Chapter 5: Electricity and Magnetism

1
Chapter 5 Electricity and Magnetism

• Alyssa Jean-Mary
• Source The Physical Universe by Konrad B.
  Krauskopf and Arthur Beiser

2
Electricity

• Electricity can not be explained by gravity or
  the kinetic theory of matter
• It is in our everyday lives making light bulbs
  glow, making motors run, making telephones and
  radios bring us sound, making our televisions
  bring us images
• This electricity that is used every day is to
  transport energy and information
• All matter is electrical in nature
• Electric forces binds electrons to a nuclei,
  which is what forms atoms
• They also are the forces that hold solids and
  liquids together

3
Positive and Negative Charge

• An experiment
• If a ball is touched with a rubber rod, the ball
  wont move
• If this same ball is touched with a rubber rod
  that has been rubbed against a piece of fur, the
  ball will now move away from the rubber rod
  because the rubber rod transferred its newly
  gained electrical charge to the ball
• If a ball that is touched with a rubber rod that
  has been rubbed against a piece of fur is placed
  next to another ball that has been touched by the
  same thing, the balls will move away from each
  other because they have the same electrical
  charge
• However, if this same ball (the one that is
  touched with a rubber rod that has been rubbed
  against a piece of fur) is placed next to a ball
  that has been touched with a glass rod that has
  been rubbed with a piece of silk, the balls will
  move towards each other because they each have a
  different electrical charge
• There are two different electric charges
• Negative charge - i.e. the charge produced by a
  rubber rod against a piece of fur
• Positive charge i.e. the charge produced by a
  glass rod against a piece of silk
• Something with a negative charge is attracted to
  something with a positive charge because opposite
  charges attract
• This same something with a negative charge is
  repelled by something else with a negative charge
  because like charges repel

4
Charge Separation

• When two objects interact with each other and an
  electrical charge is produced, one of the objects
  ends up with a negative charge and the other
  object ends up with a positive charge
• When the rubber rod is rubbed with the piece of
  fur, since the rubber rod gains a negative
  charge, the fur has gained a positive charge
• The same is true for the glass rod that is rubbed
  with the silk since the glass rod gains a
  positive charge, the silk has gained a negative
  charge
• The process of rubbing two objects together is
  not what creates the electrical charge. Every
  uncharged object actually has an equal amount
  of both positive and negative charges within it.
• The bond between these charges in an uncharged
  object can be weak, which is why rubbing could
  release some of the charges, or strong, which
  needs a lot more to release some of the charges
• An uncharged object, who has an equal amount of
  positive and negative charges is called neutral

5
What is Charge?

• Every substance is composed of tiny bits of
  matter called atoms
• Every atom, no matter the kind, is made up of
  three different elementary particles
• Protons mass 1.673 x 10-27 kg, charge
  positive
• Electrons mass 9.11 x 10-31 kg, charge
  negative
• Neutrons mass 1.675 x 10-27 kg, charge no
  charge neutral
• Protons and Electrons have the same amount of
  charge they just have opposite signs
• Protons and Neutrons have almost equal mass, and
  their masses are almost 2000 greater than the
  mass of electrons
• Every atom has a nucleus at its center. The
  nucleus contains the protons and the neutrons.
  The electrons are contained outside the nucleus,
  in energy levels or shells.
• In a neutral atom, the amount of electrons equals
  the amount of protons
• What is CHARGE? It is a fundamental property of
  certain elementary particles of which all matter
  is composed. The charge of a particle gives rise
  to electric forces just as an objects mass gives
  rise to gravitational forces.

6
The Coulomb

• The unit of electric charge is the Coulomb (C)
• Since the proton has a charge of 1.6 x 10-19C,
  and the electron has a charge of -1.6 x 10-19C,
  all charges, whether they are positive or
  negative, only occur in multiples of 1.6 x 10-19C
• Thus, equation for the basic unit of charge in
  nature (e) is
• e 1.6 x 10-19C
• Since e is such a small quantity, it appears that
  charge is continuous when looked at outside the
  laboratory (i.e. a charge of -1C would equal 6
  billion billion electrons)
• Since atoms are small, 6 billion billion atoms of
  carbon will make a piece of coal, which is almost
  pure carbon, that is only about the size of a pea

7
Coulombs Law

• The force between two charged objects depends on
  how close the objects are to each other and on
  how much charge each of the objects has
• If the distance between two objects is increased,
  the force between them is decreased AND if the
  distance between two objects is decreased, the
  force between them is increased i.e. the
  distance and force are inversely proportional
• If the charge on the objects is increased, the
  force between them is also increased AND if the
  charge on the objects is decreased, the force
  between them is also decreased i.e. the charge
  and force are directly proportional
• The equation of Coulombs Law is
• F (KQ1Q2)/R2
• where F is the electric force, Q1 is the charge
  on object 1, Q2 is the charge on object 2, R is
  the distance between the two objects, and K is a
  constant that is called the electric force
  constant and is equal to 9 x 109 Nm2/C2
• This equation shows us that if two charges, each
  with a charge of 1C, are separated by 1m, the
  electric force between them is 9 x 109N, which is
  9 billion Newtons, which is an enormous force
• A Coulomb is a very large unit it is son large
  that even the most highly charged objects will
  not contain more than a small fraction of a
  Coulomb

