Chapter 6 Electricity and MagnetismWhat is ElectricityWh.docx

Chapter 6:
Electricity and Magnetism
What is Electricity?
What is Electric Current? Amps? Voltage?
How is your electric bill calculated?
What is a Superconductor?
What are Magnets? What are Electromagnets?
Why is Iron attracted to a Magnet?
What is a Motor? What is a Generator?
9/28/2016
Prof. Michael Opferman | Temple University | Phys 0847
1
Chapter 6A:
What is Electricity?
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2
Electric Charge
Electric Charge measures the strength of an object’s attractions

and repulsions to other Charged objects
Like Charges repel, and opposite Charges attract
Electrons have a Negative Charge
Protons have a Positive Charge
http://physics.tutorvista.com/electricity-and-magnetism/electric-
charge.html
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Electric Charge
Electrons and Protons therefore attract—this is why atoms form
Neutrons have no Charge. This is why they don’t matter much
for most processes, and different Isotopes are still considered to
be the same Element
http://www.ux1.eiu.edu/~cfadd/1160/Ch17/ElChrg.html
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Electric Charge
Recall: Protons and Neutrons are made of Quarks, but Electrons
cannot be divided into smaller parts
The two most common Quarks are called Up and Down Quarks.

Up charge is +2/3
Down charge is -1/3
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Proton: +2/3+2/3-1/3=+1
Neutron: +2/3-1/3-1/3=0
Electricity
Electricity refers to the movement of Charged objects (usually
Electrons)
Electrons are very light and sometimes mobile, and they carry a
lot of charge (-1 per Electron)
Think of Electricity in analogy with flowing water
Although Charge can be stationary (like a puddle or lake), it can
also flow in currents (like a river or water flowing through a
pipe)
https://pixabay.com/en/danger-electric-faucet-electricity-
153065/
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Electric Current

Flowing Charge is therefore called “Current”
Just as water current could be measured in Gallons per Second,
Electric current is measured in Electric Charge per second
(called Amps).
http://spendergast.blogspot.com/2014/04/quantum-
measurement-of-current-one.html
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Electric Current
Just as water can flow through pipes, Electrons can flow
through metal wires
Metals are “Electrical Conductors” because they have Electrons
that can be easily detached from their atoms, so that they can
flow and form Currents.
http://sciencepedagogics.pbworks.com/w/page/69837960/Physic
s%20II%20-%20ELECTRIC%20CIRCUITS
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Voltage
Electricity also has a second property called Voltage, which is

related to the Energy of the Electrons that are flowing
This is analogous to measuring the strength of a water current
The Energy in Electricity depends on both its Current and its
Voltage
Since Current is the number of Electrons per second, and
Voltage is their Energy per Electron, multiplying them gives the
Energy flowing per second (i.e. the Power)
Power = Current * Voltage
http://www.wpclipart.com/signs_symbol/safety_signs/safety_sig
ns_2/safety_sign_high_voltage.png
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Chapter 6B:
Energy Use and
How You Pay For It
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Electricity in the Home
Electricity from wall outlets in the US is typically at 120 Volts.
The amount of Current (Amps) flowing through the

outlet depends on what devices are plugged in
The Power (Rate of Energy use) of your devices is a fixed
number, measured in Watts.
http://hunt4freebies.com/coupons/wp-
content/uploads/2009/11/GE-Energy-Smart-Energy-Star-Light-
Bulb.jpg
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Voltage * Current = Power
Current = Power/Voltage
Bulb.jpg
Example: A 20W fluorescent lightbulb would draw
20W/110V = 0.18A
Fuses blow and circuits break when the Current becomes too
high. This occurs when devices are using too many Watts of
Power
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When determining the Energy usage of your home, you can look
up the “Wattage” or Power consumption of different devices
The Energy used will be the Power multiplied by the amount of
time the device is on
In other cases, you may see the “Amps” of Current reported.
(5.2A)(110V)= 572W
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Many electronic device use AC adapters, which modify the
properties of the electricity coming out of your wall.
These will often report the Voltage and Current of the
Electricity coming out
This uses (10V)*(0.85A)=8.5W
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14
http://retrogamerjapan.com/image/cache/data/20130226133234-
600x600.jpg

