Download A. Electrical Current

Science 9
Unit D: Electrical Principles & Technology
1.0 Electrical Energy
1.1 Static Electricity
A. Review of the Atom
 protons have a positive charge
 electrons have a negative charge
 neutrons have no charge (neutral)
 atoms may gain or lose electrons and become ionized
 charged particles can either repel or attract each other
B. Electrical Charge
 most objects have an equal amount of positive and negative charge so
they are neutral overall
 when there are more protons than electrons, the object has a positive
static charge
 when there are more electrons than protons, the object has a negative
static charge
 bringing a charged object near a neutral object may cause charge
separation in the neutral object
 opposite charges attract each other
 like charges repel each other
C. Electrical Discharge
 build up of charge on one object may cause a transfer of charge to
another object
 the resulting spark is the electrical discharge
eg) “shock” from touching someone, lightning, Van de Graaf
generator, Tesla coil
 a Van de Graaf generator uses friction from a belt being rubbed to
build up a static charge
1.2 Current Electricity
A. Electrical Current
 static electricity is not useful since it doesn’t flow continuously
 electric current is the continuous flow of electrons
 the rate at which electrical current flows is measured in a unit called
Amperes (A)
 conductors allow electrical charge to move eg) copper wiring
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 small devices (cell phones, MP3 players etc) use very little current
 large devices (cars, generating stations) use a lot of current
B. Circuits
 a circuit is the continuous path that an electrical current flows
through
 it is made up of:
1. conductor – wires
2. load – device to convert energy eg) light bulb
3. energy source – battery
C. Voltage
 voltage or potential difference, is a measure of how much electrical
energy each charged particle carries
 the unit of voltage is the volt (V)
 easiest way to measure volts is with a voltmeter
1.3 Electrical Safety
A. Short Circuits
 a short circuit is an unintended path for electricity
 electric current will always take the shortest path available
 if a power line is down, electricity will not flow because the circuit is
open
 if you go too close you will close the circuit and the electricity will flow
through you!
B. Dangers of Electrical Shock
 both voltage and current can be dangerous
 skin is a bad conductor of electricity therefore a power supply voltage
must be higher than 40 volts to “push” a fast flow of charges through a
human body and cause electrocution
 however, high voltage is only dangerous if there is a flow of electrons
(current)
eg) you can get a zap of 10,000 volts from scuffing your feet on a carpet
and then touching a door knob but thankfully death doesn’t happen
because there is no current!
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Table 1: Common Voltages
Source of Electric Potential
Voltage (volts)
human cell
0.08 V
microphone
0.1 V
photocell
0.8 V
electrochemical cell
1.1 V to 2.9 V
electric eel
650 V
wall outlets in house
120 V, 240 V
generators in power stations
550 V
 the current rating must be higher than ten milliamps (10 mA or 0.01 A)
 at a current lower than 10 mA, even a high voltage power supply cannot
electrocute you
Table 2: Electric Current Ratings
Electrical Device
Electric Current (ampere)
electronic wrist watch
0.00013 A (0.13 mA or milliamps)
electronic calculator
0.002 A (2 mA)
electric clock
0.16 A (160 mA)
light bulb (100W)
0.833 A
television (color)
4.1 A
vacuum cleaner
6.5 A
oven element
11.4 A
toaster
13.6 A
car starter motor (V-8)
500 A
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 too much electricity flowing through a person’s body can cause the
following:
 pain
 loss of muscle control
 burns
 damage to the heart
 death
C. Insulators
 electricity is more dangerous when current can flow easily
 current does not flow as easily through insulators such as wood,
rubber and air
 moist materials tend to conduct better than dry materials
 if the insulator is damaged, shock from a short circuit is possible
D. Protection from Electricity
 our homes and the devices we use are equipped with safety features to
protect us from electric shocks
 many appliances and devices have three-pronged plugs
 the third prong connects the device to the ground wire of the building
to provide another pathway for electricity just in case of a short circuit
 fuses and circuit breakers interrupt a circuit when too much current
flows through it
 fuses have a wire in them that melts if the current gets too high
 breakers have a wire that trips a switch if it heats up too much
 both a blown fuse and a tripped breaker will open the circuit, not
allowing electricity to flow
 the current in a lightning strike can be 30 000 A (current as low as 0.1 A
can be fatal)
 lightning rods are placed on the top of tall buildings etc and they are
connected to the ground by a wire
 the lightning strike is then carried to the ground to be discharged
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1.4 Cells & Batteries
A. Dry Cells
 dry cells are the electricity producing cells we use as batteries in
flashlights, MP3 players etc.
 called “dry” because the chemicals are in a paste
 the paste is an electrolyte, which is a chemical that contains ions that
can conduct electricity
 also has two electrodes, which are usually metals
 electrons move through the cells from the negative electrode to the
positive electrode
B.