8
Example Calculation of Electric Force

• What is the electric force between two objects if
  one of the objects has a charge of 5.3C and the
  other object has a charge of 3.2C if they are 56m
  apart?
• Answer
• Given 5.3C, 3.2C, 56m
• Looking for electric force
• Equation F (KQ1Q2)/R2
• Solution F ((9 x 109 Nm2/C2)(5.3C)(3.2C))/(56m
  )2 4.87 x 107N

9
Force on an Uncharged Object

• An object that is charged will attract small
  uncharged particles towards it because electrons
  can have some freedom of movement without leaving
  their parent atoms or molecules
• When a comb is used in a persons hair, it gains
  a negative charge. If the comb is placed near a
  piece of paper, the paper, which is neutral, will
  be attracted to the comb because all the negative
  charge of the paper will move to the side of the
  paper away from the comb, which leaves all the
  positive charge of the paper on the side close to
  the comb. This results in attraction because the
  comb is negative and the paper near the comb is
  positive. If the comb doesnt actually touch the
  paper, the positive and negative charge will be
  returned to their normal positions. Also, since
  there is only a small amount of charge separation
  between the positive and negative charges on an
  object that comes into contact with a charged
  particle, there is only a small amount of force
  available, which means that only small objects
  will be picked up.

10
Matter in Bulk

• Coulombs Law (electric force) vs. Newtons Law
  of Gravity (gravitaitonal force)
• F (KQ1Q2)/R2 vs. F (Gm1m2)/R2
• The two laws resemble each other, but
  gravitational forces are always attractive
  forces, and electric forces can be either
  attractive forces or repulsive forces
• Because matter always attracts other matter
  gravitationally, matter always tends to come
  together into large masses in the universe. Even
  though there are other dispersive influences that
  exist (i.e. other influences that drive matter
  apart), matter must fight against this steady
  attraction. All galaxies, stars, and planets
  where made out of matter that was originally
  spread out all over space that came together
  because of this gravitational attraction.
• To collect such a large amount of electric charge
  of one sign in one place is not as easy to do as
  that. It is hard to separate neutral matter into
  its differently charged particles because
  negative and positive particles attract each
  other so strongly. Also, like charges repel each
  other, so it really is hard to collect a large
  amount of electric charge of the same sign.
• Thus, neutral particles are most stable (i.e. it
  has a minimum potential energy) when all the
  particles make up only one single body and
  electric charges are most stable when positive
  and negative charges pair off to cancel each
  other out. On a universal scale, gravitational
  forces are more significant that electric forces,
  whereas on an atomic scale, electric forces are
  more significant than gravitational forces. On an
  atomic scale, the mass of particles is so small
  to see a gravitational effect, but their charges
  are large enough to see a significant electric
  effect.

11
Conductors and Insulators

• A conductor is a substance through which electric
  charge can flow readily
• Metals are the only solids at room temperature
  that are conductors
• Each atom of a metal gives up one or more
  electrons to a gas of electrons. This gas of
  electrons can move relatively freely inside the
  metal thus, a flow of charge is created. The
  rest of the atoms of the metal dont move.
• An insulator is a substance in which electric
  charge has great difficulty flowing
• Nonmetal solids are are insulators because their
  electrons are tightly bound to their atoms, and
  therefore, cannot give up electrons to a gas of
  electrons.
• Some good insulators glass, rubber, plastic
• Semiconductors are substances that are in between
  conductors and insulators in their ability to let
  charge move through them
• Semiconductors are used to make transistors,
  which is an device that can change its ability to
  transmit charge at will. Transistors are used in
  many modern electronics, including portable
  telephones, radio receivers, and telephone
  receivers. Computers contain millions of
  transistors. These transistors act as miniature
  switches to perform arithmetic and to carry out
  logical operations. Computers also use
  semiconductor memories, where a huge number of
  memory elements are built into a chip that is
  smaller than a fingernail.

12
Ions

• The conduction of electricity through a substance
  is due to the movement of ions
• Ions are charged atoms or molecules
• If an atom or molecule loses one or more
  electron, it gains a positive charge and becomes
  a positive ion
• If an atom or molecule gains one or more
  electron, it gains a negative charge and becomes
  a negative ion
• Ionization is the process of forming ions

13
Ionization of a Gas

• A gas, which is normally a poor conductor of
  electricity, can be ionized when
• x-rays, ultraviolet light, or radiation from
  radioactive material pass through it
• an electric spark is produced
• a flame burns in it
• When the gas is ionized, electrons are stripped
  from some molecules, which means that they are
  free to attach to other molecules. Those
  molecules that lose electrons are now positive
  ions, and those that gained their electrons are
  now negative ions. At normal atmospheric pressure
  and temperature, these ions only last a few
  seconds. They opposite charges are attracted to
  each other, and so they join together, with the
  electrons from the negative ions going to the
  positive ions, to form neutral molecules again.
• In the upper part of the earths atmosphere, the
  molecules of air are so far apart that when they
  are ionized by x-rays and ultraviolet light from
  the sun, they stay ionized. Thus, they can be
  used to reflect radio waves, which makes
  long-range radio communication possible.

