Transcript intro electricity 2009

comes from the electrons in an atom when they are
transfered them between objects.
++
electrons have a negative (-) charge
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Static
Current
 Build-up of electrical
charge
 Electrons move in
non-specific directions
 Flow of electricity through
wires
 Examples – lightning, zap
from carpet when touch
something, static cling in
clothes and hair
 Examples – anything with
a cord or wire to power,
hair dryer, phone, iPod
 Electrons move in definite
paths
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Most objects tend to be
meaning that they have equal number of
positive and negative charges.
# of Protons = # of Electrons
(positive charge) = (negative charge)
=
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Net Charge of Objects
Charged Object
Neutral Object
-8
-6
+8
Net Charge:
0
+10
Net Charge:
+4
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Law of Electrical Charges
Opposite
Charges Attract
+
-
Same
Charges Repel
+
- -
-
+ +
+
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Demonstration:
How do Balloons stick to walls?
+
+
- -+ -
1. Rub a balloon on a
sweater to “charge it up.”
2. Negative charges in
the wall repel from the
balloon.
-
3. The balloon sticks
because the positive
charges left over are
attracted to the balloon.
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A battery uses chemical energy to
move electrical charges.
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+
Circuits flow in a
circular path from
positive end to
negative end
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+
Light will not go on
because the wire is
not connected
to the battery;
no current will flow.
9
+
Light bulb turns
on because
the electrical
current can now
flow through the
complete circuit.
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• an accidental path for current to flow
• often causing risk of shock or fire
• a branch with zero or very low resistance
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Safety Devices
Fuse - small piece of metal that melts if
the current becomes too high opening
switch
Circuit breaker - piece of metal that bends
when it gets over heated opening switch
because current is too high
GFI (ground fault interrupter) - usually
found in bathrooms or garages shuts off plug
when current if too high.
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• Wire
+
• Power Source (1.5 V)
• Bulb
• Resistance
open
closed
• Switch
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Series Circuits
Parallel Circuits
WATCH!
Do you notice any difference between the wirings
of the two circuits above?
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Series Circuits
The light bulbs turn on.
.75 V
.75 V
Battery
is 1.5 V
+
However,
since the
voltage drops,
the lights are
dimmer.
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Series Circuits
If this light bulb does not turn on,
Burned out light bulb
This is because the
circuit is broken here;
Battery
is 1.5 V
neither light bulb
will turn on.
so the current can’t
flow to this bulb here.
+
Christmas lights were commonly made this way.
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Parallel Circuits
Current divides and
has more than one path.
+
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Parallel Circuits
The current divides because it has more than one path.
+
Even though the
path is stopped, the
other light turns on
because its circuit is
not broken.
Burned out
light bulb
Your house is wired with parallel circuits.
Why do you think this is so?
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… house was wired in series.
What would happen if a light
bulb burned out?
How would you have to find
out which one it was to fix?
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Series Circuits
Parallel Circuits
Only 1 path for electricity to go
Many paths for electricity to travel
When 1 light in the path goes
out, they all go out because there
is no other path.
When 1 light in the path goes out
the electricity can find another way
to transfer the current
Voltage is reduce by each resistance
added like another light bulb, heater,
or appliance.
Voltage is the same at each branch
because each branch is like a
separate series circuit connected to
a source.
Current is the same in all points
Each branch does NOT always have
the same current because it depends
on how much resistance is in each
branch.
Example: Saw has more than a lamp
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