates a radial field.
Regions called
magnetic poles
produce magnetic
forces - a north
magnetic pole never
exists without a
south magnetic pole
- together called a
dipole
As for charges - like poles repel
and opposite poles attract.
Cutting a magnet in half will not isolate
a single north or south. One magnet
becomes two, then four, and so on.
ISNS 3371 - Phenomena of Nature
Magnetic Field Generation by a Moving Charge
Magnetic fields can be created by moving electric charges, i.e. currents
produce fields.
Follows right-hand rule: point thumb
of right hand in direction of current magnetic field curls around wire in
direction of curled fingers
If the current flows in a
loop, the magnetic field
produced is like a bar
magnetic - curl fingers of
right hand in direction of
current flow - north pole
is in direction of thumb.
ISNS 3371 - Phenomena of Nature
The electron spins on its axis, giving rise to a
electron current in the direction of rotation.
The electron is like a magnetic dipole, a
miniature magnet, with a north end and a
south end.
In most substances, electrons spin in random directions - magnetic fields
cancel. For iron and other magnetic substances, the spin magnetism is
not canceled. Can be permanently magnetized by placing in strong
magnetic field and permanently aligning atoms - can be demagnetized by
dropping magnet and jostling atoms out of alignment.
Electromagnetic produced by wrapping coil
around iron bar - magnetic field produced
that aligns atoms in bar - more coils or
more current - larger magnetic field and
greater atomic alignment
ISNS 3371 - Phenomena of Nature
Force on a Moving Charge
There is amazing reciprocity in nature - if moving electric charges create
magnetic fields, then magnetic fields must exert forces on moving electric
charges, just as electric charges create and are ``acted upon by''
magnetic fields. Force is perpendicular to magnetic field and direction of
motion.
A beam of electrons is
deflected by a magnetic
field.
If the beam is confined to a
wire, then the wire feels a
force.
ISNS 3371 - Phenomena of Nature
Electric Motors
This magnetic force on a wire is the idea behind how electric motors
work. Current is passed through a loop of wire (or many loops - called an
armature). The currents on either side of the loop are in opposite
directions - the forces are therefore in opposite directions - in this
picture, up on one side and down on the other. Causes the loop to
rotate. The current is reversed every half revolution by stationary
contacts - brushes - on the shaft.
ISNS 3371 - Phenomena of Nature
Current Induction
Magnetic forces on charged
particles leads to the induction of
current in a wire moved through a
magnetic field - or a magnetic
field moved past a wire.
Pass a magnet through a coil of
wire - produce a current.
Faraday’s Law - The induced voltage in a coil is proportional to the
number of loops multiplied by the rate at which the magnetic field
changes within those loops.
ISNS 3371 - Phenomena of Nature
Electric Generators
Electromagnetic induction is the principle behind electric generators motors in reverse.
Motor - electrical energy is turned into mechanical energy
Generator - mechanical energy is turned into electrical energy.
Voltage is induced in a
rotating loop when it is
rotated in a magnetic
field.
In modern power generators, an armature is forced to spin in strong
magnetic fields by a turbine - in turn spun by steam or falling water
ISNS 3371 - Phenomena of Nature
Lenz's Law
When an electric field is generated by a change in magnetic flux
according to Faraday's Law, the polarity of the induced electric field is
such that it produces a current whose magnetic field opposes the
change which produces it. The induced magnetic field inside any loop
of wire always acts to keep the magnetic field in the loop constant.
If the magnetic field is increasing, the induced field acts in opposition to
it.
If the magnetic field is decreasing, the induced field acts in the
direction of the applied field to try to keep it constant.