Physics Behind the Burglar Alarm Anna Ponce Helen Doo Joua Thao

Overview The Burglar Alarm What is Magnet? Ferromagnets and Electromagnets Basic Principals Faraday’s Law: Magnetic Induction Lenz’s Law Laws Demonstration

Burglar Alarm Types: Magnet and coils and Magnetic Switches Design- 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 relay-driven switch

More… 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? Magnets are mainly made from iron. All magnets have a North and South pole. All magnets have a magnetic field.

More Fun Facts… Like poles repel each other and unlike pole attract each other

Ferromagnets 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. 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 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 Magnetism- An invisible force that attracts or repels magnetic materials that has electromagnetic effects. 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. Magnetic Flux- is the product of the average magnetic field times the perpendicular area that it penetrates. Electromotive Force- movement generated between magnets and an electric current.

Magnetic Flux Number of magnetic field lines passing through the area bounded by the loop. where 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 where N = # of turns Φ = magnetic flux t = time 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 where N = # of turns Φ = magnetic flux t = time 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 When the magnet is moved toward the coil, an electric current is generated. Notice the direction of the current when the magnet is move inward and outward. Notice that there is no current the magnet is not moving.

More…

Sources http://www.school-for-champions.com/science/magnetism.htm http://www.ndt-ed.org/EducationResources/HighSchool/Magnetism/electromagnets.htm http://www.howstuffworks.com/