14
Ionization of Liquids

• Unlike gases, liquids can be permanently ionized
  to different extents
• Pure water has only a small amount of
  conductivity, but if even a trace of impurity is
  present, its conductivity can increase
  tremendously. Thus, since the water that we use
  on a daily basis (i.e. tap water) is impure, it
  is considered to be a conductor of electricity.

15
Superconductivity

• At ordinary temperatures, even the best
  conductors resist the flow of charge through them
  to an extent. At extremely low temperatures, they
  lose this resistance, which is a phenomenon
  called superconductivity. The temperatures that
  are needed for a substance to reach in order to
  be a superconductor are difficult and expensive,
  which is why they are not used to a great extent
  commercially.
• Aluminum, for example, is a superconductor below
  1.2K (-272C)
• Superconductivity was discovered in 1911 by
  Kamerlingh Onnes in the Netherlands
• At room temperature, if electrons are set in
  motion in a closed wire loop, they will come to a
  stop in less then a second. If the wire loop was
  a superconductor, the electrons could be circling
  for years with no extra help.
• Superconductivity is important because
• Electric current (i.e. a flow of charge) is what
  carries electric energy from one place to another
• Electric current is also used to produce magnetic
  fields
• Ordinary conductors lose some of their current as
  heat. This loss is especially great when the
  current is carried for a long distance or if the
  current itself is large. In the United States,
  about 10 percent of electric energy is lost as
  heat.

16
High-Temperature Superconductors

• Until 1986, no superconductors existed above a
  temperature of 23K
• In 1986, in Switzerland, Alex Müller and Georg
  Bednorz discovered a ceramic that is a
  superconductor up to 35K
• Others have used their approach, and there are
  now substances that are superconductors higher
  than 150K (-123C). Even though this is still a
  low temperature compared to everyday
  temperatures, it is higher than the boiling point
  of liquid nitrogen, which is 77K, and liquid
  nitrogen is cheap (cheaper than milk) and readily
  available. Superconducting cables that are cooled
  with liquid nitrogen are used usually only when a
  large current needs to be transmitted because
  they are difficult to manufacture, more
  complicated to install, and more expensive.
• For example, some underground ducts are already
  filled so much with wire that there is not room
  to expand the electric supply by normal means. In
  this case, using a superconducting cable might be
  less expensive than building a new duct.
• In Detroit, 114kg of superconductors replaced 9
  tons of ordinary conductors. The superconductors
  take up only a third of the duct space that the
  conductors used to take up.
• Research is constantly being done to find a
  superconductor at room-temperature. Such a
  superconductor would
• allow trains to be suspended above the ground by
  magnetic forces, which would result in better
  fuel efficiency and higher speeds
• waste less electric energy, which would mean that
  there would be a lower rate of depletion of fuel
  resources and less pollution

17
The Ampere

• An electric current is a flow of charge from one
  place to another
• A battery turns chemical energy into electric
  energy. If the terminals of a battery were
  connected by a wire, a complete conducting path,
  which is called a circuit, would be produced. In
  the circuit, the electrons would from the
  negative terminal of the battery to the positive
  terminal of the battery. The electrons are kept
  moving by chemical reactions that are occurring
  in the battery. The moving electrons are not
  carrying the current or producing the current,
  they are the current.
• Just like a flow of water in a pipe can be talked
  about in terms of the amount of liters that pass
  a given point in the pipe every second, a flow of
  electricity in a wire can be talked about in
  terms of the number of coulombs (the unit of
  electric charge) that go past a given point in
  the wire every second. This can be expressed in
  the following equation
• I Q/t
• where I is the electric current, Q is the
  charge transferred, and t is the time interval
• The unit of electric current is the Ampere (A).
  It was named after the French physicist André
  Marie Ampère. One ampere is one coulomb/second OR
  1 A 1 C/s.
• In the light bulb of a desk lamp, the current is
  a little less than 1A.

18
Example Calculation of Electric Current

• Example What is the electric current if 45C
  travels for 32s?
• Answer
• Given 45C, 32s
• Looking for electric current
• Equation I Q/t
• Solution I 45C/32s 1.41A

19
Potential Difference

• In a battery, a coulomb of negative charge on the
  negative terminal is repelled by the negative
  terminal and attracted to the positive terminal,
  so it has a certain amount of potential energy.
  When this coulomb is moved along a wire to the
  positive terminal, it loses its potential energy.
  Thus, the coulomb can do work when it is going
  from the negative terminal to the positive
  terminal, in an amount that is equal to the
  amount of potential energy that was lost.
• The potential difference between the two
  terminals of a battery is the decrease in the
  potential energy of a coulomb of negative charge
  is moved from the negative terminal to the
  positive terminal. The potential difference
  between two points is equal to the corresponding
  energy difference per coulomb. The unit for
  potential difference is a volt (V), which was
  named for the Italian physicist Alessandro Volta.
  A volt is equal to a joule per a coulomb OR V
  J/C. Potential difference is also referred to as
  voltage.