https://upload.wikimedia.org/wikipedia/commons/7/7f/Wall-
Wart-AC-Adapter.jpg
The Power consumption of electrical devices can be very
different from one another
Normally household electronics use about 1-100W of Power
Devices that provide Heat or cooling and devices with moving
parts tend to consume closer to 100-1000W of Power.
Air conditioners use a huge amount of Power, partly due to the
Second Law of Thermodynamics
http://www.solarpilipinas.com/wattage-chart.html
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15
The cost of using a device can be calculated by taking the
Power consumption (in kW), multiplying by the number of
hours it was running, and then multiplying by the cost you pay
for 1 kW of Electricity (which is normally around $0.15)
Example: Suppose a window AC unit uses 1000W of Power (i.e.
1 kW), and you leave it running overnight for 8 hours
Energy Used = (1kW)*(8hrs) = 8kWhrs

The cost of Energy is around $0.15 per kWhr, so the cost is
(8kWhrs)*($0.15) = $1.20
Doing this every day for a month comes to
($1.20)*(30 days) = $36.00
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Sample Electric Bill
Cost of Electricity is about $0.09 + 0.07 + $0.01 =$0.17 per
kWhr
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They often add a flat service charge that you pay no matter how
much Energy you use
as well as various taxes and fees

If the lightbulb shown were left running for a whole year, it
would cost about
(0.020kW)*($0.15/kWhr)*(24hrs)*(365 days)=$26.28
Bulb.jpg
An equivalent 75W bulb left on for the same amount of time
would cost
(0.075)(0.15)(24)(365)=$98.55
An equivalent LED bulb uses about 11W, and would cost
(0.011)(0.15)(24)(365)=$14.45
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The easiest way to reduce your Energy usage at home is to turn
off the AC and turn down the heat!
In practice, heating and cooling are the biggest Energy expenses
for most homes
http://www.wholesalesolar.com/solar-information/how-to-save-
energy/power-table
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19

Chapter 6C:
Applications of Electricity
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Electrical Energy
If a mill’s paddle wheel is placed into a flowing river, it
extracts some of the water’s Energy
Likewise, Energy can be extracted from the Charge flowing in
Currents. This is the essence of Electrical Energy.
http://media.giphy.com/media/Yc3JEbELN621W/giphy.gif
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Electrical Resistance
Electrical Energy can be stored and released by batteries and
generators
Electrical Energy becomes Heat Energy when Friction
occurs inside of a wire (or other material) where Current is
flowing (“Electrical Resistance”)

Electrical Energy becomes Heat Energy, and this is one way the
Energy can be extracted
https://funwithsnapcircuits.wordpress.com/2011/11/09/fun-with-
snap-circuits-6-resistance-is-futile-you-will-be-assimilated/
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Light Bulbs
An old-fashioned incandescent light bulb is nothing more than a
piece of wire with a high Resistance in a glass case.
The wire Heats up due to Resistance until it begins to glow “red
hot” and gives off light
http://warrenfyfenews.org/wp-content/plugins/rss-
poster/cache/f586a__76214554_bulb_think624.jpg
These light bulbs are inefficient because they create so much
Heat Energy in the process
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Toasters and Heaters
Toasters and many other devices designed to generate Heat also
work on the same principle except that we

want the Heat, and it is the Light that is the side effect.
https://en.wikipedia.org/wiki/Toaster
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Fuses
Ordinary wires may cause fires if they become too hot
A fuse is a piece of wire designed to melt if it gets too hot. The
Current then stops flowing before the heat starts a fire.
http://www.frankswebspace.org.uk/ScienceAndMaths/physics/p
hysicsGCSE/bytesize%20images/fuses1.gif
http://www.electronicrepairguide.com/images/testing%20fuses.j
pg
This fuse melts when the Current in it is 13 Amps. This would
happen if you run devices consuming Energy at a rate
(13A)*(110V) = 1430W
e.g. if you run two high wattage appliances at the same time
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Circuit Breakers

Modern houses more often use Circuit Breakers instead.
In one design, the Heat generated from Resistance causes a
Bimetallic Strip to bend (Recall: Chapter 2) which breaks the
contact and stops the Current flow
http://www.oocities.org/rjwarren_stm/College_Physics/bimetalli
cstripfuse.jpg
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Superconductors
Not every material generates Heat when Current flows through
it
A material that does not have Electrical Resistance is called a
Superconductor
For applications where the Heat generated by Resistance is
waste (like when we need to transmit Electricity through power
lines), Superconductors could be very useful
http://astrocampschool.org/superconductor/
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Superconductors

