Investigation into the resistance of a filament lamp

As the temperature of the filament changes the resistance also changes.

Problem Identified

I have decided to investigate into the resistance of a filament lamp. I will measure the current and potential difference of the filament lamp at various points, after measuring current and potential difference I will draw a graph to show the relationship between current and potential difference.

By finding 1 over gradient of the I/V characteristic graph we can work out the resistance of the filament lamp. When we find the resistance of the filament lamp we can compare resistance of the filament and the power supplied to it with the potential difference. From that we can get an estimate of the temperature for different potential difference.

Prediction

I predict that by increasing the resistance we will also increase the temperature of the filament lamp. Also the filament is not an ohmic conductor.

This is because we know that at high temperature the ions have more kinetic energy then they would have if the temperature was low, so therefore the ions vibrate and obstruct the electrons that increase the resistance.  

Key Variables

Dependant

I will be estimating the temperature of the filament lamp for different potential difference. The units for temperature will be Kelvins, for voltage it would be Voltage.

Independent

• Voltage
• Temperature
• Resistance
• Power
• Current
• Voltage

Controllable Variables

• Length of the filament
• Cross-section area in the filament
• Current of the power supply
• Material type of the filament

Safety Precautions

• Electricity is extremely dangerous so you must be extremely careful - make sure power supply is off before you begin connecting.
• Make sure current is low or not too high, if set too high the filament lamp could explode.
• Make sure you don’t run in the lab at all times.
• Make sure long hair is tied back
• Jewellery must be removed
• PD is increased the more current that flows through the circuit, because of this the user must take care when handling all equipment as it can get very hot after a short period of use.

Fair Test

To increase accuracy and make the test fair, results must be carried out at least 3 times and the mean result of these 3 should be taken as the final reading. To reduce anomalous results the experiment will be repeated as many times as necessary.

To make the experiment fair, we need to control the variables as well as making sure the same equipment is used all the time.

 

Apparatus

• Ammeter
• Voltmeter
• Variable Resistor
• Filament Lamp (12V)
• Thermometer
• Power Supply (DC 12V)

Method

1. Connect the wires to the DC Supply.
2. Set the power supply to 12V.
3. Make sure the power supply is off.
4. Now connect the ammeter to the power supply by the connecting wires.
5. Connect the ammeter to the filament bulb by the connecting wires.
6. Now connect the filament lamp to the variable resistor.
7. Connect the voltmeter in parallel to the filament lamp.
8. Once your circuit has been checked switch on the power supply.
9. Check if the circuit works, and the ammeter and voltmeter are giving you write values. If the values are negative then you have connected ammeter and voltmeter wrong way.
10. You can change the value of the voltage across the lamp by changing the resistance of the variable resistor; this will also change the value of the current.
11. Start with a low value for the voltage (0.5volts) and measure the corresponding value of the current.
12. Read the values for current and voltage and record them on a table
13. Once all the required values had been gained, the three separate results for each voltage were added together and divided by three, in order to find the average current
14. These values were then plotted into a I/V characteristic graph

Results: