Electric Currents and Charges · Force of repulsion pushes objects apart. For comparison, this...
Transcript of Electric Currents and Charges · Force of repulsion pushes objects apart. For comparison, this...
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Electric Currents and Charges
Chapter 6
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Electricity
• The conventional definition of electricity as a practical use is…
the phenomenon of energy transformation associated with electrons that have moved from one place to another.
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Electricity:
However, the science of electricity must be more
precisely defined by the following terms:
Electric field – an influence produced by an electric charge.
Electric charge - the property of subatomic particles that give rise to
the electromagnetic force.
Electromagnetism – the interaction between the magnetic field and
the motion of an electric charge.
Electric current – the flow of electrically charged particles (most
often electrons).
Electric potential – the capacity of an electric field to do work.
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Early History of Electricity:• Egyptians, ca. 2750 BC, referred to certain
electric fish as “Thunderer of the Nile”, indicating an awareness of electric shock.
• Thales of Miletus, ca. 600 BC, made the first observations that amber (electricus is Latin for “amber-like”) rubbed on cat’s fur gave rise to magnetic properties.
• The Parthians, ca. 600 – 330 BC, may have had knowledge of electroplating based on a 1936 discovery of a device in Babylonia.
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Review of Subatomic Particles:
1. Protons have a positive
electrical charge. (p+)
2. Electrons have a negative
electrical charge. (e-)
3. Neutrons have a neutral
electrical charge. (n0)
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Review of Electromagnetic
Forces:
1. Force of attraction pulls objects
together.
2. Force of repulsion pushes
objects apart. For comparison,
this force of repulsion is 1042
times stronger than the pull of
gravity.
--Like charges repel, unlike charges
attract.
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• An atom that has a balanced sum of protons
and electrons is electrically neutral and is
not attracted to objects.
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In some materials, electrons are weakly
held and can be removed.
Only electrons can easily move from atoms.
• An object gaining electrons will have a net
negative electrical charge.
• An object losing electrons will have a net
positive electrical charge.
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– Any atom with an unbalanced number of
electrons will possess an electric field or region
surrounding a charged particle in which an
electric force affects other charged particles.
– The strength of an electric field depends on the
distance from the charged particle.
– I.e. Like gravity (although much stronger), as
the distance from a charged particle increases,
the strength of its electric field decreases.
– This is why/how refrigerator magnets work!
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Static Electricity:
• Static electricity is the buildup of electric charges
on an object.
– I.e. Electricity at rest.
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Methods of Static Electric Charging:
1. Friction removes electrons from an object onto another.
e.g. Walking on wool carpet will remove electrons onto you.
2. Conduction is electric charging by direct contact with a charged object.
e.g. Touch someone after walking on wool carpet and static electricity is conducted to that person.
-In heat conduction, metals are good conductors and wood, plastic, rubber and glass are poor conductions (insulators)
-Static free rooms are coated with vinyl or glass.
https://phet.colorado.edu/sims/html/john-travoltage/latest/john-travoltage_en.html
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Methods of Static Electric Charging, cont.:
3. Induction is the rearrangement of electric charges.
-Electrons in a neutral object are moved by the influence of a nearby charged object - direct contact is not necessary.https://phet.colorado.edu/sims/html/balloons-and-static-electricity/latest/balloons-and-static-electricity_en.html
-E.g. Lightning rods attract lightning because the metal rod becomes positive when the electric field of a nearby storm cloud forces the electrons to rearrange.
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Lightning:
• Lightning is a powerful static electric
discharge.
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Lightning:
• Steps to produce lightning:
1. From 15,000 to 25,000 ft above sea level, electric charges form from turbulent winds in a matrix of water droplets and ice crystals.
2. The electric charge becomes concentrated in a region of a cloud.
3. The charge moves downward from the cloud in a series of 50-m stepladders.
4. Eventually it encounters a conductor on the ground and creates a large spark between the charge and the ground!
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Facts about Lightning:
• The fear of lightning is called astraphobia.
• Lightning tends to be attracted to:
-Metals: induction occurs easily, creating a positively charged ground.
• Benjamin Franklin, ca. 1752, invented the lightning rod based on this principle.
-Tall objects: electrons take the shortest path from one object to another object.