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Wet Cells
called “wet” cells because they use a liquid electrolyte eg) H2SO4(aq)
have two electrodes
cheaper and easier to make than dry cells but you have to be careful not
to spill the electrolyte which is corrosive
 car batteries are wet cells
C.
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Rechargeable Cells
the chemical reactions inside a rechargeable cell are reversible
called secondary cells
they can be used to store energy from an outside source
D. Batteries
 connecting two or more cells together makes a battery
E. Electrochemistry
 electrochemistry is the study of chemical reactions that involve
electricity
 electrolysis is an industrial process that is used to separate useful
elements
eg) hydrogen and oxygen gases for fuel for the space shuttle
 electroplating uses current to deposit atoms of a metal onto an
electrode
eg) silver plating
 electrorefining is a process that removes impurities from a metal
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2.0 Transfer & Control of Electrical Energy
2.1 Controlling the Flow of Electrical Current
A. Conductors and Insulators
 the electrons in insulators are tightly bound to the positive nucleus of
their atoms and they cannot flow
eg) glass, rubber
 in conductors, electrons are not tightly bound and are free to move
(with some resistance)
eg) metals
B.
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Superconductors
superconductors are perfect conductors
the electrons have no resistance to flow
the temperature must be very low (close to absolute zero) for
superconductivity
C. Resistors
 resistors allow only a certain amount of electric current to pass
 resistance is a measure of how difficult it is for electrons to flow
through a substance
 resistance is measured in Ohms ()
 the resistor gains energy from each electron that passes through it
 this energy can be released as heat or light
eg) in a space heater or light bulb
 liquids can also be good resistors
D. Switches
 switches are used to control the flow of electricity through a circuit
 when the switch is on, two conductors are pressed together, closing the
circuit and making electricity flow
 switches are enclosed in an insulating case for protection
E.
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Variable Resistors
variable resistors allow the gradual adjustment of electric current
also called rheostats
made of one single curved resistor
a dial changes the amount of the resistor that is used
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2.2 Modelling & Measuring Electricity
A. Modelling Electricity
 because electricity “flows”, we can use water to be a model for how
electricity behaves
 a waterfall is a good model for voltage
 a change in elevation allows the water to flow because of gravitational
potential energy
 in a circuit, a change in potential difference from a battery allows the
electrons to flow
 the higher the potential difference (waterfall) the higher the voltage
(energy)
 a pipe is a good model for resistance and current
 if you use a pipe to drain a pool, a longer, thinner pipe will have the
most resistance and will drain it much slower (lower current)
 if you use a short, wide pipe, it will have less resistance and will drain
faster (higher current)
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B. Ohm’s Law
 Georg Ohm found a relationship between voltage (V), current (I) and
resistance (R):
1. resistance in a conductor is constant
2. current is directly proportional to voltage
 increasing the voltage in a conductor will increase the current since
resistance is constant
C. Using Ohm’s Law
V=I×R
where:
I=
V
R
R=
V
I
V = voltage in Volts (V)
I = current in Amperes (A)
R = resistance in Ohm’s ()
Example 1
Calculate the voltage if the current is 0.5 A and the resistance is 2.0 .
I = 0.5 A
V = IR
R = 2.0 
= (0.5 A)(2.0 )
V=?
= 1.0 V
Example 2
Calculate the current if the voltage is 1.2 V and the resistance is 0.6 .
V = 1.2 V
I=V
R = 0.6 
R
I=?
= 1.2 V
0.6 
= 2.0 A
Example 3
Calculate the resistance if the current is 1.1 A and the voltage is 2.2 V.
V = 2.2 V
R=V
I = 1.1 A
I
R=?
