Electric Circuits

1
Electric Circuits
2
Battery and a Bulb--A Simple Circuit

• A circuit is a complete path from the positive
  terminal at the top of the battery to the
  negative terminal at the bottom.
• Electrons flow through devices in DC current.

3
Electric Circuits

• A gap in currents is provided to be open and
  closed to either cutoff or allow electron flow.
• Devices are commonly connected in a circuit one
  of two ways.
• In series they form a single pathway for electron
  flow between the terminals of the battery,
  generator, or wall socket.
• In parallel they form branches, each of which is
  a separate path for the flow of electrons.

4
Series Circuits

• A series circuit in which devices are arranged so
  that charge flows through each in turn. If one
  part of the circuit should break the current will
  stop.
• A break anywhere in the path results in an open
  circuit and the flow of electrons ceases.

5
Series Connection Characteristics.

• Electric current has but a single pathway through
  the circuit.
• Total resistance to current is the sum of the
  individual resistance along the circuit path.
• The current in the circuit is numerically equal
  to the voltage supplied by the source divided by
  the total resistance of the circuit.
• The potential difference across each device is
  proportional to resistance.
• The total voltage impressed across a series
  circuit divides among the individual electrical
  devises in the circuit so that the sum of the
  voltage drops across each individual device is
  equal to the total voltage.

6
Parallel Circuits

• A parallel circuit is an electrical circuit in
  which devices are connected to the same two
  points of a circuit so that any single device
  completes the circuit independently of the
  others.

7
Parallel Circuit Characteristics

• Each device connects the same two parts of the
  circuit
• Total current divides among the parallel branches
• Total current equals sum of the currents in its
  parallel branches
• As the number of parallel branches increase,
  overall resistance decreases.

8
Schematic Designs

• Simple diagrams of electrical circuit elements
  are schematic diagrams

9
Schematic Designs

• Combining resistors in a compound circuit
• The equivalent resistance is the value of the
  single resistor that would comprise the same load
  to the battery of power source.
• The equivalent resistance for a pair of equal
  resistors in parallel is half the value of either
  resistor.
• Becomes much more complicated when more resistors
  added

10
Parallel Circuits and Overloading

• Electricity is usually fed into a home by way of
  two lead wires, which are low in resistance
  (110-120 volts)
• Lines that carry more than a safe amount of
  current is overloaded
• To prevent overloading, fuses are connected in
  series along the supply line
• Circuits may be protected by circuit breakers,
  which are magnets or bi-metallic strips to open
  the switch.

11
Homework

• Chapt 35 RQ 1, 2, 3, 5, 7, 9, 12, 13, 15 T
  E 1, 5, 6, 7, 8

12
Magnetism

• Magnetic Poles
• Magnets exert force on one another
• Regions of magnetic poles produce magnetic forces
• The end that points northward is called the
  north-seeking pole and vice-versa
• Each magnet has both poles
• Likes poles repel opposite poles attract
• Electric charges can be isolated, magnets cannot

13
Magnetic Fields

• The space around a magnet in which a magnetic
  force is exerted, is filled with a magnetic field
• The shape of the field is revealed by magnetic
  field lines

14
The Nature of a Magnetic Field

• A magnetic field is produced by the motion of
  electric charge
• The magnet as a whole is stationary while the
  electrons are in constant motion
• Every spinning electron is a tiny magnet

15
Magnetic Domains

• The magnetic field of individual iron atoms is so
  strong that interactions among adjacent iron
  atoms cause large clusters of them to line up
  with each other. These clusters of aligned atoms
  are called magnetic domains
• The difference between a piece of ordinary iron
  and an iron magnet is alignment of domains
• Permanent magnets are made by simple placing
  pieces of iron or certain iron alloys in strong
  magnetic fields

16
Electric Currents and Magnetic Fields

• A moving charge produces a magnetic field, many
  of these with an electric current
• Magnetic field lines about a current-carrying
  wire crowd up when the wire is bent into a loop
• A piece of iron placed in a current-carrying coil
  of wire is an electromagnet

17
Magnetic Forces on Moving Charged Particles

• A charged particle at rest will not interact with
  a static magnetic field
• A charged particle that moves in a magnetic
  field, the magnetic character of its motion
  becomes evident, experiencing a deflected force
• The force is greatest when the particle moves in
  a direction perpendicular to the magnetic field
  lines
• At other angles the force is less
• It becomes zero when the particle moves parallel
  to the field lines

