Physical Hazards

• Physical hazards are commonly found in the laboratory.  These hazards are ubiquitous yet probably receive the least consideration because many of their consequences can be relatively minor injuries.  In spite of this, physical hazards should not be taken lightly.  

• Physical hazards originate from:

• The faulty construction of apparatus or fixtures.
• The nonpowered or nonmechanical motion of an object     
• The process of assembling, repairing, or setting up apparatus
• Ergonomic factors

• Moving objects can be dangerous.  Moving objects are typically found in physics or physical science activities, although some chemistry activities can create projectiles during the sudden release of pressure.  These hazards are also not necessarily a part of a procedure, but they can occur as an extension of or upon completion of a procedure. Any time moving objects are present, whether they are balls, carts, cylinders, or hoops, provisions must be made to contain these objects.  
• Projectiles represent the greatest problems in the physics laboratory.   Any projectile can be dangerous to students and teachers in the classroom.  Metal projectiles, however, are an especially serious concern.  Unfortunately, metal projectiles are often unavoidable in classrooms because projectile-range experiments and some demonstrations require steel balls to be launched across the room.  

• Tips to avoid projectile accidents.

• Wear impact resistant goggles during all experiments related to projectile motion.
• Use catch boxes to minimize the degree to which projectiles bounce or roll throughout the room.
• Use alternative projectile materials that are softer, especially if there insufficient or crowded space.
• Lay out firing paths that do not overlap.
• Do not lean into or walk across projectile paths.
• Announce when you are going to fire so that others can move out of the way.

• Objects that are attached to apparatus may become projectiles if they separate from the apparatus.  Examples include anything that has a hanging mass and may be spun, such as a hand-driven centripetal force apparatus or spinning disks on which objects are placed.  

• Tips to avoid accidents involving an object that becomes a projectile.

• Wear impact resistant goggles.  
• Check objects that enclose some part of the apparatus.
• Check strings and parts of apparatus for wear.
• Be sure all parts are firmly attached.
• If possible, wrap enclosing sleeves or tubes so that in the event of breakage, pieces are contained.  
• Do not use excessive rotating speeds.
• Use the lightest spinning objects possible.  
• Keep work areas free of obstructions.  
  
  

• Falling objects.  Falling objects are found in a variety of settings.  They may not even be an intended part of the activity.  Examples of objects that can be dangerous if they fall from a laboratory bench are clamped glassware such as thermometers, objects on balances, free-fall or Atwood machines, dropped magnets, or picket fences for free-fall motion studies.  

• Tips to avoid accidents caused by falling objects.

• Wear impact resistant goggles.
• Provide catch boxes to contain falling objects.  Provide padding in the boxes so that the objects will not bounce out or break on impact.
• Check clamps before each activity so that they are not too loose or too tight.
• Know how to use the equipment specified for the experiment.

• Bouncing or rolling objects.  Some activities require students to roll balls or cans down ramps or push carts across the floor.  Bouncing or rolling objects can be just as dangerous as falling objects or projectiles.  Awareness of the presence of either of these prevents accidents.  
  

Electrical Hazards

• Electrical hazards are caused by

• The improper use of machinery or apparatus
• The improper use of electrical outlets
• The improper use of electrical equipment, such as cables and power cords
• The improper maintenance of apparatus, outlets, and electrical equipment

• When used or maintained improperly, electrical equipment or devices can overheat or produce electrical fires. Frayed cords or exposed wires can easily electrify you.  

• Examples of particular hazards are

• Apparatus with deteriorated power cord insulation.
• A bent or broken prong on a plug
• A broken prong on a plug protruding from an outlet
• An overloaded circuit
• Flammable fumes near electrical apparatus
• Metal tools used near energized conductors
• Dangling jewelry near an energized conductor
• A circuit that someone is working on with both hands
• A water spill on electrical equipment

• Both alternating current (AC) and direct current (DC) can produce injury to living tissue and can destroy equipment.  However, the AC (60 Hz and 120 V) that U. S. electric companies supply to most electrical outlets disturbs human nerve impulses more readily than DC of the same  voltage or AC at other frequencies do because human nerve impulse resonate at approximately 60 Hz.  In addition, the DC circuits often used in the classroom experiments are relatively harmless.  Yet, DC can still be dangerous, and burn hazards are created in many common uses of DC.  So, all circuits should be treated cautiously.  
• Electrical hazards can burn equipment and cause a fire in the classroom.  These hazards can also cause serious injuries to you.  

