Electric thread Hi-jack
A current source is modeled (engineering wise) as containing a resistor in series with the output terminals.
So the circuit contains: perfect current source, resistor, hot or positive wire, lamp (usually the highest resistance item in the circuit), neutral or negative wire, (back to perfect current source).
If you experiment with some not so fresh batteries and flashlight lamps (bulbs) you will see that more of the lamps connected in parallel result in dimmer light from each and also if you put a voltmeter to the battery terminals you get lower voltage and when you remove some of the lamps from the circuit the others get brighter again. This can only be explained using resistance in the circuit in order to obey Ohm's Law, volts equals amperes times resistance at all times. As the resistance of the wires is usually too small to make Ohm's Law calculations come out right, we have to assume some of the resistance in the circuit is contained within the power source.
To get really analytical the wires can also be modeled as resistors to remind you that voltage drop occurs there too. With a short circuit, together with the resistance in the wires leading up to the short circuit, we still have voltage drop using Ohm's Law. Also remember that if the wires heat up as a result of the overcurrent of a short circuit, their resistance rises.
For the short circuit the "resistance within the battery" predominates; the output of a normal 1-1/2 volt battery might be measured as one quarter of a volt across its terminals with most of the voltage drop taking place in said internal resistance. Consider a flashlight battery short circuited using a piece of 14 gauge wire with 1/100 ohm resistance. Without any internal resistance, the battery would be putting out 1-1/2 volts which through 1/100 ohm means that 150 amps must flow. This does not happen, at least not for long.
The internal resistance of the battery is not constant; it varies according to the load and also according to how fresh the battery is. In actuality, hydrogen forms at the zinc shell (negative terminal) of a carbon zinc battery, the amount varies with the current draw and it is also absorbed into the manganese dioxide core. The more hydrogen produced and not yet absorbed, the higher the internal resistance. An entire engineering school semester can be devoted to studying the behavior of power sources under load.
But in broad terms the resistance in a battery in a short circuit condition may be reasonably constant over a short period of time during which the current flowing in the short circuit is also somewhat constant and can be measured.
The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.
Last edited by AllanJ; 05-28-2010 at 05:03 PM.