Download Controlling the Flow of Electrical Current

Controlling the Flow of
Electrical Current
Section 2.1 p. 297-303
Learning Outcome
• Identify electrical conductors and insulators, and compare the
resistance of different materials to electric flow (e.g., compare the
resistance of copper wire and nickel-chromium/Nichrome wire;
investigate the conduction of electricity through different solutions;
investigate applications of electrical resistance in polygraph or lie
detector tests)
• Use switches and resistors to control electrical flow, and predict the
effects of these and other devices in given applications (e.g.,
investigate and describe the operation of a rheostat)
Conductors and Insulators
• We know that for current to flow three things are required
• Load (what converts electrical energy)
• Path for current to flow (wires/conductor)
• Power Source
• Materials that allow electric current to flow are known as conductors
• The electrons are not tightly bound to the nucleus of their atom, and therefore are free to leave
• Voltage must be applied in order for current to flow
• Materials that don’t allow electric current to flow are known as Insulators
• The electrons are tightly bound to the nucleus keeping them in place
• Semiconductors: Fall somewhere in between, at high temperatures they conduct, and at low
temperatures they are insulators
• i.e. Germanium and Silicon
Insulator
Conductor with Voltage applied
Conductor
Conductors and Phases of Matter
• We know that certain solids (metals) make excellent conductors
• Certain Liquids that contain ions (+ & - charges) can conduct electricity
• Gases can also be used to conduct current
• Neon Signs
• Fluorescent Lights
• By applying electric current, the electrons of the gas become excited by the added energy and free
themselves from the positive charged ions, allowing for the flow of current electricity
• Different combination of gases creates different colors
• i.e. neon and argon create purple light
Super Conductors
• Even metals like copper, gold
and silver are not perfect when
it comes conducting electrical
current
• They still provide some
resistance
• Resistance is how difficult it is for
electrons to flow
• Superconductors
• These are the perfect conductor
• This is achieved by cooling the
conductor to almost Absolute
Zero (-273 C)
Using Resistance to Control Current
• We know Resistance is the measure of how
difficult it is for electrons to flow
• By controlling the amount of resistance in a
circuit you can control the amount of current
• Think of a scenario in a school
• The students represent the electrons
• The hallways represent the path that they travel
(wires/conductors)
• Which is easier to travel? At break when all students are in
the hallway, or during class?
• Resistance can be affected by:
• Size of the conductor (wire)
• Thinner wire makes it harder for electrons to travel
• Distance current has to travel
• Concentration of the solution (electrolyte)
• Resistance can be useful
• creates friction, which creates heat and light energy
(i.e. heaters, light bulbs)
Resistance in Polygraphs
• Polygraph is a lie detector
• It works by monitoring a persons heartrate, blood pressure, respiration, and
skin resistance
• All of these factors change when a person is under stress
• Skin resistance refers to its ability to conduct electric current
• Sweat is salty, and therefore an electrolyte
• As a person sweats more there skin will improve its ability to coduct electricity indicating
that they are stressed, and potentially lying
• This is tested by attaching electrodes
Switches
• Our simple circuit of a power source, path for current and load allow
for electricity to flow
• But there is now way to control the flow of that current (it will always be on)
• Therefore Switches are used to control when current is allowed to
flow and when it is not
Resistors
• Sometimes you need to be able to alter the amount of current
flowing in a circuit
• Not always as simple as on or off
• For this we use devices (switches) that control the current known as
Variable Resistors or Rheostats
• They work by increasing or decreasing the amount of current in a circuit by
adjusting the portion of the resistor that the current flows through
• i.e. volume controls, foot operated speed controls, dimmer switches, etc..