18
Magnetic Forces on Current-Carrying Wires

• Current-carrying wires respond to deflected
  force, moving the wire
• If the direction of the current is reversed, the
  deflected force acts in the opposite direction
• The force is maximum when the current is
  perpendicular to the magnetic field lines

19
Meters to Motors

• A current-indicating instrument is called a
  galvanometer
• A galvanometer may be calibrated to measure
  current in amperes called an ammeter
• Ammeters calibrated to measure electric potential
  (volts) is a voltmeter
• If the design of the galvanometer is modified,
  you have an electric motor
• Large motors are usually made with an
  electromagnet that is energized by a power source
  with many loops of wire that are wound about an
  iron cylinder, called an armature which then
  rotates when energized with electric current

20
The Earths Magnetic Field

• The discrepancy between the orientation of a
  compass and true north is called the magnetic
  declination
• The magnetic field of the earth is not stable

21
Homework

• Chapt 36 RQ 2, 3, 4, 6, 9, 11, 13, 14, 17, 19,
  20 T E 2, 5, 8

22
Uses of Magnetism

• Electromagnetic Induction
• The production of voltage depends only on the
  relative motion between the conductor and the
  magnetic field
• The amount of voltage induced depends on how
  quickly the magnetic field lines are traversed by
  the wire (the quicker the more voltage)
• The greater number of loops of wire that move in
  a magnetic field, the greater the induced voltage
  and the greater the current in the wire

23
Electromagnetic Induction

• It is more difficult to push the magnet into a
  coil with more loops because the magnetic field
  of each current loop resists the motion of the
  magnet.
• The phenomena of inducing voltage by changing the
  magnetic field around a conductor is called
  electromagnetic induction.
• Faradays Law states the induced voltage in a
  coil is proportional to the product of the number
  of loops and the rate at which the magnetic field
  changes within those loops.

24
Generators and Alternating Current

• If a magnet is plunged in and out of a coil of
  wire, the induced voltage alternates in
  direction. The frequency of the induced
  alternating voltage is equal to the frequency of
  the changing magnetic field within the loop. When
  a magnet enters the coil the field strength
  increases and vice versa
• Rather than moving the magnet, if the coil is
  rotated, it makes a generator, which is a motor
  running backwards. The voltage produced is
  alternating the N.A. standard changes in
  magnitude and direction during 60 complete cycles
  per second or 60 hertz
• Power plants are made up of huge coils or many
  loops that are wrapped around an iron core to
  make a armature and is rotated by a turbine
• Electricity is a form of energy that must have a
  source

25
Motor and Generator Comparison

• A motor converts electrical energy into
  mechanical energy, a generator converts
  mechanical energy into electrical energy.
• Moving charges experience a force that is
  perpendicular to both their motion and the
  magnetic field that they traverse.
• The deflected wire is the motor effect, and the
  law of induction is the generator effect

26
Transformers

• A transformer allows equal frequency of energy
  flow through two different currents
• (primary voltage/number of primary turns)
  (secondary voltage/number of secondary turns
• Power into primary power out of secondary
  (voltage current)primary (voltagecurrent)seco
  ndary

27
Power Transmission

• Almost all power today is AC and the voltage is
  transferred in different levels
• Power created at 6000 V then raised to 120000 V
• Power is reduced when it comes to user

28
Induction of Electric and Magnetic Fields

• An electric field is induced in any region of
  space in which a magnetic field is changing with
  time.
• The magnitude of the induced electrical field is
  proportional to the rate at which the magnetic
  field changes.
• The direction of the induced electric field is at
  right angles to the changing magnetic field
• A magnetic field is induced in any region of
  space in which an electric field is changing with
  time.
• The magnitude of the induced magnetic field is
  proportional to the rate at which the electric
  field changes.
• The direction of the induced magnetic field is at
  right angles to the changing electric field

29
Electromagnetic Waves

• Shake a charged object to and fro and you produce
  electromagnetic waves.
• An electromagnetic wave is composed of vibrating
  electric and magnetic fields that regenerate each
  other

30
Homework

• Chapt 37 RQ 3, 4, 6, 7, 10, 11, 13, 15, 16,
  18 T E 1, 4, 6, 8