To discover whether a filament lamp is an ohmic conductor.

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Camilla Grundy        Physics coursework        29/2/03

Electrical Properties of a filament lamp

Aim- to discover whether a filament lamp is an ohmic conductor.

Apparatus- to follow through this experiment I will use the following equipment:

  • A 12 Volt power supply
  • The ammeter setting on one digital multi-meter
  • The voltmeter setting on another digital multi-meter
  • A variable resistor used to vary the voltage from roughly 2V to 11V.  This will be finalised from my preliminary experiment.
  • A tungsten filament light bulb
  • Wires

Method- below is the method I will use to discover whether the filament lamp follows ohms law:

  • Construct the below circuit, one digital multi-meter set to voltmeter in parallel, and one digital multi-meter set to ammeter in series.

  • Move the slider on the variable resistor up in stages to vary the voltage reading from 2V to 11V
  • Take note of the ammeter reading in a table.  Repeat this at least two times for accuracy and check that the readings are similar.   Take an average of the readings and plot a graph with voltage on the y-axis and current on the x-axis using a line of best fit.
  • Where the graph bends take further readings to discover exactly what happens in that place on the graph.  I will use my preliminary experiment to find where these readings should be taken.

I will leave the bulb to get hot, and not allow it to cool between readings, as I would like to investigate the curve in the graph.

Variables- what I will keep constant to ensure a fair test, what I will measure, and what will vary as the measurement changes:

I need to keep the light bulb constant because another light bulb may have a different wattage or filament, causing the resistance to change.

I would need to keep the room temperature constant so the filament wouldn’t heat, and so that any curve in the graph can only be created by the bulb heating, not the room.

Fair test- factors to ensure my test is fair:

  • Drawing a line of best fit.
  • Using the same equipment each time I take the test
  • Repeating the test to remove any outliers
  • Take the test in the same room, under the same conditions each time it is repeated.
  • Using digital multi-meters for accuracy to measure to two decimal points.

Scientific background, explanation and prediction

Ohm’s law states that for some conductors, the current flowing is proportional to the voltage as long as the temperature remains constant.  This would give a graph like this:

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However, for our experiment I would predict a graph like this:

This is because I think the resistance of the light bulb will not remain constant, but will increase due to an increase in temperature.  As the temperature rises the filament in the bulb will begin to heat and the tungsten atom’s vibration will get larger.  This makes it harder for the electrons in the circuit to pass through the filament.  The gradient for the graph ...

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