Course:     8th grade Physics

Unit Introduction: Static Electricity and Magnetism

 

The Static Electricity and Magnetism Unit is intended to engage students in many experiences with hands-on and computer-based materials that will help them modify their existing ideas and construct new ideas about static electricity and magnetism.  Because students come into the unit with a tendency to use a similar model to account for both static electricity and magnetism phenomena, we begin this unit with a series of activities to help students recognize the similarities and differences between static electric effects and magnetic effects.  Parts 2 and 3 focuses on helping them develop a model to account for static electric phenomena.   Extensive use is made of both hands-on equipment and, particularly in Parts 2 and 3, the various static electricity simulators.

Objectives: The students learn that…

 

Part 1

      1.    Objects that interact with magnets commonly have iron in them (or nickel or cobalt).  Objects that interact with electrically charged objects can be made of almost any material.  

 

      1.    To produce a magnetized object, rub a material composed of iron (or nickel or cobalt) with a magnet.  To produce an electrically charged object, rub it with wool (or some other material).

 

      2.    A magnet always attracts non-magnet objects that are composed of iron (or nickel or cobalt).  An electrically charged object always attracts objects that are of neutral charge. 

 

      3.    Electrically charged objects have the same type of interaction (attraction or repulsion) with all parts of another electrically charged object. 

 

      4.    Opposite ends of a magnet seem to exhibit the opposite properties (attraction or repulsion) when interacting with one end of another magnet. 

 

 

      5.    Magnets and objects that can be magnetized tend to retain their magnetic properties for a long period of time.   Water does not seem to affect magnetic effects.  Electrically charged objects their static electric properties readily. Water tends to eliminate or reduce static electric effects.

 

      6.    The strength of an electric or magnetic effect decreases with distance from the object.

 

Part 2

      7.    There are two types of entities involved in static electricity. By convention, we can refer to these as positive charge and negative charge. Objects that have an excess of negative charges are negatively charged.  Objects that have equal amounts of positive and negative charges are said to be neutral.

 

      8.    Electrical conductors can transfer effects from one end to the other; insulators cannot.  If one end of a conductor is rubbed with a charged object, the entire conductor is affected.  If one end of an insulator is rubbed with a charged object, only that part is affected; the rest of the insulator is not affected.

 

      9.    When two different insulators are rubbed together, charge from one insulator is transferred to the other.  

 

     10.   Excess electrical charge is located on the rubbed surface of charged objects.

 

     11.   If you touch a charged insulator, it can lose its excess charge at the point of contact only; hence you need to touch a charged insulator all over the rubbed surface to remove all its excess charge.  If you touch a charged conductor anywhere, it loses all its excess charge.

 

Part 3

     12.   Objects with opposite types of excess electric charge will attract each other; objects with the same type of excess electric charge will repel each other. 

 

     13.   When two neutral insulators are rubbed together, the amount of excess charge accumulated on the surface of one object is equal in amount, but opposite in type, to the excess charge appearing on the surface of the other object.  The greater the amount of rubbing the greater the amount of excess charges that is produced (up to a limit).  The greater the amount of excess charges on the surfaces of two objects, the greater the strength of the electrical force between them. 

 

     14.   When a conductor is rubbed with a charged object, some of the excess charge on the object is transferred to the conductor, and the conductor acquires the same type of excess charge as the object.

 

     15.   When a charged object is brought near a neutral or charged conductor, negative charges within the conductor move toward (if the first object is positively charged) or away from (if the first object is negatively charged) the first charged object.  If the first charged object is negatively charged, then negative charges within the conductor are repelled as far away from the first object as possible leaving the end of the conductor nearest the object positively charged and the far end of the conductor negatively charged.  Thus, charges are separated (or polarized) within a neutral conductor.

 

     16.   If two conductors are in contact, electric charge may move from one conductor to the other.

 

     17.   The human body is a conductor of electric charge.  When a person touches a charged conductor, sufficient charge is transferred between the person and conductor to neutralize the conductor.  This process is called "grounding."

 

     18.   When a charged object is held near a neutral conductor, and the conductor is grounded (touched by a person), sufficient charge is transferred between the person and the conductor.  If we assume that only negative charges are mobile, then conductor either loses charge to the person (if the object is negatively charged) or gains charge from the person (if the object is positively charged).  The conductor then retains either an excess of positive or negative charges.  This process is also referred to as "charging by induction."

 

Resources:

·         www.physicsclassroom.com/mmedia/index.html has a multimedia Static Electricity section