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Facts about Lightning:• An average negative lightning strike can transfer
6.0x1018 electrons.
• The current flow is usually between 10,000 and 200,000 amperes and transfers on average 5 coulombs (500MJ) of energy.
• An average positive lightning carries 10 times more energy than an average negative strike.
• The entire flow of the charge (current) lasts less than ½ second.
• Lightning heats the surrounding air to about 20,000ºC (36,000ºF), over 3 times hotter than the Sun’s surface.
https://youtu.be/uAOSu9LFb3g
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Facts about Lightning:• A supersonic shock wave that decays to an acoustic wave
(thunder) forms from the rapid expansion of air from heat
produced by lightning.
• Florida holds the U.S. record for the most cloud to ground
lightning strikes per year.
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Electric Potential, Current and Resistance:
• Voltage () is the potential difference between the charges.
-Voltage is a measure of energy available to move electric charges.
-i.e. Voltage is the amount of work that charged particles can do, or the amount of electromotive force (emf or ).
Voc =
Where:
Voc =Voltage on an open circuit
= Electromotive force
Volt (V) is the unit for voltage.
*Electrons cannot move without energy input.
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• Current (I) is the flow of charged particles (most often electrons).
-Electrons in motion will remain in motion if they have a path and source.
-A path is any conducting material and a source is any device that transmits electrons.
-Electric current is measured in amperes (A) or the amount of electrons that pass a given point per second.
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• Resistance (R): is the opposition to the flow of
charged particles.
-Resistance is measured in ohms (Ω).
-Resistance produces heat, light, or
mechanical energy.
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Ohm’s Law
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Factors that Affect Resistance:
1. Temperature:
-Temperature has a minor effect on resistance.
-Resistance increases with a rise in
temperature in pure metals and metal alloys.
-Resistance decreases with a rise in
temperature in carbon, semiconductors and
electrolytic solutions (acids or alkalis)
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Factors that Affect Resistance:
2. Length:-Resistance of a uniform conductor is directly
proportional to the length of the conductor.
i.e. The longer the conducting wires, the more resistance.
3. Cross-Sectional Area-The resistance of a uniform conductor is inversely
proportional to its cross-sectional area.
i.e. A thick conducting wire has a low resistance;
a thin conducting wire has a high resistance.
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Factors that Affect Resistance:
4. Nature of the Materials:
-The resistance of a given conductor depends on
the material of which it was made.
-Insulators have very high resistance.
-Some metals have very low resistance.
e.g. Silver, Copper, Mercury, Aluminum
-Some metals have high resistance.
e.g. Iron, Calcium, Magensium and Tungsten
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Producing an Electric Current Using
Electrochemical Cells:
• Count Alessandro
Giuseppe Antonio
Anastasio Volta,
Italian Physicist, 1791,
invented the first
electrochemical cell
using zinc, silver and
sulfuric acid.
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Producing an Electric Current Using
Electrochemical Cells:
• All electrochemical cells produce an electric
current by using chemical potential energy.
-A chemical reaction converts chemical energy
to electric energy.
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Dry Cell:
• Dry cells contain an acid or alkaline paste
surrounding a carbon core encased in a
metal, usually zinc can.
• Electrons are released in a reaction with the
zinc and paste.
• Acid or alkaline cells are most common in
1.5V potential.
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Dry Cell:• A series of cells connected is a battery.
– E.g. Flashlight batteries
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Dry Cell:
• Nickel-cadmium rechargeable batteries use
nickel oxide hydroxide paste and cadmium
electrodes, but only carry a potential of
1.2V.
• Lithium-ion rechargeable batteries carry a
potential of 3.6V, but are more volatile,
especially if undercharged.
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Wet Cell or Voltaic Cell:
Voltaic Cell Contains:
-Electrodes – usually copper
and zinc or lead plates or
rods.
-Electrolyte – Any ionizing
solution (usually sulfuric
acid, H2SO4).
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Wet Cell or Voltaic Cell:
• Zinc reacts with H2SO4
and releases 2 electrons.
• When conductors are
connected to the
electrodes, the excess
electrons from the zinc
(negative pole) are
pushed to the copper
(positive pole).
