[cliverlong]

Hi,

(More of a physics question, really)

   I was trying to explain the "meaning" of voltage, current and resistance without reference to Ohm's law. In fact "derive" Ohm's law as a "sensible" conclusion.

   I took the only route I could think of and tried to explain voltage in terms of height of a drop or pressure of a fluid.

   Current was the "amount" of electrons flowing.

   And resistance was "ability" to stop electrons flowing.

   I got in a bit of a mess to be honest but I got as far as convincing that if is R constant, as V increase, so does I

   and

   If V constant then as R varies, I varies inversely.

   I sort of suggested that V and R were "fundamental" and I was a "dependent" quantity.

   Then I couldn't get from this to Ohm's law even though I could write out the proportionality expressions.

   So I just stated the traditional V, I , R triangle and worked "back" from that to the behaviour we had discussed earlier.

   Then we went on to batteries in series and parallel and resistors in series and parallel.

   But I feel my explanations were all haphazard and served to confuse rather than explain.

   So ... is there a good, basic, explanation on the Web of the concepts of voltage, resistance and current that don't assume Ohm's law (or the V, I, R) triangle but arrive at Ohm's law as a deduction?

   Please don't be pedantic if I have incorrectly assumed Ohm's law and V, I, R as the same concept. Please answer the "Spitir" of the question so I can explain the relationship (at a high school level) more clearly.


Thanks

Clive

[Astrokel]

Hmm, try to think in a chemistry's point of view,

A metal has a lattice of positive ions and is surrounded by a 'sea' of electrons. When these electrons drift along the lattice structure, they constantly collide with the lattice ions, thus slowing down their movement. It is these collisions that give rise to resistance in a metal, and not really the 'ability' to stop electrons flowing.

Current can be think as movement of charged particles. When there is movement of charges, there is a flow of current. Electric current is thus, the measure of the rate of flow of electrons.

Work is needed to move a unit charge from one point to the other. The difference in potential energy of the two points due to work being done on a unit charge is known as voltage.

Can you try to come out with the relationship for ohm's law based on this definitions?

[DevaDevil]

Warning: non-chemical approach! Only for explanation purposes

When I was in high school and first started learning electric circuits and such (15 or so years ago) we had an awesome teacher who had his eccentric way of making kids understand about electric circuits. As mentioned; it isn't a proper scientific explanation, but it was humorous, and easy to follow for kids who were flabbergasted about physics:

A battery is a Military camp, in it are soldiers (current) that want to move from one end to the other end of camp (the potential difference between the poles is the drive). Each soldier takes with him a backpack with provisions (the Potential Energy, Voltage). The orders are simple: go to the other end of camp following the road (the circuit) and eat ALL of your provisions on the way. And assumed is that the soldiers can plan this in advance.

If nothing is on their way they will gradually consume their provisions evenly distributed along the way (if the poles are connected with a wire the voltage drop is linearly along the wire)
If there are obstacles, however, they will eat more at the obstacle (resistance; in other words at a resistance there will be a potential drop). In fact, most obstacles are so much more difficult to travel than the paved road (the wire), that they make the provisions eaten along the road  negligible.

If there are many obstacles in a row, the provisions will be consumed along them proportionally to the difficulty of the obstacle (bigger resistance, more potential drop - or, R proportional with V) Of course ALL the soldiers have to go along all obstacles in series (current in consecutive resistances in series is constant)

Trickier: If the soldiers come across a split in the road (parallel resistances) they will march in separate columns. (current will differ along the separate parallel paths) As they need to have the same provisions after they rejoin, the provisions eaten along any of the parallel paths is the same (potential drop along the parallel resistances is constant for all the parallel paths - no matter the resistances' magnitudes)
The intelligence of the soldiers comes into play here: of course more will take the easy path than the hard one. (current will be higher along the parallel resistance that is least in magnitude; more resistance less current, or I proportional to 1/R in these parallel resistances)
After rejoining the amount of soldiers is exactly the same as they had before splitting up of course.

So from this very childish story the ohmic law is derivable:
Series: R proportional with V; or V = C₁ * R, with C₁ a constant in there
Parallel: I proportional to 1/R; or I * R = C₂, with C₂ constant in there

As along serial resistances current is constant and along parallel resistances Potential is constant, this gives the ohmic law V = I * R




Now, I do NOT recommend using this in ANY scientific situation. It is a nice thought however, and can be used to try and help people think in less abstract terms as current, potential and Resistance.

The more proper (abstract) scientific approach is given by Astrokel. This is just a small gimmick that may help

[cliverlong]

Great stuff guys.

I will work on this.

I think the soldiers will work for a 15 year old !


Clive