electric charge electric charge is a property that allows a charged object to exert a force...
TRANSCRIPT
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
http://www.tutorvista.com/content/physics/physics-iv/electric-charges/electric-field-lines.php
Lines of force
http://www.tutorvista.com/content/physics/physics-iv/electric-charges/electric-field-lines.php
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.
http://www.physicsclassroom.com/class/estatics/u8l1a.cfm
http://www.physicsclassroom.com/class/estatics/u8l1a.cfm
http://en.wikipedia.org/wiki/Portal:Physics
Summary of subatomic particles
Negative charge
No chargePositive charge
Not very massive.
MassiveMassive
Weakly boundTightly boundTightly bound
Outside nucleusIn the nucleusIn the nucleus
ELECTRONNEUTRONPROTON
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.
http://science.howstuffworks.com/transport/engines-equipment/vdg3.htm
How materials affect static charging
Polyester
Rubber
Paper
Silk
Fur
Wool
Nylon
Hair
Glass
Skin
Material
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 separationInside 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 moveeasily.For example metals in general.
• Non-conductor or insulatorsThese 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 low-resistance 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