Principle of Engineering Heating effect and magnetic effect of current. Electrostatic hazards and electrical safety

1
Principle of Engineering Heating effect and
magnetic effect of current. Electrostatic
hazards and electrical safety

• Electricity Session 4 (2 hours)

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Magnetism Force

• North and South Poles of Magnet??,????
• Attracts oppose other magnets
• Opposite Poles Attract
• Like Poles repel
• Attracts certain metals such as iron, nickel, and
  cobalt.
• Exploration hands on experimentation

3
Magnetic Field Pattern

• Magnetic field pattern can be seen using iron
  filings
• a magnetic field a region in space where each
  point influenced is influenced magnetically.

4
Mapping the Magnetic Field

• Place a rare-earth magnet on square paper
• Exploration Place the compass on different
  regions of the square paper and record direction
  of needle in terms of arrows.

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Magnetic Field Pattern of Attracting/Repelling
Magnets
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Types of Magnets

• Permanent Magnet
• Electromagnet
• Coil wound on iron core
• Field strength a current i
• Field strength a no. of turns N
• Exploration To test the effect of core, current
  and number of turns on Electromagnetic field
  strength by winding a coil with/without core and
  use it to attract / repel a hanging permanent
  magnet (taped under a table or workbench)

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Magnetic Effect of Current

• Magnetism and current are related
• Exploration Run a wire (should have a straight
  portion at least 8 inches long) through a square
  paper and plot the magnetic field pattern

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Force on Current in a Magnetic Field

• Circular magnetic field by current interacts
  with external magnetic field ? force
• Electrical ? mechanical energy conversion
• Exploration use a straight thin wire and pass a
  0.5-1 A current through it. Put a magnet near it
  and experience the attraction and repulsion.
  Verify the right hand MOTOR rule. What is the
  effect of a larger current?

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Application

• Wire loop in magnetic field ? Motor
• More turns ? coil ? stronger force

Disassemble a speaker to see how it works
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Current Induced by Motion in Magnetic Field

• Electromagnetic induction ????
• Motion produces current
• Mechanical ? electrical energy conversion

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Electromagnetic Induction Application Generator

• Exploration connect a generator (which is really
  a toy motor), preferably in a gear box, to
  another motor. Rotate the generator to drive the
  other motor to move. Alternatively use the
  generator to light up an LED.

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Electromagnetic Induction Moving Coil in
Magnetic Field

• Moving Coil in magnetic field generates currrent
• See flash animation in http//www.bbc.co.uk/school
  s/gcsebitesize/physics/electricity/electromagnetic
  inductionrev2.shtml
• Demonstration (TY only) show to the students the
  rotating magnetic wheel project that can be
  borrowed from C218

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Electromagnetic induction application flashlight

• Flashlight without battery the shake light
• Magnet shaken in out of coil/solenoid

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Electrostatic hazards

• Many people ask about shocks experienced when
  they touch the door, filing cabinet, lift, or
  other metal object
• Daily Life experiences
• Move aluminum can with balloon charged up by
  rubbing balloon with cloth
• Plastic comb and hair
• Plastic bag strips rubbed together repelling
• Rubbed plastic ruler and paper/aluminum foil

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Electrostatic hazards See Structure of Matter
first

• Matter ?? composed of Molecules ??
• Molecules composed of Atoms ??
• Structure of Atoms electrons (- charge) ??,
  nucleus protons ( charge) ??, neutrons ??

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Electrostatic hazards

• Static electricity ?? is generated whenever two
  materials are in contact with each other.
• All materials are made of electrical charges in
  the material atoms. In the universe there are
  equal amounts of negative electrical charge
  (electrons) and positive charge (protons). These
  generally try to stay in balance of equal amounts
  at every location. 

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Electrostatic hazards

• However, when two materials are in contact, some
  of the charges redistribute by moving from one
  material to the other. This leaves an excess of
  positive charge on one material, and an equal
  negative charge on the other.
• When the materials move apart, each takes it's
  charge with it. One material becomes charged
  positively, and the other negatively.
•   

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Material becomes charged positively, and the
negatively
Rub a plastic sheet ? the sheet becomes
positively charged
Rub a rubber sheet ? the sheet becomes negatively
charged
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Electrostatic hazards

• If the materials are able to conduct electricity
  away the charges will dissipate and eventually
  recombine.
• In this case, static electricity effects may be
  too small to be noticed.
• However, if the charges are separated faster than
  the material can dissipate them, the amount of
  electrostatic charge builds up.
• Eventually a high voltage, and the effects of
  static electricity, may be noticed. 

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Electrostatic hazards

• If you experience static shocks while working in
  an area where flammable atmospheres (solvent
  vapours or dust clouds) might be present, seek
  advice immediately. There may be a fire or
  explosion risk. 

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Electrostatic hazards

• Electrostatic charging has frequently caused
• Fires and explosions
• Disruption of production lines
• Degradation of products
• Equipment malfunction, computer downtime
• Electrostatic shocks to personnel 
                                                    
                        

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Electrostatic hazards

• Static charge build-up is enhanced when the air
  is dry. So, static problems and effects are often
  noticed in dry air conditions. 
• I get shocks when I'm sitting, or get up from the
  chair - and I haven't walked anywhere! Why?