20
Batteries

• A cars storage battery has about 12V of
  potential difference, and a dry cell has about
  1.5V of potential difference. This means that the
  a coulomb of charge at the negative terminal of a
  storage battery can do 8 times as much work as
  that of a dry cell. In other words, the storage
  battery can push 8 times as many electrons in a
  given time through its circuit as a dry cell,
  which means it has a current that is 8 times as
  great. Thus, the potential difference between two
  points is the amount of push available to move
  charge between two points.
• If two or more batteries are connected together,
  negative terminal to positive terminal, the
  voltage is increased because each battery will
  supply its push to the electrons that are
  flowing through the set of batteries.
• The voltage of each cell in a battery depends on
  the chemical reactions that take place in that
  cell
• For a cars storage battery, there are 6 cells,
  and each cell has a voltage of 2V, which is why
  the battery has a voltage of 12V to run the cars
  electrical equipment
• A storage battery can be recharged when it has
  used up all of its energy. On the other hand, a
  dry cell battery cannot be recharged i.e. once
  its energy is used up, the battery can no longer
  be used.
• A battery is rated according to the amount of
  charge that it can transfer from one terminal to
  the other terminal. This rating is expressed in
  Ampere-hours (Ah). The lower the amount of
  current that is transferred between terminals,
  the longer the battery will last to supply this
  current.
• For example, a cars storage battery has a
  capacity of 60 Ah. This means that it can supply
  60A for 1 hour, or 30A for 2 hours, or etc.

21
Ohms Law

• If different voltages are applied to the ends of
  the same piece of wire, the current that is in
  the wire is proportional to the potential
  difference or voltage. The current is doubled if
  the voltage is doubled, AND the current is halved
  if the voltage is halved.
• This is called Ohms Law, which was named after
  the German physicist Georg Ohm who discovered it
• Resistance is the property of a conductor that
  resists the flow of charge within it. The more
  resistance present in a circuit, the less current
  present for a certain amount of applied voltage.
• In equation form, Ohms Law is
• I V/R
• where I is still the current, V is the voltage,
  and R is the resistance.
• The unit of resistance is the ohm (O). Since an
  ampere is equal to a volt/ohm OR A V/O, then a
  Ohm equals a volt/ampere OR O V/A.
• The resistance of a metal conductor is based on
• the material it is made of (i.e. a iron wire has
  7 times more resistance than a copper wire of the
  same size)
• its length (i.e. the longer the wire, the more
  resistance it has)
• its cross-sectional area (i.e. the greater the
  area, the less resistance it has)
• the temperature (i.e. the higher the temperature,
  the more resistance it has)
• Ohms Law is not a basic physical principle like
  the law of conservation of energy because it
  applies only to metal conductors and not to
  liquid or gaseous conductors or to such
  electronic devices as tansistors

22
Example Calculation of Ohms Law

• Example What is the current if a battery has 20V
  and its resistance is 5.6O?
• Answer
• Given 20V, 5.6O
• Looking for current
• Equation I V/R
• Solution I 20V/5.6O 3.57A

23
Electric Energy Conversion

• Electric energy is useful because it is
  conveniently carried by wires and because it can
  be easily converted to other forms of energy
• In a light bulb, electric energy becomes radiant
  energy
• When a storage battery is charged, electric
  energy becomes chemical energy
• In an electric motor, electric energy becomes
  kinetic energy
• In an electric oven, electric energy becomes heat
• In all of these conversion examples, the current
  that flows through the device performs work on
  the device, which the device then turns into the
  form of energy that they need
• The electric energy that is lost because of the
  resistance of a conductor becomes heat.
• This is the basis of electric stoves and heaters.
• The filament in a light bulb gets so hot that it
  glows white.
• In an electric circuit, it is important to use
  wires that are large in diameter so that there is
  a smaller amount of resistance, which will
  prevent the wires from becoming so hot that they
  melt their insulation and thus start a fire. For
  instance, a thin extension cord might be okay to
  use for a lamp or a radio, but it might be
  dangerous if used for a heater or a power tool.

24
Electric Power

• The power of an electric current is the rate at
  which the electric current is doing work
• In equation form, this is
• P IV
• where P is the electric power, I is the
  electric current, and V is the voltage
• The unit of power is still the Watt (W)
• If the current in a power line exceeds a preset
  safe limit, a fuse or circuit breaker interrupts
  this power line. The fuses normally used homes
  have a maximum current of 15A. Since the voltage
  of the power line in a home is 120V, the maximum
  power that can be supplied without a fuse being
  blow is (15A)(120V) OR 1800W OR 1.8kW.
• Using the equation for electric power, it is
  possible to find out how much current is needed
  by a device when it is connected to a power line
  that has a certain amount of voltage. The
  equation for this is I P/V. For a device that
  has 60W of electric power and is connected to a
  120V power line, the current that it needs is
  60W/120V OR 0.5A.