= 2.2 V
1.1 A
= 2.0 
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D. Test Meters
 all test meters use a small amount of current
Test Meter
voltmeter (in volts and millivolts)
What it Measures
potential difference between two points
in a circuit (voltage)
electrical current which is the rate of
flow of electricity
small currents
resistance in a circuit
voltage, current or resistance in a
circuit
ammeters
galvanometers
ohmmeters
multimeters
2.3 Analyzing & Building Electrical Circuits
A. Circuit Drawings
 engineers and scientists use special symbols to draw, plan and analyze
electrical circuits
 all circuits have 4 basic parts:
1. source – provides energy and a supply of electrons
2. conductor – path for current
3. switch – controls current flow
4. load – converts electrical energy into another form of energy
 you can draw simple circuits using the common circuit symbols (see pg
312 of text)
Examples
Draw each of the following circuits:
1. 3 cell battery, 1 motor, 1 switch
2. 2 cell battery, 1 resistor, 2 lamps, 1 switch
3. 3 cell battery, 3 lamps, 1 ammeter, 1 switch
4. 4 cell battery, 1 fuse, 3 lamps, 1 motor, 1 rheostat
5. 3 cell battery, 2 lamps, 2 switches that must control the lamps from
either end of a room (both on/off function)
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B. Series Circuits
 a series circuit has all loads in one single loop
 electrons have only one path the follow
 an interruption results in no flow of current
 adding loads increases the resistance in the circuit
C. Parallel Circuits
 a parallel circuit has more than one pathway for the electricity to
flow through
 loads in separate loops will not interfere with each other
 loads in separate loops reduces the resistance in the circuit
D. Applications of Circuits
 house wiring uses parallel circuits…you don’t want one light bulb
burning out to shut down all power!
 houses also have a circuit in series so that you can turn off all power at
once if needed
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 Christmas lights use parallel wiring so the bulbs will stay lit even if one
burns out
 microcircuits (microelectronic circuits) are extremely small circuits
that are made up of transistors and resistors
Household Circuit
switches
loads
wires
Microelectronic Circuit
transistors
resistors
wires
3.0 Energy Conversion
3.1 Energy Types & Transformations
A. Forms of Energy
 energy is the ability to do work
 there are many different forms of energy
1. chemical – energy stored in chemical bonds
2. electrical – energy of charged particles
3. mechanical – energy of moving objects
4. thermal – kinetic energy of particles (heat energy)
B.
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Chemical Energy
energy is stored in chemical bonds
energy is released or absorbed in chemical reactions
chemical energy can be transformed into other forms of energy
eg) electrical, mechanical, sound
C. Energy Transformations
 you can use various devices to transform energy from one type to another
eg) toaster, MP3 player, flashlight
 chemical energy  electrical energy eg) batteries
 thermal energy  electrical energy eg) thermocouples
 electrical energy  thermal energy eg) oven
 electrical energy  chemical energy eg) battery charger
3.2 Energy Transformations Involving Electrical & Mechanical Energy
A. Electric Motors
 all electrical motors have 4 parts:
1. magnets along the outside
2. armature – rotating shaft of motor
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3. commutator – broken ring that breaks and reverses the flow of
current
4. brushes – conduct electricity from cell or battery to commutator
 how a motor works:
1. electric current travels from the brushes into the commutators
2. commutators magnetize the wire coil
3. magnets push or pull the wire coil
4. rotation of armature switches the position of the commutators
5. cycle continues until current is stopped
B. Direct and Alternating Current
 direct current (DC) flows in only one direction
 used in devices such as calculators, watches, cell phones, MP3 players
etc
 alternating current (AC) flows back and forth 60 times per second
 used in household circuits
 plug-in devices that require DC current come with their own power
supply that converts the socket’s AC current into DC and then
supplies the voltage required
eg) computers, stereo, TV
C. Transformers
 the most efficient way to transmit current over long distances is at a
high voltage (500 000 V)
 we can’t use this in our houses so the voltage needs to be dropped
 transformers use magnetic fields to transform one voltage to another
 there are two types:
1. step down – have less coils coming out than going in (reduces
voltage)
2. step up – have more coils coming out than going in (increases
voltage)
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D. Generators
 electromagnetic induction is the generation of electrical current by
moving a conducting wire through a magnetic field
 used for large scale generating plants
 massive coils of wire rotate in huge generators to produce electricity
 there are two types of generators:
1. DC – like a DC motor where the spinning armature produces a
one way flow of electrical current
2. AC – has 2 slip rings instead of a commutator, current flows out
through one slip ring then the other, alternating
3.3 Measuring Energy Input & Output
A. Power
 power is the rate at which a device converts energy
 measured in watts (W)
P=I×V
where:
P = power in watts (W)
I = current in Amperes (A)
V = voltage in volts (V)
Example
Calculate the power used by an MP3 player if it uses a current of 5.0 A and a
voltage of 3.0 volts.
I = 5.0 A
P = IV
V = 3.0 V
= (5.0 A)(3.0 V)
P=?
= 15 W
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B. Energy
 you can use the power rating of a device and the time it runs to
calculate the amount of energy used
 measured in Joules (J)
E=P×t
where:
P = power in watts (W)
t = time in seconds (s)
E = energy in Joules (J)
Example
Calculate the energy usage if a 15 W MP3 player operates for 2 minutes.