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Electrostatic hazards

• When you sit in a chair the contact between your
  clothes and the chair can generate a lot of
  electrostatic charge on your clothes. While you
  stay in contact with the chair your body voltage
  stays low. If you lean forward so you back moves
  away from the chair back, or if you get up out of
  the chair, then you take the electrostatic charge
  with you. Your body voltage can rise very rapidly
  to a high voltage as the charge is separated from
  it's counter charge on the chair. 

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Electrostatic hazards

• Are static shocks a health risk?

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Electrostatic hazards

• Fortunately there is little risk attached to such
  electrostatic discharges. In most cases they are
  just a common nuisance. The biggest risk is that
  a shock could cause you to have an accidental
  injury. For example, you might withdraw your arm
  suddenly and hit it against something. 

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Frictional Charges
Rub a plastic sheet ? the sheet becomes
positively charged
Rub a rubber sheet ? the sheet becomes negatively
charged
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Frictional charges

• What if two balloons were rubbed and placed
  together?

• What if two rulers were rubbed and placed
  together?

• What if a balloon and a ruler were rubbed and
  placed together?

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Van de Graaff generator

• Provide a large and continuous supply of charge
• A Charge Pump
• A Charge Separator

How does it work!?
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Principle of Van de Graaff generator
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Electrical Safety
Electrical Shocks Occur -gt People injury or dead
??????????????,?????
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Electrical Safety

• The effects of electric shock depend upon the
  type of circuit, its voltage, resistance,
  current, pathway through the body, and duration
  of the contact.  

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Electrical Safety

• Effects of Electric Current in the Human Body
•  Current  Reaction
• 1 Mill ampere - Perception level. Just a faint
  tingle.
• 5 Milliamperes -Slight shock felt not painful
  but disturbing. Average individual can let go.
  However, strong involuntary reactions to shocks
  in this range can lead to injuries.
• 6-25 Milliamperes (women) - Painful shock,
  muscular control is lost.
• 9-30 Milliamperes (men) -This is called the
  freezing current or "let-go" range.
• 50-150 Milliamperes - Extreme pain, respiratory
  arrest, severe muscular contractions.
  Individual cannot let go. Death is possible.
• 1,000-4,300 Milliamperes - Ventricular
  fibrillation. (The rhythmic pumping action of the
  heart ceases.) Muscular contraction and nerve
  damage occur.
• Death is most likely.10,000 Milliamperes
  Cardiac arrest, severe burns and probable death. 

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Preventing Electrical Hazards

1. Insulation
2. Guarding
3. Grounding
4. Circuit Protection Devices
5. Safe Work Practices
6. Training

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Preventing Electrical Hazards

1. Insulation

One way to safeguard individuals from
electrically energized wires and parts is through
insulation. An insulator is any material with
high resistance to electric current.
Insulatorssuch as glass, mica, rubber, and
plasticare put on conductors to prevent shock,
fires, and short circuits.
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Preventing Electrical Hazards

• 2.Guarding

Live parts of electric equipment operating at 50
volts or more must be guarded against accidental
contact. Guarding of live parts may be
accomplished by     location in a room,
vault, or similar enclosure     use of
permanent, substantial partitions or screens    
location on a suitable balcony, gallery, or
platform elevated     elevation of 8 feet
(2.44 meters) or more above the floor.
Entrances to rooms and other guarded locations
containing exposed live parts must be marked with
conspicuous warning signs forbidding unqualified
persons to enter.
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Preventing Electrical Hazards

• 3.Grounding

The term "ground" refers to a conductive body,
usually the earth, and means a conductive
connection, whether intentional or accidental, by
which an electric circuit or equipment is
connected to earth or the ground plane. By
"grounding" a tool or electrical system, a
low-resistance path to the earth is intentionally
created. When properly done, this path offers
sufficiently low resistance and has sufficient
current carrying capacity to prevent the buildup
of voltages that may result in a personnel
hazard. This does not guarantee that no one
will receive a shock, be injured, or be killed
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Preventing Electrical Hazards

• 4.Circuit Protection Devices

Circuit protection devices (fuses, circuit
breakers, and ground-fault circuit interrupters)
are designed to automatically limit or shut off
the flow of electricity in the event of a
ground-fault, overload, or short circuit in the
wiring system. Fuses and circuit-breakers are
over-current devices that are placed in circuits
to monitor the amount of current that the circuit
will carry. They automatically open or break the
circuit when the amount of current flow becomes
excessive and therefore unsafe.
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Preventing Electrical Hazards

• 4.Circuit Protection Devices

Fuses are designed to melt when too much current
flows through them. Circuit breakers, on the
other hand, are designed to trip open the circuit
by electro-mechanical means. Fuses and circuit
breakers are intended primarily for the
protection of conductors and equipment.
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Preventing Electrical Hazards

• 5.Safe Work Practices

Employees and others working with electric
equipment need to use safe work practices.
These include deenergizing electric equipment
before inspecting or making repairs, using
electric tools that are in good repair, using
good judgment when working near energized lines,
and using appropriate protective equipment.
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Preventing Electrical Hazards

• 6. Training

To ensure that they use safe work practices,
employees must be aware of the electrical hazards
to which they will be ex-posed. Employees must be
trained in safety-related work practices as well
as any other procedures necessary for safety from
electrical hazards.