25
Example Calculations of Electric Power

• Example 1 What is the electric power if 54A are
  doing 76V of work?
• Answer
• Given 54A, 76V
• Looking for electric power
• Equation P IV
• Solution P (54A)(76V) 4104W

• Example 2 What is the amount of current needed
  if 75W are connected to a 43V power line?
• Answer
• Given 75W, 43V
• Looking for current
• Equation I P/V
• Solution I 75W/43V 1.74A

26
The Kilowatthour

• The commercial unit of electric energy is the
  kilowatthour (kWh). A kilowatthour is the amount
  of energy that is supplied per hour if the power
  level is 1 kilowatt (kW).
• If you use electricity, you pay for the amount of
  energy that is used. For instance, if electric
  energy is sold at 0.12 per kilowatthour and a
  1.5 kW electric heater is operated for 7 hours,
  the cost would be
• Cost (price per unit of energy)(energy used)
  (price per unit of energy)(power)(time)
  (0.12/kWh)(1.5kW)(7h) 1.26

27
Magnets

• Electricity and Magnetism are closely related
• Light has an electromagnetic nature
• Electric motors and generators operate based upon
  the connection between electricity and magnetism
• The simplest magnet is a bar of iron that has
  been magnetized in one way or another (for
  instance, by having it stoked by another magnet).
  The magnetized bar of iron attracts and hold
  other pieces of iron to itself. Most of the force
  of a magnet comes from its ends, so more pieces
  of iron are attracted to the end of the bar than
  to the middle of the bar.
• If the center of a magnet is allowed to pivot
  (i.e. swing freely), the magnet turns so that one
  end of the magnet points north and the other end
  points south. The end that is pointing towards
  the north is called the north pole of the magnet,
  and the end that is pointing towards the south is
  called the south pole of the magnet. Here, a
  magnet is lining up with the earths axis. The
  reason for this is because the earth is itself a
  giant magnet.
• This is the basis of a compass, whose needle is a
  small magnet
• If the north pole of a magnet is placed near the
  north pole of another magnet, the two magnets
  will repel each other. Also, if the north pole of
  a magnet is placed near the south pole of another
  magnet, the two magnets will attract each other.
  Thus, like magnetic poles repel each other, and
  unlike magnetic poles attract each other, just
  like electric charges.

28
Poles Always Come in Pairs

• Positive and negative charges can be separated
  from each other, but north and south poles cannot
  be separated from each other. If you cut a magnet
  in half, you would think that you could separate
  the poles from each other, but each half still
  has both a north and a south pole. Thus, there is
  no such thing as a single free magnetic pole.
• An iron magnet can be cut into smaller and
  smaller pieces indefinitely. Each of these pieces
  are still a magnet. Thus, magnetism is a property
  of the iron atoms themselves. Each atom of iron
  behaves as if it has a north pole and a south
  pole. If the iron is not magnetized, the poles
  are arranged randomly so that the north and south
  poles cancel each others effects. If it is
  magnetized, the poles are aligned, so that all
  the north poles are in the same direction. Thus,
  the strength of all the tiny magnets are held
  together. To demagnetize this permanent magnet,
  it needs to be heated strongly or hammered. Both
  demagnetizing processes agitate the atoms, and
  they return to their normal random positions.
• Other material besides iron can be used to make
  permanent magnets. Nickel, cobalt, and other
  combinations of elements can also become
  permanent magnets.
• All substances are affected by magnetism, mostly
  to a small extent. Some substances are attracted
  to a magnet, but most are repelled by a magnet.

29
Magnetic Field

• Gravitational, electrical, and magnetic forces
  act without the two objects touching each other.
• For instance, a iron nail is pulled to a magnet
  when they are some distance apart. It doesnt
  wait until the magnet is touching it to be
  attracted to it.
• Thus, it seems that the properties of space near
  a mass (i.e. gravitational force), an electric
  charge (i.e. electrical force), or a magnet (i.e.
  magnetic force) are somehow altered by the
  presence of the mass, the electric charge, or the
  magnet. This altered space is called a force
  field. A force field exerts a force on an
  appropriate object. A force field cannot be seen,
  but it can be detected by its effects.
• Even those forces that are seen as being the
  result of direct contact between two objects are
  actually the result of a force field.
• For instance, when a golf ball is hit, it is
  actually the electric forces on the molecular
  level that lead to the transfer of energy and
  momentum from the golf club to the golf ball that
  is observed as a direct contact force.
• There is actually no such thing as a forced due
  to direct contact because the atoms that are
  involved in the force never touch each other.

30
Field Lines

• When iron filings are scattered on a card that is
  held over a magnet, they will form a pattern that
  suggests the form of the magnets field. The
  filings line up on the card in the direction in
  which a piece of iron would move if put there.
  The filings would gather most thickly where the
  force would be the greatest.
• A magnetic field is usually thought of in terms
  of imaginary field lines. Field lines correspond
  to the patterns that are formed by the iron
  filings.
• A magnetic field line traces the path that would
  be taken by a small iron object if it was placed
  in the field. The lines of the path are close
  together when the field is strong and far apart
  when the field is weak.
• Field lines are imaginary. A force field is a
  continuous property of the region of space where
  it is present, not a collection of lines.

31
Oersteds Experiment

• Every electric current has a magnetic field
  around it
• In 1820, the Danish physicist Hans Christian
  Oersted peformed the following, now famous,
  experiment
• A horizontal wire is connected to a battery and a
  small compass needle is held under the wire. The
  needle swings into a position that is at a right
  angle (i.e. perpendicular) to the wire. When the
  needle is placed above the wire, it also swings
  into a position that is at a right angle to the
  wire, but it is pointing in the opposite
  direction from when it was below the wire.
• Iron filings can be used to study the magnetic
  field pattern around a wire that is carrying
  current. This study shows that the filed lines
  near the wire are circles. The direction of the
  field lines (i.e. the direction in which the
  north pole of the compass points) is dependent on
  the direction of the flow of electrons through
  the wire. If you reverse the direction of the
  flow of electrons, you reverse the direction of
  the field lines too.

