ELECTRICITY MODULES Through Cognitive Learning

1
ELECTRICITY MODULES(Through Cognitive Learning)

• ENGINEERING TECHNOLOGY

2
MODULE 5 - ENERGY(Joules or Foot-Pounds)

• Devices
• Dams
• Capacitors
• Inductors
• Equipment
• Alternators
• Motors
• Generators

3
Capacitors

• A capacitor consists of two parallel conductive
  plates separated by a dielectric.
• In the circuit to the right, when the switch is
  closed
• electrons are drawn from the upper plate through
  the resistor to the power source ()VE..
• There will be an initial surge of current limited
  by the resistor and the plates will be charged as
  the diagram shows.

4
Capacitors

• Electrons will flow to the bottom plate making it
  -ve charged
• This flow of electrons continues until the p.d.
  across the plates equals the supply
• A fully charged capacitor appears like an open
  circuit to dc circuits
• Capacitance is a measure of a capacitors ability
  to store charge on its plates
• A capacitor has a capacitance of 1 farad if 1
  coulomb of charge is deposited on the plates by a
  potential difference of 1 volt. CQ/V

5
Capacitors

• The insulating material between the plates is
  called a dielectric
• Different dielectrics cause different amounts of
  charge to be stored
• The overall capacitance is determined by the size
  of the plates, the distance between the plates
  and the dielectric
• C?A/d where ? is called the permittivity of the
  dielectric
• ??r?o where ?o is the permittivity of a vacuum
  or free space and ?r is the permittivity of the
  dielectric relative to free space.

6
Capacitors

• ?o (dielectric of air) has a value of 8.85X10-12
  Farads and ?r (relative dielectric of mtl.) has a
  multiple of between 1 and 7500
• The most common dielectrics are mica, ceramic,
  tantalum and paper.
• Electrolytic capacitors are polarised which means
  they must be connected observing polarity.
  Failure to do so may cause the capacitor to
  explode.

7
Capacitors
Series

• Placing capacitors in parallel increases total
  capacitance
• Placing capacitors in series decreases total
  capacitance
• For capacitors in series, charge (like current)
  is the same in each capacitor. QT Q1 Q2
  Q3
• Applying Kirchhoffs voltage law VT V1
  V2 V3
• VT QT/CT
• Therefore
• QT/CT Q1/C1 Q2/C2 Q3/C3

Parallel
8
Capacitors

• Since the charge is the same on each series
  capacitor then 1/CT 1/C1 1/C2 1/C3
• Question Why is the charge the same on each
  series capacitor ?
• Answer
• When power is applied electrons flow to one of
  the plates of C1 causing an equal number to be
  repelled from the opposite plate.
• These electrons are attracted to one of the
  plates of C2 and repel an equal number from the
  opposite plate of C2.
• These electrons are attracted to one of the
  plates of C3 and repel an equal number from the
  opposite plate of C3 and so on...
• Hence each capacitor has the same excess of
  electrons on one plate and deficiency on the
  other plate.

9
Capacitors

• Voltage is constant for capacitors in parallel.
  VT V1 V2 V3
• Total charge is the sum of charges on all branch
  capacitors. QT Q1 Q2 Q3
• Since QT CTVT then CTVT C1V1 C2V2
  C3V3 therefore CT C1 C2 C3
• Capacitors in parallel results in increasing the
  total plate area thus increasing the total value
  of C.

Note C k (A/D) x .08842 pF A Area of
plate D distance between
plates. k dielectric constant
10
Energy Stored

• The ideal capacitor stores any of the energy
  supplied to it as an electric field between the
  plates. Be careful when handling an electrolytic
  capacitor immediately after a circuit is switched
  off. The capacitor could be charged and contain a
  considerable amount of energy.
• The energy stored is given by
• W 0.5 CV2

11
Inductors

• A magnetic field exists round a permanent magnet.
• The magnetic field is represented by flux lines
• The magnetic flux (?) is measured in webers (Wb)
• The number of flux lines in a given area is
  called the flux density (B)
• It is found that passing a wire through a
  magnetic field induces a voltage in the wire
• The same effect is caused by passing a magnetic
  field over a stationary wire.
• It is also found that passing a current through a
  wire creates a magnetic field

12
Inductors (cont.)

• Winding the wire into a coil (called an inductor)
  causes this magnetic field to increase and
  wrapping the coil around a ferromagnetic material
  further increases the field.
• The magnetic field of an inductor tries to
  prevent any change in the current
• If the current through the coil changes, the
  inductor creates a (temporary) voltage which
  opposes the current change (Lenzs Law)
• The induced voltage is given byV L (di/dt)
• The faster the change in current - the greater
  the induced voltage

13
Inductors (cont.)

• When a dc voltage is applied to a coil it
  initially acts as an open circuit (due to Lenzs
  Law) and then acts as a short circuit
• Formula for coils inductance LN2µA/l
• N is the number of turns
• µ is the permeability of the core
• A is the cross sectional area of the core
• l is the length of the coil

14
Inductors (cont.)

• Increasing inductance can be obtained by placing
  inductors in series
• Decreasing inductance can be obtained by placing
  inductors in parallel
• For inductors in series, the total inductance is
  found in the same way as for resistors
• LT L1 L2 L3

15
Inductors (cont.)

• For inductors in parallel, the total inductance
  is found in the same way as for resistors
• 1/LT 1/L1 1/L2 1/L3
• Find the total inductance of these circuits

16
RL and RLC (DC Circuits)

• For practical purposes an inductor can be
  replaced by a short circuit and a capacitor by an
  open circuit in a dc circuit, after dynamic
  change
• For the circuits at right, find all of the
  relevant voltages and currents (assume dc
  resistance of L1 is 0O).

17
Energy Stored

• The ideal inductor does not dissipate energy but
  stores it as a magnetic field.
• The energy stored by an inductor is given by
  W(joules ) 0.5 L(henries) x I2(amps)
• Find the energy stored by the inductor after the
  dynamic change.