ELECTRIC   MEASUREMENTS Learning   Objective:   Determine   voltage,   amperage, and  resistance.  Explain  Ohm’s  law  and  describe  the types of electrical circuits used in vehicles. Electricity   is   measured   in   two   ways—by   the amount  of  current  (number  of  electrons)  flowing  and by  the  push,  or  pressure,  that  causes  current  to  flow. The  push,  or  pressure,  is  caused  by  actions  of  the electrons.  They  repel  each  other.  When  electrons  are concentrated in one place, their negative charges push against  each  other.  If  a  path  is  provided  for  the electrons,  they  will  flow  away  from  the  area  where they are concentrated. The pressure to make them move is called voltage. If  there  are  many  electrons  concentrated  in  one  spot, we  say  that  there  is  high  voltage.  With  high  voltage, many  electrons  will  flow,  provided  there  is  a  path  or conductor  through  which  they  can  flow.  The  more electrons  that  flow,  the  greater  the  electric  current. Electric current is measured in amperes. Resistance is the   movement   of   electrons   through   a   substance. Resistance is a fact of life in electric circuits. We want resistance  in  some  circuits  so  that  too  much  current (too many electrons) will not flow. In other circuits, we want as little resistance as possible so that high current can flow. There  is  a  definite  relation  between  current (electron   flow),   voltage   (current   pressure),   and resistance.   As   the   electric   pressure   goes   up,   more electrons  flow.  Increasing  the  voltage  increases  the amperes ofcurrent. However, increasing the resistance decreases  the  amount  of  current  that  flows.  These relationships can be summed up in a statement known as Ohm's law. VOLTAGE Electrons  are  caused  to  flow  by  a  difference  in electron  balance  in  a  circuit;  that  is,  when  there  are more electrons in one part of a circuit than in the other, the  electrons  move  from  the  area  where  they  are concentrated  to  the  area  they  are  lacking.  This difference in electron concentration is called potential difference, or voltage. The higher the voltage goes, the greater  the  electron  imbalance  becomes.  The  greater this  electron  imbalance,  the  harder  the  push  on  the electrons (more electrons repelling each other) and the greater  the  current  of  electrons  in  the  circuit.  When there  are  many  electrons  concentrated  at  the  negative terminal  of  a  generator  (with  a  corresponding  lack  of electrons  at  the  positive  terminal),  there  is  a  much stronger   repelling   force   on   the   electrons;   con- sequently,  many  more  electrons  are  moving  in  the wire. This is exactly the same as saying that the higher the voltage, the more the electric current will flow in a circuit,  all  other  things,  such  as  resistance,  being  equal. AMPERAGE Current  flow,  or  electron  flow,  is  measured  in amperes. While we normally consider that one ampere is a rather  small  current  of  electricity  (approximately  what  a 100-watt   light   bulb   would   draw),   it   is   actually   a tremendous  flow  of  electrons.  More  than  6  billion electrons a second are required to make up one ampere. RESISTANCE A  copper  wire  conducts  electricity  with  relative ease;  however,  it  offers  resistance  to  electron  flow. This resistance is caused by the energy required to free the  outer  shell  of  electrons  and  the  collision  between the  atoms  of  the  conductor  and  the  free  electrons.  It takes   electromotive   force   (emf)   or   voltage   to overcome the resistance met by the flowing electrons. The basic unit of resistance is the ohm. The resistance of   a   conductor   varies   with   its   length,   diameter, composition, and temperature. A long wire offers more resistance than a short wire of the same diameter; this is due  to  the  electrons  having  farther  to  travel.  Some materials can lose electrons more readily than others. Copper   loses   electrons   easily,   so   there   are   always many free electrons in a copper wire. Other materials, such  as  iron,  do  not  lose  their  electrons  as  easily,  so there are fewer free electrons in an iron wire. However, fewer electrons can push through an iron wire; that is, the iron wire has more resistance than the copper wire. A  wire  with  a  small  diameter  offers  more  resistance than a wire with a large diameter. In the small diameter wire,  there  are  fewer  free  electrons,  and  thus  fewer electrons   can   push   through.   Most   metals   show   an increase in resistance with an increase in temperature, while  most  nonmetals  show  a  decrease  in  resistance with an increase in temperature. OHM’S LAW Ohm’s law is used to figure out the current (I), the voltage (E), and the resistance (R) in a circuit. This law states  that  voltage  is  equal  to  amperage  times  ohms. Or,  it  can  be  stated  as  the  mathematical  formula: E  =  I  x  R.  For  the  purpose  of  solving  problems,  the Ohm’s law formula can be expressed in three ways: 1. To find voltage: E = IR 1-9