Electricity Session 4 (2 hours)

Principle of Engineering Principle of Engineering Heating effect and magnetic effect Heating effect and magnetic effect

of current.of current. Electrostatic hazards and electrical Electrostatic hazards and electrical

safetysafety

Principle of Engineering Principle of Engineering Heating effect and magnetic effect Heating effect and magnetic effect

of current.of current. Electrostatic hazards and electrical Electrostatic hazards and electrical

safetysafety

Electricity Session 4 (2 Electricity Session 4 (2 hours)hours)

Magnetism ForceMagnetism 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 Magnetic Field PatternPattern

• 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 Mapping the Magnetic FieldMagnetic 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 Magnetic Field Pattern of Attracting/Repelling Attracting/Repelling

MagnetsMagnets

Types of MagnetsTypes 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 Magnetic Effect of CurrentCurrent

• 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 Force on Current in a Magnetic FieldMagnetic 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 Application • Wire loop in

magnetic field Motor

• More turns coil stronger force

Disassemble a speaker to see how it

works

Current Induced by Current Induced by Motion in Magnetic Motion in Magnetic

FieldField• 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 Electromagnetic Induction Application: Induction Application:

GeneratorGenerator

• 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 Electromagnetic Induction: Moving Coil in Induction: Moving Coil in

Magnetic FieldMagnetic Field• Moving Coil in magnetic field

generates currrent • See flash animation in

http://www.bbc.co.uk/schools/gcsebitesize/physics/electricity/electromagneticinductionrev2.shtml

• Demonstration (TY only): show to the students the rotating magnetic wheel project that can be borrowed from C218

Electromagnetic Electromagnetic induction application: induction application:

flashlightflashlight

• Flashlight without battery: the “shake light”

• Magnet shaken in & out of coil/solenoid

Electrostatic hazards 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 Electrostatic hazards SeeSee Structure of Matter firstStructure 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


• 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.


Material becomes charged Material becomes charged positively, and the positively, and the

negativelynegatively

Rub a Rub a plasticplastic sheet sheet the sheet becomes the sheet becomes positivelypositively charged charged

Rub a Rub a rubberrubber sheet sheet the sheet becomes the sheet becomes negatively negatively chargedcharged

• 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.


• 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 charging has frequently caused:

• Fires and explosions • Disruption of production lines • Degradation of products • Equipment malfunction, computer

downtime • Electrostatic shocks to personnel


• 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?


• 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.


• Are static shocks a health risk?


• 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.


Frictional ChargesFrictional Charges

Rub a Rub a plasticplastic sheet sheet the sheet becomes the sheet becomes positivelypositively charged charged

Rub a Rub a rubberrubber sheet sheet the sheet becomes the sheet becomes negatively negatively chargedcharged

Frictional chargesFrictional charges

What if two balloons were rubbed and placed together?

What if two rulers were rubbed and placed together?

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What if a balloon and a ruler were rubbed and placed together?

++ ++

Van de Graaff Van de Graaff generatorgenerator

• Provide a large and continuous supply of charge

• A Charge Pump• A Charge Separator

How How does it does it work!?work!?

Principle of Van de Principle of Van de Graaff generatorGraaff generator

Electrical Safety 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.


• 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.


1. Insulation2. Guarding3. Grounding4. Circuit Protection Devices5. Safe Work Practices6. Training

Preventing Electrical Preventing Electrical HazardsHazards

1. Insulation


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


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


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


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


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


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


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.

Documents

Electricity Session 4 (2 hours)