Heat Transfer in an Incandescent Lamp

by | Oct 6, 2022 | Physics, Experiments


In this experiment, we will be determining the amount of electric power & heat transfer in an incandescent lamp that is wasted by the thermal conduction from the filament.

If we could find the loss and reduce it, then we will be able to increase the efficiency of the incandescent lamp.


To find out the loss in heat in electrical energy of an incandescent lamp by the thermal conduction from the filament.


1. The lamp filament material, tungsten, has a low vapour pressure at maximum temperature.

2. The filament also has a very high melting temperature.


1. 25-watt evacuated light bulb

2. Two high precision digital meter

3. Digital thermometer

4. Notebook


Step 1: Connect a 25 watt evacuated light bulb to the power supply.

Step 2: Using two digital meters, measure the amount of current that passed through the light bulb and also check the EMF across it.

Step 3: Using a digital thermometer, measure the temperature of the light bulb.

Step 4: Via GPIB-to-USB interface, connect the four devices to the computer.

Step 5: To control the experiment, write a programme in Visual Basic.

Step 6: Bring this programme to Microsoft Excel for analysis.

Step 7: This programme contains precise current and EMF resistance in every condition.

Step 8: Calculate the resistance and lamp power for each pair of experiments.

Step 9: Calculate the filament temperature from resistance.

Step 10: Decompose the input power thermal radiation and thermal conduction components using power, ambient temperature and filament temperature.

Step 11: The experiment is possible because Fourier’s Law says that the conduction is linearly proportional to the filament temperature. And also because of Stefan-Boltzman Law that says that radiation is proportional to the fourth power of the temperature of the filament.

Step 12: Record your observations.


1. We obtained results over seven decades of lamp power between 0.002 mW and 22.7 W.

2. We observed that the lamp resistance increased almost thirteen times (45 to 567 ohms).

3. At 0.5 mW (T = 315 K) the filament temperature acquired a slope of 40 K/mW that varied linearly with input power.

4. At a temperature equal to 850 K (136 mW), the conduction and radiation conduction were equal.

5. At 136 mW (850 K), we observed a faint incandescent.

6. At the point of faint incandescent, thermal conduction accounted for 10% of the input power.

7. At 22. mW (2395K) or higher power, loss fraction of conduction was only 0.2%.


1. We found that the aspect of increasing the efficiency of electrical lamps by reducing the amount of thermal conduction in filament was not good.

2. The existing lamps used this technology extremely well.


1. Take help in measuring the current and resistance.

2. Make sure all the devices are working properly.

3. Record observation precisely.


In this experiment, we tried to find out the heat loss in an electrical bulb through the thermal conduction of filament.

Viva Questions with Answers

Q.1 What was the aim of your experiment?

ANS. We aimed to determine the heat loss in an electric bulb.

Q.2 What was the result of your experiment?

ANS. Our result suggested that at the higher power, the conduction loss fraction is very less.

Q.3 What is Fourier’s law?

ANS. It states that the time rate of heat transfer and its negative gradient is directly proportional to the gradient area through which the heat flows.

q = -k∆T

Q.4 What is Stephan-Boltzmann’s law?

ANS. This law states that the emission of total radiant heat power is directly proportional to the fourth power of the temperature.


You May Also Like To Create…


Submit a Comment

Your email address will not be published. Required fields are marked *