Electricity
Static and Current
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Electricity
The flow of electricity is simply a stream of
tiny particles called electrons. These
electrons flow from one substance to another.
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Kinds of Electricity
Static: Electricity at rest. No movement.
Current: Electricity moving.
Current Electricity can be alternating (AC) –
obtained from generators and used in houses for
lighting and power.
Or it can be direct (DC) obtained from batteries or
dry cells and is used in flashlights and ipods.
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Static Electricity
There are two types of electric charges: Positive
and negative
The Law of Electric Charges
Charged objects attract neutral objects.
Objects with like charges repel each other
Objects with unlike charges attract to each other.
The interactions of these laws occur inside of
electric fields which are the areas over which the
electric charges can exert force. An electric field is
strongest near the charged particle.
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Creating Charge Differences
There are three basic ways to create charge
differences that drive electrical flow:
Charging by friction
Charging by contact
Charging by induction
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Charging by Friction
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Charging with Static Electricity
In our model of the atom we know that atoms
are made of protons and neutrons in the
nucleus and electrons held weakly outside of
the nucleus. Electrons are the only particles
transferred during friction.
The transfer of electrons from one object to
another results in one object having excess
electrons and a negative charge and the
other object having a deficit of electrons and
a positive charge.
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Electrostatic Series
The electrostatic series is a list that can be
used to predict the transfer of electrons from
friction.
A substance higher on the list will always
lose electrons and become positively
charged while a substance lower on the list
becomes negatively charged as it gains
electrons.
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Electrostatic Series
(Weak hold on electrons)
Items further apart on the list will transfer
Acetate
Glass more electrons than items closer on the list
Wool and build up a greater electric charge.
Fur
Calcium, magnesium, lead
Silk
Aluminum, zinc
Cotton
Question:
Paraffin wax
Ebonite By mistake a silk blouse and a
Polyethylene (plastic)
pair of wool socks are put into a
Carbon, copper, nickel
Rubber clothes dryer. What charge will
Sulphur appear on the blouse when it
Platinum, gold rubs against the socks?
(Strong hold on electrons)
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Lightning
Static electric charge builds up during
a storm. Positive charge builds up at the
top of the cloud and negative charge
builds up at the bottom. When the force
of attraction between these unlike
charges becomes too great, there is a
discharge called lightning. (This
discharge is current electricity.)
The negative charges at the bottom of
the cloud can also discharge to the
ground below. (Current) This is what
Science@NASA :
happens when lightning strikes the science.nasa.gov/...
ground. /y2006/13sep_electr
icice.htm 2006
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Charging by Contact
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When a charged object comes in contact
with a neutral object, some of the
electrons are transferred between the
objects.
When a negatively
charged rod touches
this neutral sphere,
+ -
some of the electrons
+ -
-
+
+ - + move from the rod into
the sphere. The
sphere becomes
negative.
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+ - +
- + +
When a positively charged rod touches this
neutral sphere, some of the electrons from the
sphere move into the positively charged rod. The
sphere becomes positive.
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Charging by Induction
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When a charged object is brought close to
but not touching a neutral object, electrons in
the neutral object will respond to the force of
the charged object.
This creates a
charge on either end
of that object that is
temporary.
If the object is
grounded, so that
the electrons have a
pathway to escape,
a permanent
charge can be
established.
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Electroscopes
Electroscopes can be used to demonstrate changes
in charge by contact or induction.
Metal-leaf Electroscope
A small metal ball connected to a
metal rod. Hanging from the rod
are two thin metal strips called
leaves.
Because the parts of the
electroscope are made of metal
electrons can move freely within it.
Physlink Science Store.
http://www.physlink.com/estore/cart/GoldF
oilElectroscope.cfm. 2007
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Pith-ball Electroscope
A small ball hanging by a
thin thread.
Because the ball is very
light it moves in response
to small electric forces.
School Store.
http://store.schoolorders.com/images/me
dium/3134-01.jpg. 2007
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Current Electricity
Current electricity flows in
electric circuits.
Electricians and scientists
draw electric circuits using
circuit symbols.
All circuits must have at least
one source (e.g. battery), one
load (e.g. light bulb) and
connecting wires between
them.
Simple circuits have a single
source and load.
Symbolic representation of
the above circuit
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Common Circuit Symbols
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Series Circuits
In series circuits, there is only one path for
the electrons.
Note there are two
battery symbols,
representing the two
batteries.
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Parallel Circuit
There is more than one path in a parallel
circuit.
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Electric Current
The flow of electrons around a circuit is called
electric current.
The current (I) is measured in Amperes (Amp - Å)
by an ammeter, which is connected in series in the
part of the circuit where current is being measured.
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Voltage (Electric Potential Difference)
Voltage is the amount of energy that is gained by each
electron from a source and lost over a load. It is the
difference in electrical potential energy between two
points in a circuit.
It is measured with a voltmeter which is connected in
parallel with the load. The unit is the Volt (V)
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Ohm’s Law Summary
Unit Physics
Value Unit
Symbol Symbol
Voltage Volt V V
Amphere
Current Å I
(Amp)
Resistance Ohm Ω R
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Series and Parallel Circuits Lab Summary
Series Parallel
Definition Circuit with one Circuit with multiple
path for electrons paths for electrons to
to flow through flow through
Current
Voltage
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Series and Parallel Circuits Lab Summary
Series Parallel
Definition Circuit with one Circuit with multiple
path for electrons paths for electrons to
to flow through flow through
Current The same Split up among the
everywhere in the branches of the circuit
circuit
Voltage
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Series and Parallel Circuits Lab Summary
Series Parallel
Definition Circuit with one Circuit with multiple
path for electrons paths for electrons to
to flow through flow through
Current The same Split up among the
everywhere in the branches of the circuit
circuit
Voltage Split up among The same in each
the loads of the branch of the circuit
circuit
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Resistance
Resistance is the opposing force to the flow of
electrons through a part of the circuit. Its unit is the
ohm (Ω).
Ohm’s Law: The resistance of an object determines
the amount of current through the object for a given
voltage across the object.
Current = voltage/resistance
I = V/R
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Electric Power
Electric power can be calculated using the
current and voltage.
Power = Current x Voltage
P=IxV
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Fuses and Circuit Breakers
These devices protect circuits from over-
heating due to too much current.
Fuses
Inside the fuse is a narrow wire that will melt when
the current gets too high. This will break the
circuit and stop the current.
Circuit Breakers
In a circuit breaker there are two types of metal on
one strip. When the current gets too high the strip
bends, opening a switch and breaking the circuit.
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Safety with Electricity
While dealing with electricity you have to be
very careful. An electric shock could
seriously hurt or kill a person.
Don’t work with electrical circuits while the power source
is on.
Avoid water while using or working with electricity.
Avoid overloading circuits which could cause fires.
Maintain all electrical circuits in your home.
Have a qualified electrician do all electrical wiring work.
Never become a part of the circuit!
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