🚀 go-pugleaf

In article <02bc4421$13618920$31b815a5@sni>,
Shu ShinLa <shinla@mbox2.singnet.com.sg> wrote:

[Unnecessary quoted text deleted by sci.physics.research moderator]

>I have a problem understanding what the teacher is explaining when it
>comes to electric current.  He defines p.d as the energy converted from
>electrical to other forms when 1 coloumb of charge passes between two
>circuit, 
>
>volt as the potential difference between 2 points in a conductor such that
>1 joule of work done in taking 1 coloumb of charge from in point to other,
>
>and the formula V=W/Q,where V is p.d, W as energy converted from
>electrical to other forms, Q as the charge.
>
>I REALLY DON"T UNDERSTAND HIM!!!! 

Try it this way.  Potential difference is just like gravitational
potential energy, electric field is just like gravitational force.  Look
at a ball sitting on the floor.  There's a force of mg pulling it down,
the mass of the ball times gravity.  Lift the ball up to head-height.
There's a force of mg pulling it down.  If you let the ball go, it will
fall down to the floor.

What is the potential energy of the ball on the floor?  Well,
gravitational potential energy is sort of an arbitrary thing because you
can decide where "zero" is.  That's because you only ever use potential
energy when you're talking about a change in the height.  When the ball is
on the floor it has potential energy equals zero, because we says it does.
When you lift it up, it gains the energy mgh, where h is the height you
lifted, because that's how much work you did on the ball to lift it.
(Force times distance equals the change in potential energy.)  We could
have said the energy of the ball on the floor equals 10 (units), then when
you lift it, the ball will have mgh+10.  But the change in potential
energy, or the difference, is still mgh.  So it doesn't matter where you
start, just how far you go.  When you drop the ball, it will fall down
because that's where it will have lower potential energy.

Go back to electricity.  The electric field is the force that moves the
electrons, it's pretty constant throughout the circuit.  And it's not just
an analogy.  The electric field really does exert a force F=Eq on an
electron, where E is the strength of the field and q is the charge of the
electron.  And if you move an electron a distance h against the force, it
will gain a potential energy of V=Eqh relative to the position where it
started.  If you "drop" the electron, it will "fall" to the point of lower
potential energy, because that is the direction the force is pushing it.

We can define the zero potential anywhere we want.  By convention we say
the ground has zero potential energy, or is at zero volts.  But we don't
have to.  We could give the ground whatever value we want, because the
only thing that matters is the potential difference, or the force times
the distance the electron moves.

So we could say the positive terminal is at +20 volts relative to ground.
If we plug two resistors into the circuit we could say the middle, where
the two are joined, is at +10 volts relative to ground, or -10 volts
relative to the positive terminal.

The other way of looking at it, which turns out to be really the same
thing, is that voltage is pressure.  Say the wires are pipes, and the
power supply is a fan.  Does the fluid flow if the end is capped?  Nope,
because the pressure at the end is the same as the pressure at the pump.
It is not pressure that makes air flow, but a pressure difference.  One
side at high pressure, one side at low pressure.  Unplug the pipe and the
pump end might be at 4 atmospheres while the end is at 1 atmosphere.
That's a pressure difference of 3 atmospheres, which means air is going to
move really fast.

Pressure difference, potential difference.  Get it?

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