• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month
Page
  1. 1
    1
  2. 2
    2
  3. 3
    3
  4. 4
    4
  5. 5
    5
  6. 6
    6
  7. 7
    7
  8. 8
    8
  9. 9
    9
  10. 10
    10
  11. 11
    11
  12. 12
    12
  13. 13
    13
  14. 14
    14
  15. 15
    15

Resistivity of Lightbulbs

Extracts from this document...

Introduction

image00.png

Lower 6th Quality of Measurement:

25 January 2009

image01.png

Introduction:

In this experiment I am aiming to investigate how different bulbs emit different light intensities depending on the potential difference applied to them. I will create my own sensor using a potential divider circuit using a Light Dependent Resistor (LDR) along with a lux-meter to calibrate it and then collect readings.

What is a Potential Divider Circuit?

Potential Dividers are part of many sensors. They consist of a voltage source along with some resistors in series. The potential of the voltage source[1], for example a power supply, is divided in proportion to the ratio of resistances. This means that you can choose the resistance to get the voltage you want across one image02.pngof them. In the case of this experiment, the resistance across the LDR[2] varies with light intensity thus causing the potential difference across it to vary. The other resistor in series is a fixed resistor[3] which is there to compare the resistance of the LDR thus completing the ratio. The Voltmeter[4] records the potential difference across the fixed resistor thus giving the sensor a value. As the light levels increase, the resistance of the LDR falls and thus the potential difference across the LDR falls. In turn this increases the potential difference across the fixed resistor which is measured by the voltmeter. Increasing light levels increases output voltage and this is the light sensor. The Diagram below shows this:

Choosing the correct Fixed Resistor:

...read more.

Middle

image07.pngy regardless of whether the ambient lights were on or off. This meant that there would be no need to tape the box to the workbench.

Second Calibration:

The second method again consisted of three main circuits:

  • The potential divider circuit
  • The bulb and the VARIAC
  • The Xlogger Lux Meter connected to the laptop

They were arranged in the following setup:

This improved method incorporates the new Xlogger lux meter which is integrated in the top of the cardboard box, right next to the LDR. This means that they will be both recording the same light intensity thimage04.pngus resulting in the most accurate calibration possible. The gaps in the cardboard box have been sealed and the bulb stand is also secured to the desk to prevent movement and a bias towards either sensor. To ensure a fair test, the same VARIAC was used for all experiments as actual figure of voltage through the light bulb may depend on the accuracy of the VARIAC. Of course this is not a problem so long as the same apparatus is used each time.

The above graph shows a much more regular set of results when compared with the first calibration. This is excellent as it ensures that when the real experiment is done, the light intensity figures collected by the LDR will be as close as possible to the true values. If the graph was more curved, then any values recorded at the lower light intensity (voltages)

...read more.

Conclusion

As explained at the beginning a potential divider circuit works by using a variable resistor (the centre of the sensor) in conjunction with a fixed resistor along with a power supply. By measuring the potential difference across the fixed resistor, the LDR, a sensor reading can be recorded. The LDR achieves this by being a semi-conductor, that is to say that it is a material with a conduction rating between that of a conductor an an insulator. The atoms inside the LDR have bound electrons. These electrons require different amount of energy to escape from the atom and pass a current through the semiconductor. Light, which is made up of packets of photons, determines how many of these electrons are free to flow and pass a current. Therefore the higher the light intensity, the lower the resistance of the LDR and thus the lower the potential difference across the LDR.

Limitations to the method:

The method can be used to measure only a certain range of light intensities for which a) the LDR is calibrated for and b) the range that the LDR can accurately interpret. This is due to the nature of the calibration curve: there is only a specific range range of input potential differences for which the LDR can operate when used with a specific fixed resistor. This is why the experiment only measured light intensity between 80 and 140 volts because outside of that spectrum, the ratio between the 250Ω fixed resistor and the LDR is too small or too great to give an accurate reading. When presented on a graph it is at the extremities: when the gradient is close to 0 and where the gradient is close to 1.

...read more.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Electrical & Thermal Physics essays

  1. Peer reviewed

    Measurement of the resistivity of Nichrome

    5 star(s)

    Add a straight best fit line across those points. Graph plotting Error bar To find the error bar on the y-axis, I need to calculate the percentage uncertainty of resistance. But how to work out that? From Table A, I could find that the wire is not uniform.

  2. Measuring The Resistivity Of A Pencil Lead.

    error of ?3.5% * In calculating the averages with a percentage error of ?3.5% Anomalies My main anomaly is at length 145mm as the pencil lead snapped not allowing me to take any readings for that length. My other set of anomalies is at 125mm-135mm.

  1. Physics - Resistivity

    The atom remains as a positively charge ion. Since there are equal numbers of free electrons -ve and ions +ve, the metal has no overall charge and it is neutral. When a power source is connected to the wire and switched on, voltage is produced which provides a push to make the conduction electrons flow around the circuit.

  2. I am going to investigate what the resistivity is of a pencil lead. ...

    I will try to make it as accurate as possible by using an electric ammeter and a voltmeter where the intervals are 0.1V. Preliminary Experiment I will be carrying out a preliminary experiment so that I know exactly what I am doing and can eliminate any problems that I may come across.

  1. Characteristics of Ohmic and Non Ohmic Conductors.

    The thermistor and the LDR have not been used so the differences involving them cannot be justified. Plan Prior Test Length of Nichrome wire 60 cm 80 cm Thickness of wire 0.5 mm 0.45 mm D.C Supply Batteries/Cells Besides these the rest of the apparatus remains the same.

  2. To Investigate How the Resistance of the Light Dependent Resistor Depends On the Current ...

    So the resistance of the LDR increase four times. Fair test: But there are some variables, which can also affect the brightness of the light bulb: 1. As the reasons show above, the brightness of the light bulb plays the main role in affecting the resistance of the LDR. 2. The distance between the light bulb and the LDR.

  1. Investigating The Characteristics Of A Light Bulb.

    graph is a straight line, which means that the temperature has not increased. My results for the 12-volt bulb were good enough to support my prediction and for this reason I think my investigation was worth carrying out. As you can see from these results, that the current and voltage increases.

  2. Making, Calibrating and Testing a Sensor

    However because of the use of electricity there are always risks. The major one is at the plug because the voltage coming out of them is 240V therefore you should avoid contact with any live points. Then within the experiment mostly the voltage will be safe, however it could accidentally

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work