Transcript Lecture5 - GEOCITIES.ws

Engineering
Fundamentals
Week 5 (3 hours)
Events this week
• Quiz (45 minutes)
• Electricity and Magnetism
Exploration
Magnetism Force
• North and South Poles of Magnet磁石,磁鐵;
磁體
• Attracts & oppose other magnets
• Opposite Poles Attract
• Like Poles repel
• Attracts certain metals such as iron,
nickel, and cobalt.
• Exploration: hands on experimentation
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.
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.
Magnetic Field Pattern of
Attracting/Repelling Magnets
Types of Magnets
• Permanent Magnet
• Electromagnet
– Coil wound on iron core
– Field strength α current i
– Field strength α 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)
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
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?
Application
• Wire loop in
magnetic field 
Motor
• More turns  coil
 stronger force
Disassemble a
speaker to see
how it works
Current Induced by Motion
in Magnetic Field
• Electromagnetic induction 感應電磁
• Motion produces current
• Mechanical  electrical energy conversion
Direction of
motion of
wire
S
N
Direction
of induced
current
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.
Electromagnetic Induction:
Moving Coil in Magnetic Field
• Moving Coil in magnetic field generates
currrent
• See flash animation in
http://www.bbc.co.uk/schools/gcsebitesi
ze/physics/electricity/electromagneticind
uctionrev2.shtml
• Demonstration (TY only): show to the
students the rotating magnetic wheel
project that can be borrowed from C218
Electromagnetic induction
application: flashlight
• Flashlight without
battery: the “shake
light”
• Magnet shaken in &
out of coil/solenoid
Electric Charges 電荷
• Static electricity 靜電
• 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
• Like charges repel, unlike
charges attract
Van de Graaff generator
• Provide a large and continuous supply of
charge
• A Charge Pump
• A Charge Separator
How does
it work!?
Frictional Charges
Rub a plastic sheet  the
sheet becomes positively
charged
Rub a rubber sheet  the
sheet becomes negatively
charged
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?
+
Principle of Van de Graaff
generator
Capacitance (Optional)
• Capacitor
• Capacitance – how much
charge is created for each
Volt.
• Exploration
• Use the given circuit in
which the sound is inversely
proportional to the
capacitance C.
• The C is made up of 2
alluminium plates
• C directly/inversely α area?
5V
• C directly/inversely α
distance between the plates?
16
10
9
1 2 3 4 5 6 7 8
buzzer
470K
resistors
Equipment List
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DC Power Supply
Rare-earth magnets
Graph paper
Compasses
Iron filing (optional)
Wires
Iron core (nails)
4049 CMOS inverter (optional)
aluminium plates (2” X 4”) 2/group (optional)
Balloons, Rulers, Garbage Bags,aluminium cans