Presentation on theme: "Principle of Engineering Heating effect and magnetic effect of current. Electrostatic hazards and electrical safety Electricity Session 4 (2 hours)"— Presentation transcript:
Principle of Engineering Heating effect and magnetic effect of current. Electrostatic hazards and electrical safety Electricity Session 4 (2 hours)
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 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 http://www.bbc.co.uk/schools/gcsebitesi ze/physics/electricity/electromagneticind uctionrev2.shtml 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
Electrostatic hazards Many people ask about shocks experienced when they touch the door, filing cabinet, lift, or other metal object 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
Electrostatic hazards See Structure of Matter first Matter 物質 composed of Molecules 分子 Molecules composed of Atoms 原子 Structure of Atoms: electrons (- charge) 電子, nucleus: protons (+ charge) 質子, neutrons 中子
Electrostatic hazards Static electricity 靜電 is generated whenever two materials are in contact with each other. All materials are made of electrical charges in the material atoms. In the universe there are equal amounts of negative electrical charge (electrons) and positive charge (protons). These generally try to stay in balance of equal amounts at every location.
However, when two materials are in contact, some of the charges redistribute by moving from one material to the other. This leaves an excess of positive charge on one material, and an equal negative charge on the other. When the materials move apart, each takes it's charge with it. One material becomes charged positively, and the other negatively. Electrostatic hazards
Material becomes charged positively, and the negatively Rub a plastic sheet the sheet becomes positively charged Rub a rubber sheet the sheet becomes negatively charged
If the materials are able to conduct electricity away the charges will dissipate and eventually recombine. In this case, static electricity effects may be too small to be noticed. However, if the charges are separated faster than the material can dissipate them, the amount of electrostatic charge builds up. Eventually a high voltage, and the effects of static electricity, may be noticed. Electrostatic hazards
If you experience static shocks while working in an area where flammable atmospheres (solvent vapours or dust clouds) might be present, seek advice immediately. There may be a fire or explosion risk. Electrostatic hazards
Electrostatic charging has frequently caused: Fires and explosions Disruption of production lines Degradation of products Equipment malfunction, computer downtime Electrostatic shocks to personnel Electrostatic hazards
Static charge build-up is enhanced when the air is dry. So, static problems and effects are often noticed in dry air conditions. I get shocks when I'm sitting, or get up from the chair - and I haven't walked anywhere! Why? Electrostatic hazards
When you sit in a chair the contact between your clothes and the chair can generate a lot of electrostatic charge on your clothes. While you stay in contact with the chair your body voltage stays low. If you lean forward so you back moves away from the chair back, or if you get up out of the chair, then you take the electrostatic charge with you. Your body voltage can rise very rapidly to a high voltage as the charge is separated from it's counter charge on the chair. Electrostatic hazards
Are static shocks a health risk? Electrostatic hazards
Fortunately there is little risk attached to such electrostatic discharges. In most cases they are just a common nuisance. The biggest risk is that a shock could cause you to have an accidental injury. For example, you might withdraw your arm suddenly and hit it against something. Electrostatic hazards
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? ++
Van de Graaff generator Provide a large and continuous supply of charge A Charge Pump A Charge Separator How does it work!?
Electrical Safety Electrical Shocks Occur -> People injury or dead 執波而觸電男童送院時面呈紫黑，口吐白沫。
The effects of electric shock depend upon the type of circuit, its voltage, resistance, current, pathway through the body, and duration of the contact. Electrical Safety
Effects of Electric Current in the Human Body Current Reaction 1 Mill ampere - Perception level. Just a faint tingle. 5 Milliamperes -Slight shock felt; not painful but disturbing. Average individual can let go. However, strong involuntary reactions to shocks in this range can lead to injuries. 6-25 Milliamperes (women) - Painful shock, muscular control is lost. 9-30 Milliamperes (men) -This is called the freezing current or "let-go" range. 50-150 Milliamperes - Extreme pain, respiratory arrest, severe muscular contractions. * Individual cannot let go. Death is possible. 1,000-4,300 Milliamperes - Ventricular fibrillation. (The rhythmic pumping action of the heart ceases.) Muscular contraction and nerve damage occur. Death is most likely.10,000 – Milliamperes Cardiac arrest, severe burns and probable death. Electrical Safety
1.Insulation Preventing Electrical Hazards One way to safeguard individuals from electrically energized wires and parts is through insulation. An insulator is any material with high resistance to electric current. Insulators — such as glass, mica, rubber, and plastic — are put on conductors to prevent shock, fires, and short circuits.
2.Guarding Preventing Electrical Hazards Live parts of electric equipment operating at 50 volts or more must be guarded against accidental contact. Guarding of live parts may be accomplished by: location in a room, vault, or similar enclosure use of permanent, substantial partitions or screens location on a suitable balcony, gallery, or platform elevated elevation of 8 feet (2.44 meters) or more above the floor. Entrances to rooms and other guarded locations containing exposed live parts must be marked with conspicuous warning signs forbidding unqualified persons to enter.
3.Grounding Preventing Electrical Hazards The term "ground" refers to a conductive body, usually the earth, and means a conductive connection, whether intentional or accidental, by which an electric circuit or equipment is connected to earth or the ground plane. By "grounding" a tool or electrical system, a low-resistance path to the earth is intentionally created. When properly done, this path offers sufficiently low resistance and has sufficient current carrying capacity to prevent the buildup of voltages that may result in a personnel hazard. This does not guarantee that no one will receive a shock, be injured, or be killed
4.Circuit Protection Devices Preventing Electrical Hazards Circuit protection devices (fuses, circuit breakers, and ground-fault circuit interrupters) are designed to automatically limit or shut off the flow of electricity in the event of a ground-fault, overload, or short circuit in the wiring system. Fuses and circuit-breakers are over-current devices that are placed in circuits to monitor the amount of current that the circuit will carry. They automatically open or break the circuit when the amount of current flow becomes excessive and therefore unsafe.
4.Circuit Protection Devices Preventing Electrical Hazards Fuses are designed to melt when too much current flows through them. Circuit breakers, on the other hand, are designed to trip open the circuit by electro- mechanical means. Fuses and circuit breakers are intended primarily for the protection of conductors and equipment.
5.Safe Work Practices Preventing Electrical Hazards Employees and others working with electric equipment need to use safe work practices. These include: deenergizing electric equipment before inspecting or making repairs, using electric tools that are in good repair, using good judgment when working near energized lines, and using appropriate protective equipment.
6. Training Preventing Electrical Hazards To ensure that they use safe work practices, employees must be aware of the electrical hazards to which they will be ex-posed. Employees must be trained in safety-related work practices as well as any other procedures necessary for safety from electrical hazards.