Many materials become Superconductors when they become
very cold
Even “High-Temperature Superconductors” require
temperatures around -200°F (-125°C) to work
They have to be cooled with very cold liquids such as Liquid
Helium and Liquid Nitrogen to keep their Superconducting
properties
Liquid Helium is fairly rare and expensive, but Liquid Nitrogen
can be made from air and is about the same price as milk
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High Tension Power Lines
Since we presently don’t use Superconductors in Power Lines,
Resistance causes a lot of Electrical Energy to be wasted
The waste is reduced by transmitting the Electricity over long
distances at high Voltage (as high as 500,000 Volts), but low
Current
These Power Lines are usually called High Tension Power Lines
In this context Tension means Voltage
http://www.forbes.com/sites/stevensalzberg/2014/09/01/do-
high-voltage-power-lines-cause-cancer/
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Static Electricity
Static Electricity occurs when Charges become stuck to things
(Static = stationary)
This can occur when Electrons on the ground become stuck to
your feet
If the air is humid, the Electrons can stick to water in the air
and leave your body. But if the air is dry, they tend to leave in a
large bunch (a spark). We call this Static Shock.
A Static Shock can be around 50,000 Volts! But the Current is
so low, and it is so short in duration that not much Energy is
transferred.
http://www.nachi.org/images10/staticjpg.jpg
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30
Lightning
Lightning is like a Static Shock that flows between the clouds
and the ground.
The Voltage of a Lightning strike can be around 10 Million
Volts
The Current is also high—around 100,000 Amps
http://www.lovethispic.com/uploaded_images/111744-City-
Lightning.gif
The duration is very short, but the amount of Energy transferred

it still significant—around the amount released by 7kg of high
explosives
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Electricity in the Body
In 1786, Galvani (inventor of the battery) discovered that
Electricity could be used to make dead frog legs move
This would serve as the inspiration for Mary Shelley’s
Frankenstein (1817), where a “mad scientist” reanimates a dead
body using Electricity.
http://stream1.gifsoup.com/view2/20140218/4981960/frankenste
in-lightning-o.gif
We would later learn that this is because the body uses
Electricity to carry signals, including in the brain
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32
Chapter 6D:

What is Magnetism?
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Cause of Magnetism
The Electric Force is caused by attraction and repulsion
between Charges.
The Magnetic Force is caused by attraction and repulsion
between *moving* Charges (i.e. Currents)
But what does this have to do with Magnets?
http://astarmathsandphysics.com/ib-physics-notes/120-the-
history-and-development-of-physics/1300-ampere-s-
experiments-on-currents-in-wires.html
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Cause of Magnetism
The atom is made of a Negative Electron orbiting around the
nucleus
The orbit of the Electron is like a tiny microscopic Current

http://wps.prenhall.com/wps/media/objects/3311/3390683/blb06
07.html
Since Currents create Magnetism, every Electron is like a tiny
Magnet
The direction of the Current is described by the North and South
Poles of the Magnet
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35
What is a Magnet?
A Magnet is actually made of many tiny microscopic Magnets.
Each of the microscopic Magnets is aligned so that the effects
add up and have a cumulative effect.
http://qph.is.quoracdn.net/main-qimg-
fa8f726798c09da7949360b3b54fba38
N
S
N
S
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What is a Magnet?
On the other hand, if a material is not a Magnet, the tiny
microscopic Magnets are not aligned, so they cancel out
http://qph.is.quoracdn.net/main-qimg-
fa8f726798c09da7949360b3b54fba38
N
S
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Poles and Attraction
Note: Since like Currents attract, opposite Poles attract
http://wps.prenhall.com/wps/media/objects/3311/3390683/blb06
07.html

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Bar Magnets
Magnets come in all shapes and sizes.
Shown here are “bar magnets”
http://s3.amazonaws.com/engrade-
myfiles/4004657025287470/8892624_orig.jpg
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39
Refrigerator Magnets
Glue a bunch of very thin Bar Magnets together to form a
rectangle, and you have a typical Refrigerator Magnet
The alternating pattern of N and S poles gives the magnet a
more uniform feel.

