12.1 Introducing Current electricity

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Transcript 12.1 Introducing Current electricity

p.432-443
 Current
 Static
Electricity –
electrons stay in
one place on the
surface of objects
Electricity
– continuous flow
of electrons flow
through a
conductor in a
controlled path
 Electrons
are always moving
 The
steady flow of electrons in a circuit can be
directed and used to power devices
 Flow
of electrons comes into wires and then into
the device
A
continuous path in
which electrons can
flow
 Analogy: circulatory
system – a continuous
path where blood
travels through
“wires” or blood
vessels to where it is
needed
1.
2.
3.
4.
Conducting wire
Switch
Load
Power source
 Also
known as connecting wires
 Join all parts of a circuit
 Provides a path for electrons to flow
 E.g. Copper, aluminum wires
 Controls
current flow in an electric circuit
 When the switch is “on” the circuit is CLOSED
and the path is COMPLETE
 When the switch is “off” the circuit is OPEN and
the path is INCOMPLETE
A
device that transforms electrical energy
into other usable forms of energy
 Examples of loads: lamps, heaters, fans,
computer hard drives, motor, iPOD
 Provides
power for the electric device
 There are small and large scale sources of
energy
 Example of small scale power source:
batteries
 Example of large scale power source:
generating station
A
battery is a combination of
electrochemical cells
An
electrochemical cell is a package of
chemicals that converts chemical energy
into electrical energy that is stored in
charged particles
 Portable
device that
converts chemical energy
into electrical energy
 Made up of two electrodes
(conductors) in a
conducting solution
(electrolyte)
 One electrode is positive,
the other negative
 In
everyday language an electric cell is called a
battery
 In science, “battery” refers to two or more
electric cells in combination
 Electric cells are used in cameras, cell phones,
flashlights
TYPE
1.WET CELL
Description
2 metal electrodes are placed
in a liquid (electrolyte) and
react
Electrolyte conducts
electricity
Continuously replace
electrodes and electrolyte
2. DRY CELL
The electrolyte is a paste
Electrons stop flowing as the
negative electrode is used up
 Electric
cells that cannot be recharged over and
over; can only be used once
 Examples: zinc chloride & alkaline
 Electric
cells that can be recharged and reused
many times; they are recycled
 Examples: Lead-acid batteries (in cars) & nickel
metal hydride batteries (NiMH), Lithium Ion used in
cell phones
 Special
type of battery through which a
continuous supply of chemicals is pumped as
the cell operates
 A fuel cell can operate much longer than a
typical electric cell.
 Is an electrochemical cell that generates
electricity directly from a chemical reaction
with fuel, such as hydrogen.
 Much of energy made is wasted as heat
 Example:
hydrogen fuel cell – produces
electrical energy by converting hydrogen &
oxygen into water.
It
is the difference in electric potential
energy per unit of charge measured at two
points
Also
known as voltage and Electric
Potential
The
electric potential energy per unit
charge is often abbreviated to: electric
potential
Unit for voltage is volts (V)
There
is potential difference
between two terminals of an
electric cell.
Electrons leave the negative
terminal with electric potential
energy that can be used to operate
the motor
As
a result, the elections return to
the positive terminal of a cell with
less electric potential energy then
they started because some of the
energy was used to start the motor
Once inside the cell chemical
reactions re-energize the electrons
and send them out to the negative
terminal again
 In
this way the electric cell acts like a pump
A
voltmeter is a device used to measure potential
difference

A voltmeter must be connected in parallel with a
load or an energy source. The reason for this is
because the energy is relative to two points.
Electric Current is the rate of electron
flow past specific point in a circuit.
The unit for electric
current is called the
ampere (A). The
symbol for current is I.
An ammeter is
used to measure the
current flow.
An ammeter must be connected
in series to properly measure
current flow in a circuit.
1. Direct
Current
2. Alternating
Current
 Electrons
flow in one direction only
 Is produced by an electric cell such as a battery to
power portable electrical devices




Electrons move back and forth, alternating their
direction
Is produced by generators at electric generating
stations
Used at electric generating stations because it is a
more efficient method of distributing electrical
energy over long distances than DC
In an AC circuit, as the electrons move back and
forth, there is an instant at which the electrons
stop to change direction and at this point there is
no electric current
Electrons move
freely through the
wire until they
reach the light
bulb's filament.
Chemical reactions
within the battery cause
electrons to be stripped
away from the carbon
electrode. Electrons will
try to flow from a
battery's negative
terminal to its positive
one, if allowed.
A loop of wire spinning
through a magnetic
field will create an
alternating current.
Note: current will flow
only if the circuit
connected to the
generator is complete.
Is
the ability of a material to oppose the
flow of electric current; measured in
ohms (Ω)
Symbol of Resistance: R
Symbol of Units of Resistance: Ω
 Insulators
tend to minimize the amount of electron
flow and it has a high resistance
 Conductors have a very low internal resistance (this
is why the electrons flow so easily in copper wires)
1. Type
of Material
2. Cross-Sectional Area
3. Length
4. Temperature
The
type of material will affect
how freely the electrons move
within the material
E.g. copper is an excellent
conductor due to its low electrical
resistance
The
diameter of the wire will indicate how
well electrons flow
The thicker the wire the less internal
resistance and electrons have more room
to move freely
E.g. the greater the diameter of a water
pipe, the greater the water flow
 The
longer the wire, the
greater the internal resistance
and slow down because
electrons have to travel
through more material
 The longer the wire the
warmer it gets
 E.g. dangerous for very long
extension cords (to avoid
overheating manufacturers
make a larger-diameter wire)
When
a wire gets warmer,
the atoms in the wire gain
energy and vibrate faster
More collisions , resistance
increases with
temperature
Ohmmeter
– device used to
measure resistance *ADD: OR can
calculate
When connecting an ohmmeter it
must be connected in parallel with
a load
Resistors
are devices that are put into a
circuit that reduce the flow of electric
current so as to avoid overheating of
the wire
Some may consider any load a resistor
but true resistors are specifically used
for this purpose
E.g.
heavy ceramic resistors in larger
circuits, lightweight carbon resistors in
electronics OR dimmer switches and volume
controls (called variable resistors)
Read 11.1 and supplement
your notes
Complete p. 436 #1-3
p. 438 #1,3,4
p. 442 #1-3
p. 447 ?’s #2-5, 7-11