29-01-2017, 03:37 PM
Your high voltage supply had what is called a "high internal resistance". It could be it actually had a resistance in series with the output, or the resistance of the transformer secondary itself was too high to allow enough current to light the bulb.
On the other hand the battery had a very low internal resistance, so although the voltage was low enough current could be pushed round the circuit by the 1.5v to ensure the bulb glowed.
When the bulb was connected to the high voltage supply, if you had measured the voltage across the bulb, you could do a rough calculation to work out the internal resistance of the supply or battery by comparing it to the open circuit voltage of the supply or battery.
The internal resistance of a battery may be calculated from its open circuit voltage VNL, load voltage VFL, and the load resistance RL:
For more information on "internal resistance" see https://en.wikibooks.org/wiki/A-level_Ph...Resistance
Mike
On the other hand the battery had a very low internal resistance, so although the voltage was low enough current could be pushed round the circuit by the 1.5v to ensure the bulb glowed.
When the bulb was connected to the high voltage supply, if you had measured the voltage across the bulb, you could do a rough calculation to work out the internal resistance of the supply or battery by comparing it to the open circuit voltage of the supply or battery.
The internal resistance of a battery may be calculated from its open circuit voltage VNL, load voltage VFL, and the load resistance RL:
For more information on "internal resistance" see https://en.wikibooks.org/wiki/A-level_Ph...Resistance
Mike