32
Oersteds Experiment the Right-Hand Rule

• In general, to find the direction of the magnetic
  field around a wire, encircle the wire with the
  fingers of the right hand so that the extended
  thumb points along the wire in the direction of
  the current. The magnetic field lines are in the
  same direction as your hand is in. This is know
  as the right-hand rule. Remember that the current
  and the field are perpendicular to each other.
• Oersteds experiment is so famous because it
  showed the connection between electricity and
  magnetism for the first time. Also, it was the
  first demonstration on the concept on which the
  electric motor is based.
• Thus, magnetism and electricity are only related
  thought moving charges. If an electric charge is
  at rest, it doesnt have any magnetic properties.
  A magnet is not influenced by a stationary
  electric charge near it and a electric charge is
  not influenced by a stationary magnet near it.
• When a current passes through a wire that is bent
  into a circle, the magnetic field that results is
  the same as the magnetic field that results
  around a bar magnet. One side of the loop of the
  wire acts as its north pole, and the other side
  acts as its south pole. If the loop was free to
  turn, the loop will swing into a north-south
  position just like a bar magnet that is free to
  turn does. Also just like a bar magnet does, a
  current loop attracts pieces of iron.
• Thus, resulting from Oersteds and others
  experiments
• All moving electric charges give rise to
  magnetic field

33
The Electromagnetic Field

• An electric charge at rest is surrounded only by
  an electric field, but an electric charge in
  motion is surrounded by a magnetic field in
  addition to its electric field.
• If we use instruments to travel alongside a
  moving charge, in the same direction and with the
  same speed as the charge, we find that there is
  now only an electric field present the magnetic
  field that was present is no longer there. But,
  if we move past a charge that is stationary with
  the instruments, we find both a magnetic field
  and a electric field.
• Thus, a relative motion between a charge and an
  observer is needed to produce a magnetic field.
  If there is no motion between a charge and an
  observer (i.e. relative motion), then there is no
  magnetic field.
• The theory of relativity says that whatever it is
  in nature that shows itself as an electric force
  between charges at rest must also show itself as
  a magnetic force between moving charges. One
  effect is not possible without the other one.
  Thus, an electric field and a magnetic field are
  not separate - they are both part of a single
  electromagnetic field that is surrounding every
  electric charge. The electric field is always
  present when a charge is present, but the
  magnetic field is only present when there is
  relative motion present.
• In a wire that carries an electric current, a
  magnetic field is only present because the wire
  is itself electrically neutral. In the wire, the
  electric field of the electrons is canceled out
  by the opposite electric field of the positive
  ions. But, since the positive ions arent moving
  and the electrons are moving, there is no
  magnetic field around the positive ions to cancel
  the magnetic field around the electrons. Now, if
  we move a wire that has no current in it, the
  electric and magnetic fields of the electrons
  will be canceled by those of the positive ions.

34
Electromagnets

• If several wires that carry currents in the same
  direction are placed side by side, their magnetic
  fields will add together and thus produce a
  stronger TOTAL magnetic field.
• This effect is used often to increase the
  magnetic field around a current loop
• Many loops of a wire are wound into a coil. The
  strength of the magnetic field that results
  depends on the amount of turns in the coil. The
  number of turns in the coil tells how many times
  stronger the filed is than if there was only one
  turn in the coil (i.e. a coil with 50 turns
  produces a magnetic field 50 times greater than a
  coil that has only 1 turn).
• If a rod of iron is placed inside a coil, the
  magnetic filed is tremendously increased. This
  combination of a coil and iron is called an
  electromagnet. An electromagnet only exerts a
  magnetic field when there is a current flowing
  through the coil. Because of this, its action can
  be turned on and off with the current. Also, if
  many turns are used with enough electric current,
  an electromagnet can be made much more powerful
  than a permanent magnet.
• Electromagnets are widely used. They range in
  size from the tiny coils that are in telephone
  receivers to the giant coils that are used to
  load and unload scrap iron. In Rotterdam, a Dutch
  port is installing powerful electromagnets in
  order to hold the steel hulls of cargo ships to
  piers to make the loading and unloading of these
  ships faster and to involve less labor than using
  ropes for this purpose.

35
Using Magnetism

• Many things use magnetic fields to turn one form
  of energy into another form
• An electric motor uses a magnetic field to turn
  electric energy into mechanical energy
• A generator uses a magnetic field to turn
  mechanical energy into electric energy
• Magnetic fields also play essential roles in
  television picture tubes, in sound and video
  recording, and in the transformers used to
  distribute electric power over large areas.

