Transcript ELECTRICITY

Electric charge
• Electric charge is a property that allows a
charged object to exert a force (electric
force) on another charged object without
touching it.
• The force exerted can be attraction or
repulsion depending on the charges
3 phenomena relates to electric
charge
• Electricity
• Static electricity
• Magnetism
Electricity
Source of power to many things:
• Electrical appliances
• Lights
• computers
Static electricty
-Clothes sticking together
-Pieces of paper attracted by a
plastic object
Magnetism
• Magnets attracting metal objects
• The magnetic field of Earth
• Little magnets that holds to a door of a
refrigerator
Electric charge can be of 2 types
• Positive (+)
• Negative (-)
Interactions between charges
• Like charges repel each other, so:
Positive charges repel positive charges.
Negative charges repel negative charges.
• unlike charges attract each other, so:
Positive charges attract negative charges.
Electric field
• The space in which electric forces
between particles can be exerted.
• The strength of the electric field gets
smaller as distance is increased
Lines of force
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Lines of force
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Materials are made of small
particles.
Those particles, depending on the
material, can be:
• Atoms
• Molecules
• Ions
Atoms
• Atoms are made of even smaller particles.
Three of these particles are: Protons,
electrons and neutrons.
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http://en.wikipedia.org/wiki/Portal:Physics
Summary of subatomic particles
PROTON
NEUTRON
ELECTRON
In the nucleus
In the nucleus
Outside nucleus
Tightly bound
Tightly bound
Weakly bound
Positive charge No charge
Massive
Massive
Negative
charge
Not very
massive.
Neutrality of atoms
• In atoms the number of protons is the
same as the number of electrons so atoms
are neutral particles.
• For example a carbon atom has 6 protons
(6 positive charges) and 6 electrons (6
negative charges).
• So (6+) + (6-) = Zero
Mobility of particles
Electrons can move from atom to atom,
protons cannot.
This movement of electrons in a material
is called conduction.
The extension of conduction depends on
the material.
Ions
Particles in which the number of protons is
different from the number of electrons.
Cations
Positive ions in which the number of
protons is greater than the number of
electrons.
Anions
Negative ions in which the number of
electrons is greater than the number of
protons.
Static charges
Normally materials have an equal number
of positive and negatives charges (protons
and electrons) but they can develop static
charges. A static charge is a buildup of
charges of the same type, positive or
negative.
How does this happen?
As electrons have mobility
they can move from:
a) One atom to another. This process results
in formation of ions. The atom that loses
electrons becomes a cation and the atom
that gains electrons becomes an anion.
b) Electrons can move from one material to another. This
process results in the formation of static charge (buildup
of electric charges)
• If the material has more protons than
electrons it has a positive net charge.
• If the opposite occurs, the material has a
negative net charge.
• The net charge is the number of
unbalanced positive or negative charges in
an object.
Charging by contact
• Electrons can go from one object to
another when they are in contact.
• The object that receives the electrons
become negative and the other object
becomes positive.
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How materials affect static charging
Material
Skin
Glass
Hair
Nylon
Wool
Fur
Silk
Paper
Rubber
Polyester
People can get charged when
walking on carpets
Induction
http://www.physicsclassroom.com/class/estatics/u8l2b.cfm
Charge polarization
Charge polarization
Potential energy
Energy related to the position of an object
in a field, for example in a gravitational
field.
The higher an object is, the higher is its
gravitational potential energy.
Kinetic energy
Energy related to motion.
Electric potential energy
It’s the energy a charged particle has due
to its position in an electric field.
It takes energy to put a positive charge close to
another positive charge because of repulsion.
Electric Potential
It is the electric potential energy per unit
charge at a certain position in an electric
field.
Water does not flow between 2
places at the same height
Electric charges don’t move
between places with the same
electric potential
Charges move from a higher
electric potential position to a
lower electric potential position.
During this, part of its electric
potential energy is being
converted to kinetic energy and
part of it can be used to do work
or to produce heat.
Differences in electric potential also
explains why you get a shock
when touching a doorknob if your
body is charged.
The electric potencial in your body
is higher than the electric potential
in the doorknob. So electrons will
move from you to the doorknob.
As the electrons are moving, their
electric potential energy are being
converted do kinetic energy but
some of the energy is also
converted to heat, light and sound.
Lightning
It”s a high-energy static discharge
Lightnings in the world
Steps in the formation of
lightnings
Charge separation
Inside clouds collisions between
particles of moisture and with air
components molecules cause
them to become charged. Wind
and gravity will separate the
charges. Positive particles at the
top of the clouds and negative
ones at the bottom.