https://en.wikipedia.org/wiki/Refrigerator_magnet
N
N
N
N
S
S
S
S
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40
Horseshoe Magnets
Bend the Bar Magnet into a horseshoe shape, and you will have
the type of magnet most commonly shown in cartoons.
Both poles attract certain materials, such as iron.
http://www.wisegeek.org/what-is-a-horseshoe-magnet.htm
9/28/2016
41

Magnets Attract Iron
Iron (and a few other materials) are attracted to a Magnet
because the Magnet can temporarily align their microscopic
Electron magnets and turn it into a Magnet
Microscopic Magnets in nail rotate
http://www1.curriculum.edu.au/sciencepd/light/colr_what.htm
9/28/2016
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Magnets Attract Iron
If the microscopic magnets in the iron become stuck in this
configuration, the iron becomes “Magnetized” and is now a
permanent Magnet
See e.g. https://www.youtube.com/watch?v=4Joh80s_DeM
about how to Magnetize a screwdriver.

Can you explain what’s going on better than the guy who made
the video?
http://i.ytimg.com/vi/4Joh80s_DeM/maxresdefault.jpg
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43
Magnetic Levitation
When Iron is exposed to a Magnet, it becomes a Magnet itself,
and it is attracted.
Some other materials, when they are exposed to a Magnet, are
repelled instead of attracted.
This allows Magnets to be used for levitation.
Example: Superconductors have this property.
But recall that Superconductors require very cold temperatures
(e.g. from Liquid Nitrogen)
https://amyrobinson.files.wordpress.com/2013/03/meissner-
effect.gif
9/28/2016
44

Maglev
The levitation effect can be achieved with ordinary materials if
the Magnetic Field is created by Electromagnets which are
triggered at the right time
This is the idea behind Levitating Trains, or Maglevs
This allows the train to move with very little friction and
without a conventional engine
http://www.atom.rmutphysics.com/charud/specialnews/3/maglev
/maglev5.htm
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Chapter 6E:
Magnetic Recording
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Magnetic Recording
Suppose we want to store some information (like a credit card
number) as a series of bits such as 1,0,1,1,0
One way to do it would be to divide a material into small

sections and Magnetize each section either N or S depending on
the bit
N
S
N
N
S
1
0
1
1
0
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47
Your credit card number in binary
http://ipensatori.com/wp-content/uploads/2014/01/credit-card-
track-2-data.png

Magnetic Recording
Many electronic storage devices such as computer hard drives
(magnetic, not solid state), VHS tapes, audio tapes, floppy
disks, and credit cards work in exactly this way.
The data is stored by changing the direction of the
Magnetization in different regions of the storage medium with a
strong Magnet.
http://encyclopedia2.thefreedictionary.com/Perpendicular+magn
etic+recording
9/28/2016
48
Magnetic Recording
The data can be read by using a weaker Magnet to see the
pattern of where it is attracted and repelled from the Magnetic
strip
9/28/2016
49

https://www.chargify.com/images/uploads/Square-Credit-Card-
Reader-1.jpg
This is what’s going on when you swipe a credit card or a
computer hard drive spins
http://www.escapistmagazine.com/articles/view/scienceandtech/
columns/forscience/12908-How-Big-Will-Hard-Drives-Get-
Hard-Disk-Sizes-over-Time
The Earth is a Magnet
It was discovered that certain rocks (called Lodestones or
Kissing Stones) will spontaneously point in a certain direction
if allowed to move.
This is because the stones are Magnets. The North pole of the
stone’s internal magnet is attracted to the Earth’s South
Magnetic pole and vice versa.
I have just described a Compass!
http://www.explainthatstuff.com/how-compasses-work.html
The Earth also has a North and South pole. This is no
coincidence!
9/28/2016
50

The Earth is a Magnet
This is because opposite poles attract. The North Pole of the
Compass Magnet points toward the South Pole of the Earth.
The Earth’s Magnetic Field actually changes as time passes (due
to changes in the Earth’s molten core), and once or twice every
million years, the directions flip. We are expecting a flip to
happen fairly soon.
http://www.explainthatstuff.com/how-compasses-work.html
Note that the Earth’s Geographic North Pole is actually (near)
the Magnetic South Pole and vice versa
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Chapter 6F:
Electricity + Magnetism
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Electromagnets
A Magnet can also be formed by using ordinary Electric
Currents from a battery or wall outlet.
A piece of wire twisted into a cylinder of loops gives rise to a
Magnetic Field similar to a Bar Magnet
This type of Magnet is used in many devices since the Magnet
can be switched on and off.
The Earth’s Magnetic Field is believed to be caused by Currents
flowing in the molten core
http://www.electronics-
tutorials.ws/electromagnetism/electromagnets.html
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Electric Motors
In an Electric Motor, Electrical Energy is used to create an
Electric Current
This Current is pushed by the Magnetic Force from a Magnet,
creating mechanical motion (Kinetic Energy).
http://www.explainthatstuff.com/electricmotors.html
9/28/2016