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Thermocouple:
• Thermocouples convert heat energy into electric energy.
• Thermocouples are used as an electrical source for temperature gages and valve switches.
• The differences in specific heat of copper/iron wire creates a low voltage electric current.
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Current Direction:
• Direct Current (DC) – Electrons flow in the
same continuous direction.
– E.g. Cell phones, flashlights. From batteries = DC
• Alternating Current (AC) – electrons flow
back and forth through the conductor.
– Your Refrigerator, washer and dryer, oven, lights,
use AC. From the wall = AC
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Current Direction:
• Nikola Tesla, 1882, Croatian
born American, is responsible
for the invention of the AC
indcution motor; George
Westinghouse bought Tesla’s
patent and promoted AC use in
the United States.
• Standard AC frequency in the
US is 60Hz (cycles per second).
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Electric Circuits:
• Electrons cannot flow unless they have a
closed path.
– i.e. Any break in the current creating an open
path does not allow electrons to flow.
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Parts of a Circuit:
1. A source is any generator of electromotive force.
e.g. Battery, Inductive Generator, etc
2. A conducting path is usually a metallic wire.
3. The resistance (load) is any appliance
e.g. Lamp, Machine, or Motor
• In the absence of a load, a short circuit results and often results in a malfunction of the source or path.
4. A switch is necessary to open or close a circuit.
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Series Circuit: all devices are connected one after the other. There is only one path.
Advantages:
• Several devices may be switched on simultaneously.
• Less wiring is required.
• Voltage may be divided without the use of a transformer, therefore less energy loss by unwanted resistance.
Disadvantages:
• If a break occurs at any point in the
circuit, the entire circuit is open.
e.g. If one light on a Christmas tree light
string goes out, they all go out!
Parallel Circuit: all devices are on different branches of the path. There is more than one path.
Advantages:
• Any device can be operated independently of other devices.
Disadvantages:
• The circuit requires more conducting paths (more wires involved).
Types of Circuits:
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Series and Parallel:
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Electric Power:
• Electric power is a measure of the rate of
work (energy) provided by electricity.
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Electric Power:• All electric appliances convert electric energy into
light energy, heat energy, and/or mechanical energy.
• Electrical power is rate of this conversion.
• The amount of electricity an appliance uses depends on its power, which is determined by voltage, current flow, and time of usage.
i.e. P = VI
Where: P = Power (W)
V = Voltage (V)
I = Current (A)
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Electric Power:• The amount of energy used is calculated by:
e = Pt
since P = e/t
or work/time
• Large quantities of electric power sold by electric power companies are measured in kilowatt-hours (kWhr).
• Current cost is about 10-12 cents/kWhr depending on where you live.
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Electrical Safety:
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Electrical Safety:
• Fuses and circuit breakers are devices used
to protect a circuit from overload.
– They switch off the power automatically if
amperage (causing excessive resistance in the
path) exceeds the allowed rate.
• A fuse is a thin strip of wire that melts
(opening the circuit) if too much current is
drawn through the wire.
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Electrical Safety:
• A circuit breaker or
reset switch is a device
in the form of a relay
that can open or close
an electric circuit.
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Electrical Safety:• A ground fault circuit
interrupter (GFCI) is a
specialized circuit breaker
designed to open the
circuit if current leakage
exceeds .006A (6MA)
– GFCIs should always be
used where the potential
for wet conditions are
present.
e.g. outside, kitchen and
bathroom receptacles
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Electrical Safety:
• Electricity can be dangerous and should be
respected!
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Rules of Electrical Safety:
1. Always use electrical devices in dry conditions.
2. Stay away from power service lines, especially during wet weather.
3. Use a fiberglass or wooden ladder when working near electric lines.
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Rules of Electrical Safety:
4. Never run wires under rugs or carpet.
5. Never modify a receptacle for multiple
appliances.
6. Correctly repair frayed electrical wires or
throw away the device.
7. Never stick objects in receptacles or
appliances.
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Rules of Electrical Safety:
8. Avoid using extension cords inside a home.
9. Call a knowledgeable electrician if you are unsure if a condition is safe.
-Don’t do it yourself if you are unsure of what you are doing!
10. Never by-pass fuses or circuit breakers.