Voltage Reverses, Yet
Current Doesn't? Wha?!
W. Beaty 1999

On Tue, 30 Mar 1999, Wes M. wrote:

> How can the Voltage polarity reverse, and the flow of electricity 
> (talkin' about electric charge here)  not change direction?  
> This statement was made in the article entitled "Electricity is not
> Electrical Energy".  If anyone can answer my question, I'd be eternally
> greatful.  Thanks! 

Hi Wes.  Glad you like my "stuff!"

  [Lurkers, see http://amasci.com/miscon/energ1.html]

The direction of electric charge-flow vs. the direction of energy flow... 
it's easy to mix up these two.  I suspect that this is a major reason why 
people wrongly believe that electric current is flow of energy.

In most electric circuits, whenever we want to reverse the direction of 
the flowing energy, we must reverse the direction of the electric current. 
For example, to "recharge" a battery, we force the charge to reverse its 
direction of flow.  Doesn't this prove that electric current is a flow of 
energy?  No, because the flow of energy is not the current, instead it's 
the current MULTIPLIED BY voltage.  To reverse the energy flow, we can 
reverse the current, but we can also reverse the voltage.  Maintaining the 
current while reversing of the voltage is a common trick in electronic 
equipment, such as...

  - when coils are driven by capacitors

  - when coils dump energy into resistors

  - and when standing waves appear on radio transmitter cables
    (known as 'SWR' with CB and ham radios.)

As you suspected, this effect does NOT appear when batteries drive purely 
resistive loads such as light bulbs and electric heaters.  In that case 
the polarity of the voltage directly controls the direction of the charge 
flow.  Therefore the battery voltage and charge flow must change 
polarity together, leaving the energy flowing in the same direction during 
the change.  For example, when a light bulb is plugged into the AC lines, 
the alternating voltage and charge-flow both reverse together, and so the 
energy keeps going forward and does not vibrate back and forth.  The 
energy always flows FROM the generators and INTO the light bulbs, even 
though the charge carriers in the metal are wiggling back and forth.

Here's one situation where energy flow is revered by the change of VOLTAGE 
polarity, rather than by changing the CURRENT direction.  Suppose we have 
an enormous coil of wire wrapped around an iron core.  We can store energy 
in this coil by temporarily connecting it to a battery.  At the moment we 
connect it, electric charges in the coil begin to slowly flow, and a 
magnetic field forms around the coil.  As time goes on, the current builds 
up faster and and faster, and the magnetic field grows in strength and 
energy.  If we multiply the voltage across the battery by the current in 
the circuit, we get watts, and the polarity of the wattage shows us that 
electrical energy is flowing OUT of the battery and INTO the coil.  Makes 
sense.  The electrical energy from the battery is being stored in the 
coil's magnetic field.

                       ------>     Charge-flow is circular
              |                         |
              |                         |_
            __|__  +                      --____
             ___                        ----____----
            _____                       ----____----
   BATTERY   ___                        ----____----  LARGE
            _____                       ----____----  COIL
             ___                        ----____----
              |    -                    ----____----
              |                                 --_
              |        <------                     |

Now let's reverse the direction of the energy by letting the coil power a 
light bulb. Connect a light bulb across the coil, then disconnect the 
battery.  The flow of charge continues through the coil in the same 
direction as before, and the light bulb lights up.  (Coils behave somewhat 
like flywheels: they provide a sort of "electrical inertia" which resists 
any change in the flow rate of the charge.)  When the light bulb lights 
up, the current still has the same direction as before, but THE VOLTAGE IS 
REVERSED.  The polarity of voltage across the light bulb (below) is 
opposite that of the battery above.

                     current  ------>
              |                         |
              |                         |_
            __|__  -                      --____
           /  \  \                      ----____----
  LIGHT   |   /   |                     ----____----
  BULB   |    \    |                    ----____----  LARGE
          |   /   |                     ----____----  COIL
           \__\__/                      ----____----
              |    +                    ----____----
              |                                 --_
              |    <------  current                |

Which way does the energy flow?  When the coil is lighting the bulb, if we 
multiply the current by the voltage, we discover that the direction of the 
energy flow has reversed: energy is now flowing FROM the coil and INTO the 
light bulb.  Yet the flow of charge has not reversed.  "Electricity" still 
flows the same, and only the voltage and the energy has been reversed.  
The coil is powering the light bulb, the magnetic field collapses, current 
becomes less and less, the light bulb grows dimmer, and the coil's 
magnetic field eventually disappears.


Here's another, more complicated situation:  coil/capacitor oscillations. 
When a coil drains a charged capacitor, the flow of charge in the coil
does not stop, and the coil will "charge up" the capacitor again, but with
reversed voltage.  The voltage across the capacitor reverses, and the
energy flow reverses, yet the direction of charge flow stays the same
while this is happening. 

Suppose we have a coil of wire and a capacitor.  Suppose that we "charge" 
the capacitor at the start, then we connect it to the coil.  If we measure
the voltage across the capacitor, we will find that it is going plus and
minus over and over, and the coil/capacitor circuit is "ringing"  like
some sort of electromagnetic bell.  Clear enough?  Charge up capacitor,
touch it to the coil, and the circuit goes "dingggggggg....."  This
happens because the energy in the capacitor "sloshes" into the coil, and
then the energy in the coil "sloshes" back into the capacitor, over and
over very rapidly. (Eventually the ringing dies away.)

Now for the fun part.  When we touched the capacitor to the coil, the
capacitor's voltage caused charge to begin flowing in the circuit. 
However, the coil acts as a kind of electrical flywheel, and the charge
does not flow all at once, instead the flow rate slowly builds up, and a
magnetic field balloons into existence around the coil. The voltage across
the capacitor falls and falls, and the charge flow in the circuit slowly
gets faster (more amps.)  THE CAPACITOR IS LOSING ENERGY.  Where does the
energy go?  It is stored in the coil, because a large magnetic field is
building up in the coil at the same time that the electric field between
the capacitor plates is decreasing.  The energy is moving from the
capacitor to the coil.
                     current  ------>
              |                         |
              |                         |_
              |                           --____
              |                         ----____----
 CAPACITOR ___|___ +                    ----____----
           _______                      ----____----  LARGE
              |    -                    ----____----  COIL
              |                         ----____----
              |                         ----____----
              |                                 --_
              |    <------  current                |


Finally the capacitor voltage reaches zero, and no more energy is left in
the capacitor. Yet the flowing charge in the circuit is at maximum.  All
the energy has moved into the coil (stored in its magnetic field.)  

              |                         |
              |                         |_
              |                           --____
              |                         ----____----
 CAPACITOR ___|___                      ----____----
           _______                      ----____----  LARGE
              |                         ----____----  COIL
              |                         ----____----
              |                         ----____----
              |                                 --_
              |        <------                     |


current does not change direction. 

              |                         |
              |                         |_
              |                           --____
              |                         ----____----
 CAPACITOR ___|___ -                     ----____----
           _______                      ----____----  LARGE
              |    +                    ----____----  COIL
              |                         ----____----
              |                         ----____----
              |                                 --_
              |        <------                     |


This is not a rare situation: coil/capacitor pairs are used throughout the
circuitry of radio receivers and transmitters, in TV sets, etc.  The
electric companies even use them, adding "phase correcting" capacitors to
their substations when industrial customers have too many large motor
coils as part of their factories.  It makes the electrical energy slosh
between the coils and nearby capacitors, rather than wasting energy by
heating up the long power lines that reach back to the distant generators. 

((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billbeskimo.com                                  http://amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
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