MadSci Network: Engineering

Re: What is meant by comlpetion of a circuit?

Date: Tue Oct 9 18:20:13 2012
Posted By: Martin Smith, Engineering, B.E., M.EngSc., Uni of Qld / airline pilot
Area of science: Engineering
ID: 1349710995.Eg

For charge to flow there must be an area of higher electrical potential
energy, an area of lower electrical potential energy and a conduction path.

So for a simple circuit of say a battery and a wire.  If we connect the
wire from the positive to the negative terminals of the battery, charge can
then flow along the wire from the area of high potential to the area of low

Remember potential is relative not absolute.  So potential is measured
between two points.

An analogy is water flowing down a hill.  If we have a bucket of water half
way up a hill it is at lower gravitational potential energy to all the
points higher up the hill.  It is at a higher gravitational potential to
points lower down the hill.  The potential difference is given by the
height between your bucket and the other place on the hill.

If we connect a hose to the bucket with the other end lower down the hill
water will flow from the area of higher gravitational potential to the area
of lower gravitational potential.  The height difference between the two
points is what is crucial and what will force the water down the hose.  Not
the height of either place say in reference to the ground.


I am not sure what you mean when asking about standing on the ground
barefoot.  If you feel a current flowing then there must be a potential
energy difference between your feet.  This can certainly happen, and is an
important thing to remember if say there is lightning about.

If lightning hits the ground near you, where it strikes will most likely be
at a greatly different potential to where you are.  Current will flow
through the ground according to that potential difference, and the
resistance of the ground.

V=IR  So the current that flows is proportional to the potential difference
V, and inversely proportional to the resistance R.  The R of the earth is
reasonably low, and the V near a lighting strike is likely to be large.
So there will be a large potential difference from where the lightning
struck to some point a long way away.  For every point in between there
will also be a potential difference, generally getting smaller the closer
the two points are together.

So if the potential difference between the point of the strike, which we
will call 0m, and a point 1000m away was say 100 000V (just a made up
number), the the potential difference between 0m and 500m might be 70 000v
(again, just a made up number but the drop off is unlikely to be linear).

Now at all points in between there will also be a potential difference.  So
the point 100m away and the point 101m away will have a potential
difference between them and current will be flowing.  If you have one foot
at 100m and the other at 101m then your feet will be at different
potentials and a current may flow from one foot through your body to the
other foot.

For this reason it is recommended if caught in a lightning storm to crouch
low with your feet close together.  This minimises the likelihood of
lighting striking you, and if lightning strikes close by  minimises the
potential difference that will be between your feet.

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