• Current passing through the body may produce one or more of the following symptoms:

• Shock
• Involuntary muscle reaction
• Muscle paralysis

• Burning of tissue and organs
• Death by electrocution

• Some critical values of current and their corresponding physiological effects are given here.  These approximate values are based on 60 Hz AC passing through the intact skin for one second.
  

Critical values of 60 Hz AC(mA)	Physiological effects
Female                          Male
0.7                                   1.1	threshold of sensation
1.2                                   1.8	threshold of painless shock
11                                    16	paralysis of voluntary muscles
19                                    30	respiratory arrest
50                                    75	ventricular fibrillation
4000                             4000	cardiac arrest
5000                             5000	burns that are severe enough to be fatal

• Most electric shocks are caused when people come in contact with defective power cords or with energized instruments whose cases are removed.  A person receives an electric shock when he or she becomes part of a live electric circuit---when current enters the body at one point and exits at a different point.  You will receive an electric shock if you are in contact with

• Both energized receptacle slots of an outlet or power cord wires.
• One energized wire and a ground
• A metallic piece that is in contact with an energized conductor and a ground.  

• When instrument cases are open or electric components are exposed, an electric shock is likely to occur.  Always make sure that an instrument is unplugged before you inspect the internal components of an instrument.  
• Several factors may affect the severity of electrical shock.  Those factors include the following:

• Magnitude and type of current
• Current path through the body
• Length of time the electric shock acts on the body
• Location on the body of the electrical contact
• Skin resistance
• Other physiological and psychological factors such as age and physical condition

• If someone is electrified, tell the teacher to call emergency personnel.  Remove the person from contact with the energized conductor by using a nonconductor or by turning off the power.  Do not touch the person.  After the person has been removed, check for breathing and heartbeat.  Artificial respiration may be required.  Use blankets to keep the person warm until emergency personnel arrive.  
• Resistive heating is the thermal energy that current produces while moving through circuits.  This type of energy is also known as Joule heating or I²R loss and can be a hazard to you and your equipment.  Charges colliding with atoms or ions of the conductor can loose kinetic energy when collisions occur.  The conductor gains thermal energy.

Resistive heating can occur if

• Equipment components or conductors are not rated for the amount of current in their circuit.
• Outlets or circuits are overloaded
• Electrical connections are poorly or improperly made
• Apparatus is not properly ventilated

• The effects of resistive heating are

• Burns
• Ignition of combustible materials nearby
• Vaporization of or explosion of components

• Resistive heating can be prevented by

• Planning circuits by drawing schematics identifying all components.
• Estimating the current that each component will have, based on the voltage of the power source.
• Comparing your estimates of current with the ratings for the components you will use to prevent overloading the components.
• Connecting all components in the circuit except the power source, and verify that all connections are properly made.  
• Opening the switch or turning off the power source while connecting the components and the power source.
• Leaving the power source on for the minimum time necessary.

• Static electricity is caused by an imbalance of electrons between two surfaces.  The imbalance can be corrected by transferring electrons from one surface to another surface by conduction or induction.  When the excess charge on a surface is discharged, a person standing near the discharge can be electrified.  These shocks are normally not hazardous.  
• One serious concern related to static electricity is the accumulation of charge near a hazardous material such as a flammable gas or vapor.  Ignition can occur.  
• Other issues to be concerned about include reacting to electric charge by jumping and knocking something over, sparks that can ignite something, and electric arcs that may occur if a switch is closed slowly.
• Rules for preventing electrical hazards

• Inspect the insulation on power cords, etc., for deterioration.
• Do not splice cords.
• Use power cords rated for the device with which they are to be used.  
• Never remove the ground prong from a plug.  
• Do not use three-to-two adapters.
• Have any two prong outlets replaced with three prong outlets.  The third prong is the ground.
• Use GFI circuits if possible.
• Do not overload circuits.
• Report broken or damaged apparatus immediately.  
• Never staple or nail power cords.  
• Never carry a device by its power cord.
• Provide adequate ventilation for electrical devices.
• Keep activity areas dry.  
• If you smell gas when entering a room, do not turn on the lights or other electrical apparatus.
  

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