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 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 S N Direction of motion of wire 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 ze/physics/electricity/electromagneticind uctionrev2.shtml 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? C directly/inversely α distance between the plates? V buzzer 470K resistors

Equipment List 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