P = 15 W
E = Pt
T = 2 min x 60 s/min
= (15 W)(120 s)
= 120 s
= 1800 J
E=?
C.
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The Kilowatt Hour
in our households, it doesn’t take long to use up a lot of Joules of energy
we use a more practical unit called the kilowatt hour
it is calculated with the same formula E = Pt
power is measured in kilowatts (1 kW= 1000 W)
time is measured in hours (h)
D. Energy Dissipation
 Law of Conservation of Energy says that energy cannot be created or
destroyed, it can only change from one form to another
 energy is lost (dissipated) as heat, sound and/or light in every
conversion
E. Efficiency
 the efficiency of a device is a comparison of the input energy and
useful output energy
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percent efficiency = Joules of useful output
Joules of input energy
x 100
Example
Calculate the efficiency of a light bulb that emits 20 J of light energy for
every 80 J of input energy.
percent efficiency = Joules of useful output x 100
Joules of input energy
= 20 J x 100
80 J
= 25 %
3.4 Reducing the Energy Wasted by Devices
A. Limit to Efficiency
 devices used for heat are very efficient
 any device that does not use the heat produced in the energy
transformation is not as efficient
B. Increasing Efficiency
 efficiency can be increased by:
1. decrease friction – can use bearings and oils with moving parts
2. increase insulation – improve for devices that you don’t want to
lose heat from eg) refrigerator, oven
4.0 Society & the Environment
4.1 Electrical Energy Sources & Alternatives
A. From Heat to Electricity
 about 65% of the electricity in the world is generated by burning oil,
coal or natural gas
 these are called fossil fuels because they formed by the decomposition
of ancient plants and animals
 most electricity in Alberta comes from burning coal
 can also use nuclear fission, in which a heavy element like uranium is
split into smaller atoms
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 both combustion of fossil fuels and nuclear fission release a lot of
energy
 this energy is used to turn water into steam
 the steam then turns large turbines
 each turbine rotates wire coils in a generator which produces
electricity
 can also get energy to turn water into steam from geothermal energy or
burning biomass
 with hydro-electric power, the energy of the falling water turns the
turbines
B. Alternative Energy Sources
1. Tides
 moving water can power turbines that run generators
eg) in Nova Scotia in the Bay of Fundy
2. Wind
 moving air (wind) can turn turbines that run generators
eg) in Pinscher Creek Alberta
3. Sunlight
 solar energy is converted into electrical energy by silicon-based solar
cells
4. Fuel Cells
 chemicals continuously feed a chemical reaction that produces
electricity
eg) hydrogen gas + oxygen gas  water + electricity
C. Renewable vs. Nonrenewable Resources
 renewable resources are those that can be replaced in a short period of
time
eg) solar energy, wind energy
 nonrenewable resources cannot be replaced as they are used up
eg) fossil fuels, uranium
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4.2 Electricity & the Environment
A. Air Pollution
 burning fossil fuels releases pollutants
 coal-burning power plants release small amounts of fly ash into the
atmosphere which contains mercury
 SOx, NOx, and CO2 released through combustion lead to acid rain
formation
 CO2 also causes the enhanced greenhouse effect
B.
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Other Environmental Effects
strip mining of coal results in a disruption of the environment
oil and gas wells may release deadly gases
steam turbines release warm water into rivers and lakes, which can
harm organisms
nuclear reactors produce radioactive wastes
hydroelectric dams flood river valleys
wind farms make large areas of land not useable for anything else
building solar cells requires harmful chemicals
tidal generators displace marine organisms
“green” sources of energy (tidal, geothermal, wind) harm the
environment much less than burning fossil fuels
C. Conservation
 higher demand for energy means higher consumption of energy
resources
 lowering energy use conserves nonrenewable resources
 conserving energy means using resources at a rate that can be
maintained indefinitely
 called sustainability
 personal decisions by all people influence sustainability
4.3 Electrical Technology & Society
A. Benefits
 improved standard of living
 can complete tasks faster, giving us more free time
B.
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Disadvantages
technology can be expensive
sustainability is threatened
old discarded technologies adds to solid waste
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C. Computers
 computers use binary numbers (one’s and zero’s)
 different technologies store and transmit computer information
eg) CD’s, hard drives, microcircuits
 “hackers” can break into computer systems
 “viruses” exist and slow down computers
 internet has lots of information…not always correct
 can get “information overload” from the amount of material that is
available through computers
D. Viewpoints
 you can look at how things affect society through a variety of viewpoints:
1. economical – money
2. political – laws and governments
3. technological – technology
4. environmental – effect on ecosystems, organisms etc
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