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The aim of this experiment is to investigate the relationship between the current, voltage and resistance through the use of a fixed resistor and a filament lamp.

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Abdul Mufti                                               Centre Number: 13329                                 Candidate Number: 4138  

10. F


The aim of this experiment is to investigate the relationship between the current, voltage and resistance through the use of a fixed resistor and a filament lamp.


Based on knowledge of Ohm’s law it can be hypothesised that when increasing voltage and current is passed through a filament lamp the resistance would increase in a non-linear fashion, such that a graph similar to the one given below would be obtained (figure 1). This non-linear graph would be expected due to temperature increases in the filament lamp.

It can also be hypothesised that when current is passed through a fixed resistor the relationship between V and I would be expected to be linear such that a straight line through the origin would be obtained (figure 2). In addition the readings on the ammeter and voltmeter would both change accordingly as expected.


The shape of a fixed resistor current-voltage graph (I-V graph) is explained in figure 3 since the three variables are related through Ohm’s law.


Circuit Diagrams





Fixed resistor &

Filament Lamp         to impede and obstruct current flowing through circuit

Ammeter                 to measure current flowing through the circuit

Voltmeter–         to measure the voltage present in the circuit and to make sure the power supply is correctly calibrated.

Power Supply–         to act as the adjustable power source for the circuit

Wiresto connect the circuit components.

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In order to apply Ohm’s law we need to know two of the 3 variables. In this experiment we will know the voltage and current. Therefore by rearranging Ohm’s law we can calculate the resistance.

  • A steady increase in resistance, in a circuit with constant voltage, produces a progressively (not a straight-line if graphed) weaker current. 
  • A steady increase in voltage, in a circuit with constant resistance, produces a constant linear rise in current.

In this case, Ohm’s law is needed to calculate the resistance of the fixed resistor and the change in resistance as the filament lamp gets hot. For each case gradient from the appropriate graphs will be used. Resistance can be varied by using a variable resistor, by altering the gauge or by length of the wire or by changing the temperature. If a longer wire is being used then the resistance increases as the electrons have to travel further than in a short wire. If a thicker wire is being used then the electrons have more space to move and therefore resistance is decreased. If a thinner wire is used then the resistance will increase as the electrons cannot get around the circuit easily.


The work surface was insured to be dry and clean. Bags, stools and other possible obstacles were removed from the work area. The equipment was collected according to the list given and verified by close inspection to be clean, non-corroded and undamaged.

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It would have been practical in the interest of conducting a fair test to use averages for series of current readings. This could be done using two different sets of equipment, conducting the experiment on each set and averaging the values. This could help us reduce error margin in any anomalies found. It may have been interesting to investigate the same aim with a wider range and more sensitive set of equipment. Smaller graduations of voltage on the PSU would have allowed us to plot the graphs with more accuracy. If possible it would have been interesting to use a diode. However given the amount of time we had, it would not have been possible to further complicate the experimental design.

We may also have adjusted variables such as the gauge, length and type of wire used to investigate the effect these factors have on ohms law. It may also have been of interest to us if we investigated how adjusting the circuit diagram would have affected our results. However this may have been a little advanced for our level at present.


GCSE double Science Physics – CGP – ISBN: 1-84146-401-5

New Modular Science for GCSE, (Vol 1), - Heinemann (1996) – ISBN: 0-43557-196-6

AQA GCSE Science syllabus (2001).

...read more.

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