Transcript Physics Behind the Burglar Alarm

Physics Behind the
Burglar Alarm
Anna Ponce
Helen Doo
Joua Thao
Overview
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The Burglar Alarm
What is Magnet?
Ferromagnets and Electromagnets
Basic Principals
Faraday’s Law: Magnetic Induction
Lenz’s Law
Laws Demonstration
Burglar Alarm
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Types: Magnet and coils and Magnetic
Switches
Design
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Battery powered circuit
Spring driven metal switch built into door
frame
Magnet embedded in the door, lined up to the
switch
Separately powered buzzer with a relaydriven switch
More…
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When the door is closed the magnet pulls the metal switch
closed so the circuit is complete.
The current powers the relay’s electromagnet, so the buzzer
stays open.
When you move the magnet by opening the door the spring
snaps the switch back into an open position. This cuts off the
current and closes the relay, sounding the alarm.
So When the alarm is armed and a intruder pushes the window
open, the magnet slides out of line with the switch, and the
buzzer is activated.
What is a magnet?
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Magnets are mainly made from iron.
All magnets have a North and South pole.
All magnets have a magnetic field.
More Fun Facts…
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Like poles repel each other and unlike pole
attract each other
Ferromagnets
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Metals which, when magnetized by a magnetic field are able to hold
onto the magnetization even when the field is turned off .
Only ferromagnets can be made into a permanent magnets.
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Placing items in an external field which will retain some magnetism
Placing item in Solenoid which are loops of wire with direct current
Stroking a existing magnet along a ferromagnetic material from one end to
the other repeatedly in same direction.
Placing a steel bar in magnetic field , heating high temperatures and
hammering as it cools.
Word Ferromagnetic comes from the Latin word Ferrus meaning Iron.
Element sign is ‘Fe’
Ferromagnetic Materials: Iron, Steel alloys of Iron, Nickels, Samarium,
Neodymium, and cobalt.
Electromagnets
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Coil of wires which, when a current is passed
through, generate a magnetic field.
Can be turned on and off by a switch
To increase the strength of the electromagnet
- Increase the current flow
- Increase the number of coils
- Putting a core of a magnetic material inside
the solenoid. The iron core becomes
magnetized itself and makes the field stronger.
Definitions – Basic Principles
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Magnetism- An invisible force that attracts or repels
magnetic materials that has electromagnetic effects.
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Electromagnetic Induction- is the process by which a
voltage is generated in a circuit when there is a
magnetic flux, and that magnetic flux changes.
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Magnetic Flux- is the product of the average magnetic
field times the perpendicular area that it penetrates.
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Electromotive Force- movement generated between
magnets and an electric current.
Magnetic Flux
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Number of magnetic field lines passing
through the area bounded by the loop.
where
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B = Magnetic field strength
A =Area of the coil
It is at it maximum value when the field lines
are perpendicular to the plane of the loop.
It is at zero when the field line are parallel to
the loop.
Magnetic Flux Illustration
Faraday’s Law:
Magnetic Induction
Faraday’s Law:
Magnetic Induction
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where N = # of turns
Φ = magnetic flux
t = time
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It states: “A voltage (electromotive force) is
induced in a circuit when there is a
changing magnetic flux passing through the
circuit.”
Lenz’s Law
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where N = # of turns
Φ = magnetic flux
t = time
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It states: “An induced electromotive force
generates a current that induces a counter
magnetic field that opposes the magnetic
field generating the current.”
Lenz’s Law Experiment
When the switch is closed, current passes through the solenoid. A
current in opposite direction is induced in the can. The two
currents repel each other leading the can to crush.
Galvanometer Demonstration
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When the magnet is moved toward the coil,
an electric current is generated.
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Notice the direction of the current when the
magnet is move inward and outward.
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Notice that there is no current the magnet is
not moving.
More…
Sources
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http://www.school-forchampions.com/science/magnetism.htm
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http://www.ndted.org/EducationResources/HighSchool/Magne
tism/electromagnets.htm
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http://www.howstuffworks.com/