Steps in the formation of
lightnings
Charge buildup
Because of induction the negative
particles at the bottom of the clouds will
repel electrons on the ground making it
positive.
Steps in the formation of
lightnings
• Static discharge
• When the electric potential difference
between the bottom of the cloud and the
ground is large enough an electric
discharge of high energy will occur. This is
lightning. The energy released produces
the light and the sound (thunder).
Materials affect charge
movement
• Conductors
Materials in which electrons can move
easily.
For example metals in general.
• Non-conductor or insulators
These materials don’t conduct electrons
well. The electrons can’t move easily in
those materials. Examples: Plastic and
rubber
Resistance
• This property of a material is related to the
easiness or difficulty of motion of
electrons.
• It’s measured in ohms.
• All Materials have some degree of
resistance.
For example: Good conductors have low
resistance and non-conductors (insulators)
have high resistance.
Resistance depends on:
•
•
Amount of the material
The shape of the material.
Examples: A thin wire has more
resistance than a thick one.
A longer wire has more resistance than a
short one.
Unit for resistance
• The unit used for to measure resistance
is the ohm and the symbol is the greek
letter omega ()
Resistance can be used in
useful ways
Light bulbs
Electric heating
SUPERCONDUCTORS
Materials that have very low resistance at
very low temperatures.
Can be used in:
• Power lines (increase efficiency and
conserve energy).
• High-speed trains (reduce friction).
• Faster computers.
GROUNDING
• If a charge can pass through 2 different
materials, it will pass through the one with
lower resistance.
• Grounding means to provide a lowresistance path for charges for safety
reasons.
• Generally the charges go to the ground.
GROUNDING
Objective:
Protect buidings from lightning.
How it works.
Because lightning rods have lower
resistance the lightning will preferably
strike the rod instead of the roof.
ELECTRIC CURRENT IS A
FLOW OF CHARGE
• When a conductor has 2 points with
different electric potential a flow of charge
will flow from the highest potential to the
lowest one. If this flow of charge is
continuous we have an electric current.
• Electric current is a continuous flow of
electrons in a given direction.
UNIT OF ELECTRIC
CURRENT
• Ampere (amp)
• It’s a measure of amount of charge that
pass a certain point per unit time.
• One ampere is 1 coulomb per second.
ELECTRIC CURRENT CAN BE
COMPARED TO THE FLOW OF
WATER THROUGH A PIPE
The difference in the electric potential
(voltage) between 2 points is like pressure
pushing water through a pipe. Instead of
water, the difference in electric potential
makes the electrons move from the point
of higher potential to the point of lower
potential.
RESISTANCE IS LIKE THE
DIAMETER OF A PIPE.
Resistance affects the flow of electrons,
just like the diameter of a pipe affects the
flow of water.
HOW POTENTIAL AND RESISTANCE
AFFECT CURRENT
Current increases as potential increases.
Current decreases as resistance increases
Ohm’s Law
• Current = Voltage/Resistance
• I = V/R
• Amps (A) = Volts (V)/Ohms (Ω)
MEASURING ELECTRICITY
•
•
•
•
Volts: Voltmeter
Ohms: Ohmmeter
Amps:Ammeter
Combination of 3 instruments: Multimeter.
ELECTROCHEMICAL CELLS
They produce electricity as a result of
chemical reactions.
ELECTROCHEMICAL CELLS
CONTAIN
• Electrodes
They are electrical conductors
• Electrolyte
Materials containing free ions(they
conduct electricity because of this)
HOW DO ELECTROCHEMICAL
CELLS WORK?
Chemical reactions occur between the
electrodes and the electrolyte and a flow
of electrons is produced as a result of
these reactions.
DIRECTION OF THE ELECTRON
CURRENT IN
ELECTROCHEMICAL CELLS
The electrons go from the negative
electrode to the positive one through an
external wire connected to them.
BATTERIES
A battery is 2 or more cells connected
together but usually single cells are also
called batteries.
WET CELLS AND DRY CELLS
• Wet cells have a liquid electrolyte (page
31).
• Dry cells have a solid paste as the
electrolyte (zinc-carbon dry cell, page 33).
PRIMARY CELLS
They work until the chemicals are used up.
They are not rechargeable.
Example: Zinc-carbon batteries, they work
until zinc and manganese dioxide are used
up.
STORAGE CELLS
In this type of cell the reactions that
produce electric current can be reversed.
Other names for this type of cells:
Secondary cells or storage batteries.
THE LEAD-ACID BATTERIE IS A
STORAGE CELL