Chapter 5: Electricity and Magnetism Part 3

1
Chapter 5 Electricity and Magnetism Part 3

• Alyssa Jean-Mary

2
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.

3
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

4
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.

5
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.

6
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.

7
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.

8
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.

9
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.

10
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.

11
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

12
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.

13
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.

14
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

15
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

16
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.