University of Virginia
Physics Department

## The Energy Content of Fuels

A Physical Science Activity

### Student Activity

Materials

• CBL System
• TI-83 graphing calculator
• Temperature probe
• Tin can, approximately 300 mL
• Stand and 2 clamps
• Stopper with slit
• Stirring rod
• Cold water
• Electronic scale (the accuracy should be at least 0.1 g)
• Two types of fuel:
1. A regular candle
2. Isopropyl alcohol in a 30 mL beaker with a wick (If you are using 70% isopropyl (rubbing alcohol), you need to make a correction in the data found to find the pure isopropyl energy content. This can be done by dividing the value found by 0.7)

Procedure

1.   Put on your safety glasses. NOTE: Goggles must be worn throughout this laboratory.
2. Plug in the temperature probe to the CBL Unit and connect the CBL to calculator with link cable.
3. Run the Vernier Program PHYSICS on the calculator.
4. Select SET UP PROBES from menu.
5. Select 1 PROBE.
6. Select TEMPERATURE PROBE.
7. Select USE STORED from calibration menu.
8. To set up the calculator for data collection:
• Change the data collection speed by selecting COLLECT DATA from the main menu, then select TIME GRAPH from the data collection menu.
• Enter 5 as the time between samples, in seconds
• Enter 95 as the number of samples
• Press enter, then select USE TIME SETUP to continue.
• Select LIVE DISPLAY
• Enter 0 as the minimum temperature (Ymin)
• Enter 60 as the maximum temperature (Ymax)
• Enter 5 as the temperature increment (Yscl). Hit enter and you are now ready to log temperature.
9. Arrange the setup as shown in the figure above. Attach the tin can to the stand so that the fuel can rest underneath it.
10. Mass the tin can alone. Use the graduated cylinder to measure out the amount of water to put in the tin can (100 mL for the candle and 200 mL for the alcohol). Pour into the can and then mass the tin can with the water inside. Enter values on the data table.
11. Place the temperature probe through the slit stopper and clamp above the tin can so that the probe falls into the water in the can but does not hit the bottom or the sides.
12. Mass the fuel and its container using the digital balance. Make sure you are very precise because the change in mass will be very small. Write down the mass in the data table. If the candle drips, be sure to scrape the dripped portions up and mass them also, because this fuel was not burned and therefore should not be part of the mass of fuel used.
13. Place the thermometer in the tin can and wait about 45 seconds. This equilibrates the thermometer to the surrounding water.
14. Start logging time by hitting enter on the calculator and then light the fuel source.
15. Continually stir the water so there are no spots that are hotter than others.
16. WHEN THE TEMPERATURE REACHES 40 °C, blow out the flame and CONTINUE TO STIR. Soon after this the temperature will start to decrease and you will have your maximum temperature reading.
17. After the calculator is done logging data, the CBL will read DONE. To determine the initial temperature of the water and the maximum temperature of the water, hit ENTER to display the graph and trace through the graph by using the left and right arrow keys. Enter the maximum and minimum water temperatures in the data sheet. arrow keys. Enter the maximum and minimum water temperatures in the data sheet.

18. Measure the mass of your fuel sample and enter it into the data sheet.
19. To repeat the measurement, hit ENTER and select YES when asked if you want to repeat the experiment. Perform steps 8 - 16 using the second fuel sample.

Data Sheet

 Trial #1 Trial #2 1. Fuel Used 2. Initial Mass of Fuel (g) 3. Final Mass of Fuel (g) 4. Dm (line 3 - line 2) 5. Mass of Empty Can (g) 6. Mass of Can + Water (g) 7. Mass of Water (6-5) (g) 8. Initial Water Temp (°C) 9. Final Water Temp (°C) 10. DT (line 9 - line 8) (°C)

Data Processing

Calculate the heat gained by the water. Qwater=DTwater * mwater * c where specific heat capacity of water is c = 4.18 J/g°C.

We now have the amount of heat energy that each fuel gave to the water. We want to know how much heat the fuel produces for each gram burned, so divide the heat increase of the water by the change in mass of the fuel during burning.

Questions

1. Which fuel has the greatest energy content?

2. What other factors must we consider when choosing to use a particular fuel source?