ELECTRODYNAMICS

1
ELECTRODYNAMICS
2
Force on a current-carrying wire in a magnetic
field.

• When a wire carrying a current is placed inside a
  magnetic field, it experiences a force that
  causes the wire to move.
• The force is the result of the interaction
  between the magnetic field of the magnets and the
  magnetic field of the wire.

3

• The force is at a maximum when the wire moves at
  90o to the magnetic field and the force is zero
  when the wire moves parallel to the magnetic
  field.
• The direction of the force can be found by using
  right hand screw rule
• Conventional current is used,
• that is, current that flows from
• the positive towards the
• negative terminal.

4
The Direct Current (DC) Motor

• Electrical energy (current) in the motor is
  converted into mechanical energy (the movement of
  the motor).
• Using the Left Hand Motor Rule, the left side is
  forced up and the right side is forced down.

5

• A simple DC motor consists of a rectangular coil
  of wire mounted on an axle that can rotate
  between the two poles of a magnet.
• Each end of the coil is connected to half of a
  split-ring commutator that consists of two copper
  segments that rotate with the coil.
• Two carbon blocks, the brushes, are pressed
  lightly against the split rings.
• The brushes are connected to the power supply.
• The split-ring commutator ensures that the coil
  turns in one direction only.

6

• Step 1 shows the coil in the horizontal position.
  ab experiences an upward force and cd a downward
  force.
• Step 2. The torque causes the coil to rotate into
  a vertical position. Now the openings between the
  half-rings of the split-ring commutator are
  opposite the brushes and the commutator loses
  contact with the brushes. The current stops
  flowing through the coil. However, the momentum
  of the coil carries it past the vertical position.

7

• Step 3 The commutator makes contact with the
  brushes again, but the current in the coil is
  reversed (in the direction abcd). This allows the
  torque to continue acting in the same direction.
  The side ab now experiences a downward force, and
  side cd an upward force.
• Step 4 The coil continues to rotate until it
  reaches a vertical position again and the current
  is broken.
• The rotating shaft is usually connected to other
  rotating parts in the system, by means of gears
  or pulleys.

8
The turning effect on the coil can be increased
by

• Increasing the current in the coil.
• Increasing the number of turns on the coil.
• Increasing the strength of the magnetic field.

9
Motors in everyday life

• The changing force experienced by the coil as it
  rotates through 360o, results in the simple motor
  not running smoothly.
• In practice, motors turn very smoothly and at
  high speeds.
• In these motors the coil consists of a soft iron
  core, surrounded by many loops.
• Such a coil is called an armature.
• Most armatures have many coils which are placed
  at different angles.
• Each coil in the armature has its own commutator.

10

• Some motors, such as those in electric drills,
  can run on AC, because they contain
  electromagnets, rather than permanent magnets.
• As the current flows through the coil, the
  magnetic field changes direction to match it.
• This enables the motor to keep turning in the
  same direction.
• Electric motors have almost limitless useful
  applications. These vary from the tiny motors
  found in moving electric toys and disc players,
  to the driving force behind water pumps all the
  way to the giant motors that drive the winches on
  construction cranes.

11

• The design of an electric motor depends on the
  task it must perform sometimes it must turn
  fast as in the dentist drill, or slower as in a
  clock and sometimes in steps as in the motor in a
  printer which feeds the paper line by line.

12
ELECTROMAGNETIC INDUCTION

• When a magnet moves near a conductor or when a
  wire moves in the magnetic field of a magnet, the
  change in the magnetic field induces an emf and a
  current flows in the conductor.
• This phenomenon is called electromagnetic
  induction.
• The induced current will be maximised when the
  motion of the conductor is perpendicular to the
  direction of the magnetic field, and minimised
  when it travels parallel to the field.

13
Moving a conductor in a magnetic field
14
Faradays Law

• The size of the induced current is directly
  proportional to the rate of change of magnetic
  flux linkage.
• What this means is that the induced current is
  most effectively produced when the number of
  magnetic field lines being cut by the conductor
  is greatest.
• The size of the induced emf (and hence the
  induced current) can be increased by
• Moving the conductor faster
• Using stronger magnets
• Increasing the length of the conductor (more
  turns on the coil)

15
THE AC GENERATOR

• N S are the field magnets that provide the
  magnetic flux.
• abcd indicates one turn of a rectangular coil of
  insulated copper wire and represents the
  armature.
• S1 S2 are a pair of slip-rings consisting of
  copper around an insulated cylinder. Each end of
  the coil remains connected to its own slip ring.

