Determining the Best Requirements for a Bulb

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Determining the Best Requirements for a Bulb

A torch is powered by four dry cells placed in series. There is a

choice of bulbs that can be used. The user wants a bulb that will

provide a maximum amount of power. The aim is, by performing

appropriate experiments, determine the best requirements for the bulb.


From previously performed experiments in lessons and personal research

I have deduced that in order to obtain the maximum amount of power in

a particular circuit the internal resistance (r) has to equal the load

resistance (R). This theory can be presented on the following graph of

load resistance (x-axis) against power (y-axis).


This graph, which can be obtained from most modern textbooks, proves

that the maximum power can be achieved when the load resistance is

equal to the internal resistance (r). I can therefore use this

knowledge to develop a method by which to conduct an appropriate

experiment that will allow me to obtain the best requirements for a

particular bulb.

In an electrical circuit, power is related to current and to potential

difference. The current (I) is the number of coulombs per second

flowing past a certain point in a circuit, and the potential

difference (V) across a load is the number of joules each coulomb

transfers to the load. Therefore the rate of energy transfer in the

load, the power (P), is the product of current and voltage. This can

be written in the form of the simple equation:

P = IV

By taking this simple equation into consideration it would be possible

to set up a simple circuit in which the current and voltage could be

measured across the bulb. By altering the resistance of the circuit I

would obtain several sets of measurements of voltage and current which

would allow me, by using the equation P = IV to find the power from

the voltage and current readings, to plot a graph similar to the one


Theoretically, I would then be able to read off the peak of the graph

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