CONTROLLING THE FLOW Of Electrical Current

CONDUCTORS INSULATORS  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.  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