16

• B1 B2 are carbon or copper brushes for
  collecting the induced current.
• A is an insulated shaft that enables the coil and
  slip-rings to rotate as a single unit.
• The handle stresses the fact that kinetic energy
  must be supplied to the armature and the lamp,L,
  represents the electrical device in which the
  generated current is used.
• Generators use mechanical energy to produce
  electrical energy.

17
Working of an AC generator

• When the coil is in the vertical position, no
  magnetic field lines are cut by ab and dc no emf
  is induced and no current flows.
• While the coil rotates it cuts the field lines
  and causes a change in magnetic flux, an emf is
  induced which causes an electric current flow in
  the circuit.

18

• As the coil stats moving to the vertical position
  again, the induced emf decreases so the current
  also decreases, till it is zero.
• When the coil is rotated further, there is a
  change in magnetic flux again, but now the
  induced current flows in the opposite direction.

19

• When the coil of an AC generator rotates in a
  magnetic field, a constantly varying emf is
  induced across the coils ends.
• One complete change in the direction of the
  current during one revolution of the coil is
  called a cycle of alternating current.
• The induced emf varies according to a sine wave
  and the induced current is slightly less than the
  induced emf.

20
Uses of the AC generator

• Alternators in cars, the movement of the cars
  engine produces electricity via the alternatort
  to recharge the cars battery.
• Back-up power generators are used to produce
  electricity where there is a power outage. Places
  that are left vulnerable or cannot function in
  the event of a power cut, such as hospitals,
  fresh produce distributors and high security
  areas, use generators.
• Power stations huge dynamos are turned using
  steam from heated water, to produce electricity.

21
The DC Generator (dynamo)

• The slip-rings of the AC generator are replaced
  by a split-ring commutator to convert the AC
  generator to a DC generator.
• The DC generator produces current that flows in
  one direction only.
• The carbon brushes are arranged in such a way
  that contact is broken between the coil and the
  brushes for a brief instant when the coil is
  vertical.
• When contact is re-established, the brushes come
  into contact with a different part of the coil.
• So the current continues to flow through the
  brushes with no switch in its direction.

22
(No Transcript)
23

• Uses of AC Generators
• The main generators in nearly all electric power
  plants
• are AC generators. This is because a simple
• electromagnetic device called a transformer makes
  it
• easy to increase or decrease the voltage of
  alternating
• current. Almost all household appliances utilize
  AC.

• Uses of DC Generators
• Factories that do electroplating and those that
  produce
• aluminium, chlorine, and some other industrial
• materials need large amounts of direct current
  and use
• DC generators. So do locomotives and ships driven
  by
• diesel-electric motors. Because commutators are
• complex and costly, many DC generators are being
• replaced by AC generators combined with
  electronic
• rectifiers.

24
Alternating Current

• Alternating current (AC) is an electric current
  which reverses its direction of flow fifty times
  every second it has a frequency of 50 Hz.
• Why do we use AC and not DC?
• Electricity needs to be distributed through the
  country at high voltages to reduce energy losses
  in the power cables.
• To increase the voltage at the power stations and
  reduce it again before it reaches your home,
  transformers must be used.
• Transformers can only work on AC, since a
  changing magnetic field is required to induce a
  current.

25

• In addition, generating AC at the power stations
  is easier no complex modifications to the AC
  generator is necessary.
• Most appliances can operate on AC, including all
  those with a heating element (light bulbs,
  kettles, toasters, stoves), but some require the
  AC to be converted to DC first as they are
  direction sensitive (laptops, cell phone
  chargers)
• Characteristics of AC
• One of the characteristics of alternating current
  is that it causes self-inductance in the wires
  that carry it.
• It means that when the current changes direction,
  the magnetic field associated with it is in such
  a way as to oppose that change (Lenzs Law).

26

• Because of self-inductance, some of the
  electrical energy of the current is wasted
  appliances will get a lower maximum voltage than
  the peak value.
• This lower maximum voltage is known as the
  root-mean-square value (RMS value)

27

• In SA our mains supply is 220V (rms) AC (50 Hz).
  What is the peak or maximum voltage?