36
Magnetic Force on a Current

• If a horizontal wire that is connected to a
  battery is suspended so that it is free to move
  from side to side, and a bar magnets north pole
  is placed directly under it, the reverse of
  Oersteds experiment occurs. What Oersted did was
  place a moveable magnet near a wire that was in a
  fixed position, but here we have a fixed magnet
  near a moveable wire. The prediction of this
  experiment, using Oersteds results and Newtons
  third law of motion, is that the wire will move.
  The wire swings out to one side as soon as its
  current is turned on. The wire moves in a
  direction that is perpendicular to the magnetic
  field of the bar magnet. Thus, which way the wire
  swings depends on the direction of the flow of
  electrons in the wire and also on which pole of
  the bar magnet is under the wire.
• This shows that the force a magnetic field exerts
  on an electric current is not just attraction or
  repulsion - it is actually a sidewise push. The
  maximum sidewise push occurs when the current is
  perpendicular to the magnetic field. At angles
  that arent perpendicular, the push is less, and
  if the current is parallel to the magnetic field,
  there is no sidewise push.
• Because every current has a magnetic field around
  it, nearby currents exert magnetic forces on each
  other. When the two currents are moving in the
  same direction, the force between them is an
  attractive force. When the two currents are
  moving in opposite directions, the force between
  them is repulsive.

37
Electric Motors

• The sidewise push of a magnetic field on a wire
  that is carrying current can be used to produce
  continuous motion. A magnet has a magnetic filed
  inside which a wire loop is free to turn. If the
  loop is parallel to the magnetic field, there is
  no force on the two sides of the loop that lie
  along the magnetic field. The two sides of the
  loop that dont lie along the magnetic field do
  experience a push the left side receives a
  downward push, and the right side receives an
  upward push, which turns the loop
  counterclockwise.
• In order to produce a continuous motion, the
  direction of the current in a loop must be
  reversed if the loop is in a vertical position.
  The reversed current then interacts with the
  magnetic filed to continue to rotate the loop
  through 180. Now, the loop has to again swing
  around through a half-turn to reverse the
  direction of the current again. A commutator is a
  device that automatically changes the direction
  of the current. A commutator is a copper sleeve
  that is divided into segments. It is located on
  the shaft of a direct-current motor. Usually,
  more than two loops and commutator segments are
  used, which yields the maximum turning force.
• Actual direct-current motors, like the starter
  motor of a car, are much more complicated, but
  follow the same basic operating principle. The
  magnets that are used to create the magnetic
  field are usually electromagnets and not
  permanent magnets. In some motors, the magnet is
  actually the one that rotates, with the coil
  remaining fixed. A motor that is based on
  alternating current instead of direct current
  does not need commutators because the direction
  of their current changes back and forth many
  times per second.

38
Electromagnetic Induction

• A lot of the electric energy used today comes
  from generators that are powered by turbines. The
  turbines in turn are powered either by running
  water or by steam. When they are powered by
  steam, the boilers that supply the steam obtain
  heat from coal, oil, natural gas, or nuclear
  reactors. Ships and isolated places run on
  smaller generators that are powered by gasoline
  or diesel engines. In both cases, it is the
  kinetic energy of moving machinery that is turned
  into electricity.
• In the nineteenth century, English physicist
  Michael Faraday discovered the principle of the
  generator. He was interested in the work of
  Ampère and Oersted on the magnetic fields around
  electric currents. He reasoned that if a current
  can produce a magnetic field, then a magnet
  should be able to produce an electric current.
  But, when he placed a wire in a magnetic field
  and connected it to a meter, since there was no
  sign of a current, he concluded that a current
  is produced in a wire when there is relative
  motion between the wire and a magnetic field. As
  long as the wire continues to move across
  magnetic field lines, the current continues. If
  the motion stops, the current also stops. This
  type of current is called an induced current
  since it is produced by motion through a magnetic
  field. This entire effect is called
  electromagnetic induction.

39
Faradays Experiment

• A wire is moved back and forth across the field
  lines of force of a bar magnet. A meter that is
  connected to this wire will show a current first
  in one direction, and then in the other
  direction. The direction of this induced current
  depends on the relative directions of the wires
  motion and of the field lines. If the motion of
  the wire is reversed, or if the opposite magnetic
  pole is placed under the wire, which changes the
  direction of the field lines, then the current is
  reversed. The strength of the current depends on
  two things
• The strength of the magnetic filed
• How rapidly the wire is moving
• Electromagnetic induction is related to the
  sidewise force that a magnetic field exerts on
  the electrons that are flowing along a wire. In
  Faradays experiment, electrons are also moving
  through a magnetic field, but in this case, they
  are moving because the wire is moved as a whole.
  The electrons are still pushed sidewise, and thus
  they move along the wire as an electric current.

40
Alternating Current

• To obtain a large induced current, a generator
  uses several coils (instead of a single wire as
  in Faradays experiment) and several
  electromagnets (instead of a bar magnet as in
  Faradays experiment). When the wires of the
  coils are turned rapidly between electromagnets,
  they cut lines of force first one way, and then
  the other way.
• A generator works by using a coil that is turning
  between two magnets. During one part of each
  turn, each side of the coil cuts the field in one
  direction. During the other part of the turn,
  each side of the coil cuts the field in the
  opposite direction. Thus, the induced current
  flows first one way, and then the other way. A
  current that has this back and forth motion is an
  alternating current.
• The pressure variations of a sound wave can be
  changed into an alternating current by use of a
  microphone. There are several types of
  microphones. One type uses electromagnetic
  induction. A loudspeaker, which turns alternating
  current into sound waves, is this type of
  microphone. The operation of a loudspeaker is
  based on the force exerted on a wire that is
  carrying a current in a magnetic field

41
Direct Current

• Electric currents that come from batteries,
  photoelectric cells, and other such sources are
  always one-way currents, which are called direct
  currents. Direct currents can only be reversed by
  changing the connections.
• Thus, a direct current cannot easily change the
  direction of the electrons like an alternating
  current can. For instance, in a 60-Hz (1 Hz 1
  hertz 1 cycle/second) alternating current,
  electrons change their direction 120 times each
  second. Direct current is abbreviated dc and
  alternating current is a abbreviated ac.
• When commutators like those used on dc motors are
  used, generators can be built that produce direct
  current. Direct current can also be obtained from
  an ac generator, which is called an alternator,
  by using a rectifier, which is a device that
  permits current to pass though it in only one
  direction. Because alternators are simpler to
  make and more reliable than dc generators, they
  are often used with rectifiers to produce the
  direct current that is needed to charge the
  batteries of cars.