54
Electric Motors
Electric Motors are at the heart of many everyday devices that
turn Electrical Energy into Kinetic Energy including kitchen
appliances, electric cars, and power tools
9/28/2016
55
http://www.electrical4u.com/electrical-motor-types-
classification-and-history-of-motor/
Electric Generators
An Electric Generator is the opposite of an Electric Motor.
It turns mechanical motion (Kinetic Energy) into Electrical
Energy using the reverse process.
A piece of wire is moved through a Magnetic Field (e.g. by a
hand crank or a Turbine)
Electrons in the wire are pushed by a nearby Magnet, and this

generates a Current
Electric Generators create almost all of the Electrical Energy we
use (e.g. from Power Plants, where steam turns the Turbine)
http://www.smartlearner.mobi/science/VideoPastPapers
/Electrodynamics/Electrodynamics.htm
9/28/2016
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Rail Guns
We can shoot bullets out of a gun using magnetism.
The Rail Gun gets its name from two parallel rails that comprise
the barrel. The payload is accelerated using magnetism
generated by an electrical current that flows up one rail and
down the other.
https://www.youtube.com/watch?v=eObepuHvYAw
http://science.howstuffworks.com/rail-gun1.htm
9/28/2016
57

http://i.dailymail.co.uk/i/pix/2011/11/02/article-0-
0EA4280300000578-652_634x681.jpg
From: http://www.dailymail.co.uk/sciencetech/article-
2094413/Rail-gun-US-Navy-takes-step-hi-tech-electromagnetic-
weapon.html
9/28/2016
58
Rail Guns can fire bullets at dramatically higher speeds than
conventional guns. This gives them superior power and range.
But presently, the power source required is far too large to be
mobile, except perhaps on large naval ships
Rail Gun in Sci-Fi
http://vignette3.wikia.nocookie.net/rvb/images/2/2e/Locus_with
_Railgun_2.png/revision/latest
9/28/2016
59
Two parallel rails
Bullet is accelerated between the rails
Transformers

A transformer is a device that raises or lowers the Voltage of
Electricity (e.g. to go from High Tension to ordinary lines)
One coil of wire is used as an Electromagnet and generates a
Magnetic Field
Another coil of wire is placed nearby, and its Electrons are
pushed by the Electromagnet, like in the Generator.
The Voltage and Current in the two pieces of wire will be
different.
http://www.scienceaid.co.uk/physics/electricity/electromagnetis
m.html
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60
Transformers
In practice, transformers normally look like this
This transformer is changing the Voltage from whatever it is in
the power line to the 120V needed in your home
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61

Chapter 6: Summary
Protons and Electrons have a property called Charge, which
makes them attract and repel. This is called the Electric or
Electrostatic Force.
When Charges move around, we refer to it as Electric Currents.
The Voltage of the Electricity is related to the Energy of the
moving Charges, and the Current is related to their number.
When Current flows through a material, some Energy is lost and
changed to Heat. This is called Resistance.
Unless the material is a Superconductor, which have no
Resistance.
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Chapter 6: Summary
Moving Charges (Currents) experience another force called the
Magnetic Force.
In a Permanent Magnet, the moving charges are due to the
microscopic motion of Electrons
In most materials, the motion of the Electrons is random. If the
Electrons move in the same direction, you get a Magnet
When a Magnetic material (such as Iron) is placed near a
Magnet, the Electron motion aligns, the material temporarily
becomes a Magnet, and it is attracted.

Ordinary Electric Currents also act like Magnets, and are called
Electromagnets
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Chapter 6: Summary
Electric Motors convert Electrical Energy to mechanical motion
Electric Generators convert mechanical motion to Electrical
Energy
Transformers change the Voltage of Electricity
Magnets can be used for applications such as Magnetic
Recording (computer hard drives) and Rail Guns
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64

Chapter 6 Electricity and MagnetismWhat is ElectricityWh.docx

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