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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 • • • • • • • • • • 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