Transcript Section 2 - Lamont High

Section 2.1
Controlling the Flow of Electrical
Current
Insulators:
• Electrons are tightly bound to the nucleus
allowing little movement. Eg. wood
Conductors:
• Electrons are free to move to the positive
terminal when a voltage source is applied.eg.
metal
• The more charged particles (or ions) present
in a solution, the better it conducts.
• Pure water will not conduct electricity. Tap
water will.
Neon Signs
• Neon signs are a circuit where electrons must
flow through a gas.
• Usually the gas would be an insulator, but
when current is applied the electrons are
excited and free themselves from the atoms.
• This creates a mixture of charged particles
which conducts electricity.
• Different gases emit different colors of light.
Semiconductors
• At high temps they act like conductors.
• At low temps they act like insulators.
• Examples: Germanium and Silicon
Superconductors
• Perfect conductors (no resistance)
• Some metals act as superconductors at
extremely low temperatures.
Video: How Superconducting Levitation Works
Resistance: A measure of how difficult it is for
electrons to flow. Measured in ohms (Ώ)
• High resistance creates heat or light.
• Resistors: allow current to pass through but
provides resistance, limiting the current.
• Read P. 301. How does a polygraph work?
Put your answer into your notebook.
Switches and Variable Resistors
A switch is a device that allows the flow of
electrons or stops the flow. When the switch is
open, there is no flow, because there is a gap
in the conductor.
When the switch is closed, the switch becomes
the 'gap replacement' and allows the flow of
electrons to continue.
To change the electron flow gradually, a
variable resistor, or rheostat is used
(a dimmer switch, volume control knob).
Dimmer Switch
• Do p299 Act D-3 Investigating Conductivity
• Do 2.1 Check and Reflect P. 303 # 1-3,5-9
2.2. Modeling and Measuring Electricity
Modeling Voltage
A waterfall is used, as a model, to demonstrate
voltage. Water flows when there is a change in
the gravitational potential energy
(elevation). Electricity will not flow unless
there is a change in electrical potential
(voltage).
Modeling Resistance and Current
Flow of water in pipes is used, as a model, to
demonstrate resistance. The size of pipe
determines the volume of water allowed
through it. The amount of resistance, in a
circuit, determines the size of the current.
Ohm's Law
Georg Simon Ohm, a mathematician, proved a
link between voltage (V), current (I) and
resistance (R). The unit of resistance was
named after him, the ohm.
Ohm's Law states that as long as temperature
stays the same:
• the resistance of a conductor stays
constant, and
• the current is directly proportional to the
voltage applied
Ohm's Law
V = I x R,
I = V/R,
V = voltage in volts (V)
I = current in amperes (A)
R = resistance in ohms (Ω)
Do 1-3 p. 307
R = V/I
If the temperature of a resistor changes, the
resistance changes as well (resistance is usually
low when the resistor is cool, and as the
temperature increases, so does resistance).
Types of Resistors
Different resistors are used for different
applications, especially in electronics. There are
many styles, sizes and shapes. The two most
common are the wire-wound and carboncomposition types.
• Wire wound: The longer and thinner the wire the
higher the resistance.
• Carbon-composite: By varying the composition and
diameter different resistances are available.
Using Test Meters
Voltmeters measure voltage difference (voltage
drop).
Ammeters measure current (rate of flow) in amperes.
Small currents are measured using galvanometers.
Multimeters can measure voltage, current and
resistance in a circuit.
2.3 Analyzing and Building Electrical
Circuits
• Engineers and designers of
electrical circuits use
symbols to identify
components and
connections. A drawing
made with these symbols
is called a schematic or
schematic diagram.
Parts of a Circuit
1. A source provides energy and a supply of
electrons for the circuit.
2. A conductor provides a path for current.
3. A switching mechanism controls current flow,
turning it on and off, or directing it into
different parts of the circuit.
4. A load converts electrical energy into another
form of energy.
Circuits
• A circuit which is open
is said to be off
• A circuit which is closed
is said to be on (the
load is working)
because the current is
able to flow freely
• Open (off)
• closed (On)
Circuit Analysis Example - Bulldozer
The toy bulldozer has 2 loads, a motor and a bulb.
2 1.5V cells act as the energy source.
A switching mechanism connects to 4 wires.
Label these on the diagrams.
forward
backward
What are "series" and "parallel"
circuits?
• There are two basic ways in which to connect
more than two circuit components:
series and parallel.
*Check it out yourself at:
http://www.autoshop101.com/trainmodules/elec_circuits/circ101.html
• In a series circuit there is only one pathway
for the current to flow. If there is a break in a
circuit the whole circuit cannot function.
Adding components to a series circuit
increases the total resistance in the circuit and
decreases the current.
eg. lights get dimmer as you add more.
• In parallel circuit there are multiple pathways
for the current to flow. Components in the
circuit can have their own pathway to the
current source.
• A break in one section of the circuit will not
affect components in another section of a
circuit. Adding resistors in parallel decreases
the total resistance of the circuit.
Applications of Series and Parallel
Circuits
House Wiring - uses parallel circuits
Microcircuits are made up of transistors and resistors
built on an extremely small scale.
Microcircuits (Integrated Circuits) - transistors are used
with three layers of specially treated silicon, with the
middle layer receiving a small voltage, allowing it to
control the voltage in the outer layers
This allows the transistors to act as switches.
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