42
Transformers 1

• In order to induce a current, magnetic field
  lines need to move across a conductor. There are
  three ways to accomplish this, two of which are
• 1. Move a wire past a magnet
• 2. Move a magnet past a wire
• Coil A is connected to a switch and a battery and
  coil B is connected to a meter.
• When the switch is closed, a current flows
  through A, which builds up a magnetic field
  around it. The current and the field do not reach
  their full strength all at once. A fraction of a
  second is needed for the current to increase from
  zero to its final value. The magnetic field
  increases along with the current. As the current
  and the field are increased, the field lines from
  coil A spread outward across the wires of coil B.
  This motion of the field lines of coil A across
  coil B produces a momentary current in coil B.
  Once the current in coil A reaches its normal,
  steady value, the magnetic field becomes
  stationary, and the induced current in coil B
  stops.
• If the switch in opened to break the circuit, in
  a fraction of a second, the current in coil A
  drops to zero and thus its magnetic field
  collapses. Again, field lines from coil A cut
  across coil B, which induces a current in coil B.
  This current is in the opposite direction as
  before because the filed lines of A are now
  moving the other way past B.
• Thus, starting and stopping a current in coil A
  has the same effect as moving a magnet in and out
  of B. An induced current is generated wherever
  the switch is opened or closed.

43
Transformers 2

• Now, coil A is connected to a 60-Hz alternating
  current. In this case, no switch is needed since
  120 times each second the current automatically
  comes to a complete stop and starts off again in
  the other direction. The magnetic field produced
  expands and contracts at the same rate as before
  when coil A was connected to a battery and a
  switch, and the field lines from coil A are still
  cutting coil B, first in one direction, and then
  in the other direction, which induces an
  alternating current in coil B that is similar to
  that in coil A. The ordinary meter that was
  connected to coil B when coil A was connected to
  a battery and a switch will not respond to these
  rapid alterations in current. An instrument that
  is meant for ac will need to be connected to coil
  B to show the induced current.
• Thus, an alternating current in one coil (coil A)
  produces an alternating current in a nearby
  unconnected coil (coil B).
• A transformer is a combination of two such coils
  (i.e. where an alternating current in one coil
  produces an alternating current in a nearby
  unconnected coil) and an iron core.
• To generate an induced current most efficiently,
  the two coils need to be close together, and they
  need to be wound around a core of soft iron.
• The coil that obtains electricity from an outside
  source (i.e. coil A) is the primary coil, and the
  coil where an induced current is generated (i.e.
  coil B) is the secondary coil

44
Why Transformers are Useful 1

• Transformers are useful because the voltage of
  the induced current can be raised or lowered by
  suitable windings or turns of the coils
• If the secondary coil has the same amount of
  turns as the primary coil, the induced voltage
  will be the same as the primary voltage
• If the secondary coil has twice as many turns as
  the primary coil, the induced voltage will be
  twice as much as the primary voltage
• If the secondary coil has half as many turns as
  the primary coil, the induced voltage will be
  half as much as the primary voltage
• Thus, if the coil has more turns, it has more
  voltage and if it has less turns, it has less
  voltage
• Thus, by using a suitable transformer, any amount
  of voltage can by obtained from a given
  alternating current

45
Why Transformers are Useful 2

• When the secondary coil has a higher voltage than
  the primary coil, the secondary coil has a lower
  current than the primary coil AND when the
  secondary coil has a lower voltage than the
  primary coil, the secondary coil has a higher
  current than the primary coil. This is so that
  the power (P IV) is the same in both coils
• N1/N2 V1/V2 I2/I1
• where N1 is the number of turns in the primary
  coil, N2 is the number of turns in the secondary
  coil, V1 is the voltage in the primary coil, V2
  is the voltage in the secondary coil, I1 is the
  current in the primary coil, and I2 is the
  current in the secondary coil
• Transformers are useful because it is sometimes
  desirable to change the voltage of alternating
  currents
• The most valuable use for transformers is that
  they permit the efficient long-distance
  transmission of power. A current in a
  long-distance transmission has to be as small as
  possible because a large amount of current would
  mean a lot of energy lost in heating the
  transmission wires. Thus, at a power plant, the
  electricity from the generator is led into a
  step-up transformer that increases the voltage
  and decreases the current, sometimes several
  hundred times. High-voltage lines, which
  sometimes carry currents at voltages that exceed
  1 million V, carry current to local substations.
  At the local substations, other transformers
  step-down the voltage to make it safe for local
  transmission and use.