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CONTROLLING
THE FLOW
Of Electrical Current
CONDUCTORS
 Electrons not as tightly
bound.
 They are freer to move.
 When a conductor is
connected to an electrical
source, the electrons
move toward the positive
end of the voltage source.
INSULATORS
 Electrons tightly bound to
the positive nucleus of
their atoms.
 They resist moving away
from the nucleus.
ELECTRONS MOVE WHEN VOLTAGE IS
APPLIED
The atomic structure of a substance affects
how well it conducts or insulates.
Some substances are more resistant to
electron flow than others.
SEMICONDUCTORS- AT HIGH TEMP THEY
CONDUCT; LOW, THEY INSULATE
SUPERCONDUCTORS
Superconductors are perfect conductors-
they have no resistance to electron flow.
Metals such as silver, copper, mercury, and
gold are all excellent conductors, but they
are not perfect conductors.
Electrons travelling through them still
encounter some resistance.
HEIKE KAMERLINGH ONNES
THE PROBLEM WITH
SUPERCONDUCTIVITY
 Onnes brought temperature of mercury down to absolute zero
using liquid helium, and found was a perfect conductor, with no
resistance to current flow.
 Substances have been found to superconduct at temperatures
above absolute zero, but still to low for any practical applications.
 Absolute zero: the lowest temperature that is theoretically
possible;
 –273.15°C
ELECTRICAL ENGINEERS
 Need to know how well different materials conduct electricity so they
can design devices that are both safe and effective.
RESISTORS
 Conductor which allows electric current to pass, but provides
resistance to it.
 More current flows through a resistor with low resistance, than one
with a high resistance.
 Resistance is a measure of how difficult it is for electrons to flow
through a substance.
 Resistance is measured in ohms.
THE GREATER THE RESISTANCE
Greater the energy gained.
Energy gained by the substance is
radiated as heat or light energy.
SOLUTIONS CAN ALSO BE RESISTORS
 The more charged particles, the
greater it conducts.
 Tap water & water in the
environment are good
conductors because they
contain dissolved minerals.
 Distilled water is not a good
conductor because it contains
only water molecules.
LIE DETECTOR TEST
 Polygraph machine is a practical application of resistance.
 Measures skin resistance, blood pressure, and respiration; all of these
change when people are under stress.
 Sweat (salt) contributes to a change in skin resistance.
 A lie should cause an increase in conduction between 2 electrodes
attached to the skin.
 In theory, people sweat more when they lie.
 This shows up as a peak on the graph plotted by the polygraph machine.
SWITCHES & VARIABLE RESISTORS
SWITCH
 Best method for turning electricity on and off.
 When a switch is on, two conductors are pressed together
so that current can flow from one to the other.
 When the switch is off, the conductors are separated and
no current flows.
 Most switches are in an insulated casing or a metal box to
prevent shocks and short circuits.
VARIABLE RESISTOR (RHEOSTAT)
 Sometimes you want to change the current flow gradually in
a circuit, instead of just turning it on or off… like you may
have a dimmer switch in your home for certain lights.
 Rheostats can increase or decrease the amount of current
in a circuit by adjusting the the portion of the resistor that
the current travels through.
CHECK & REFLECT
Page 303, #s 1-3, 6, 7 & 9