Find out how the current through a filament lamp varies with the potential difference across it.

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Investigation into the resistance of a filament lamp

Physics A 3883

Planning

Aim

 For this investigation I have been asked to find out how the current through a filament lamp varies with the potential difference across it and hence determine how the resistance of the filament and the power supplied to it vary with the potential difference across it. I will also make estimates of the temperature of the filament for different potential differences.

Key factors

Independent variables:

        

  • Current (Current will be measured with a digital ammeter/multi meter)
  • Temperature (Thermometer to measure surrounding temperature)

Dependent variable

  • Resistance
  • Voltage (Will be measured with a digital voltmeter)

        

Controlled variable  

  • Filament lamp

The range of readings that I am going to take will be from 0 volts to 10 volts this is because it will give me a good set of data to work with.

 I will increase the voltage by one volt each time.

To make sure I get good accurate fair results I will repeat the process at least 3 times.

When I do repeat the process I will make sure that I leave all the equipment as it is and not replace bits or add or remove components.

To make sure that I don’t have to replace any components, I will before I start the test make sure that all my equipment is working correctly and properly calibrated to the range of readings that I will take in the test. This will include testing the ammeter, voltmeter, and power supply and filament lamp.

Prediction

“I predict as the potential difference is increased the current will increase and so will the resistance and the temperature”

This is because at low temperatures the electrons in the metal of the filament lamp are flowing relatively freely (see diagram) this is because the ions are not vibrating very much so the electrons can flow easily past them without getting diverted.

        

        

                                           

But as the potential difference is increased the current also increases because there is and increased flow of electrons this causes the filament in the lamp to heat up, this is where the ions vibrate rapidly and cause the electrons to be diverted this makes the electrons lose energy due to heat and light.

Also the filament of the lamp may be impure and have ions that obstruct the flow of the electrons causing them to take another route round the ion.

Preliminary work

For my preliminary work I carried out my planned procedure to see if there had to be any alterations made to the method. I carried out the test and I changed the analogue ammeter to a digital one because it was very had to read to get a good reading on the needle, also the analogue meter was not 100% accurate. My following results (see table) have confirmed that resistance increases when the voltage and current increase.

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Diagram

                         Power pack        

                

Ammeter

        

        

Filament lamp

Voltmeter

Method

  • Set up the equipment as shown above making sure to connect up the voltmeter in parallel and the ammeter in series.
  • Just before you are about to start take the surrounding temperature of the lab around the experiment area, you will use this later in an equation to find the temperature.
  • Plug in the power supply and turn the voltage up until it reads 0.1 volts on the voltmeter, ...

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