Can Energy Be Created? (First Law of Thermodynamics)
To verify the first law of thermodynamics by calculating specific heat of water
To determine light bulb efficiency
Electricity results from the movement of electrons. The charge on one electron is equal to 1.602 x 10-19
coulombs. Electrons moving along a path create an electrical current, measured in the unit of amperes
(A). The filament of a light bulb has a high resistance, which forces the electrons to slow down. As the
electrons slow down, heat is generated, and in the case of the light bulb, both heat and light are
For this experiment, an incandescent lamp will be immersed in water. Remember that the First Law of
Thermodynamics says that energy can change from one form to another, but that the total amount of
energy remains constant. So in this case all the energy given off by the incandescent lamp should be
absorbed by the water and converted into heat or light energy.
Part A – Calculating the Specific Heat of Water
In the first part of this experiment, we determine the specific heat of water.
Specific heat (c) = amount of energy to increase the temperature of a body by 1 degree, per unit mass
In the SI system it has units of J/kgC, which gives Q units of J (Joules). In this part of the experiment,
the water will be mixed with India ink so that the light energy is also absorbed by the water, producing
First, determine the total electrical energy that flows into the lamp (E), which is equal to the total heat
absorbed by the water (Q). The electrical power delivered to an incandescent light bulb can be
Power IV Eq. 1
where I is current in amps, V is voltage in volts, and the resulting power is in Watts. With this power,
how can we determine the total amount of energy added to the water, in Joules, over a certain time
The heat energy produced by the incandescent light bulb can be calculated by measuring the change in
temperature of the water and jar in which the light bulb is immersed using Equation 2.
Q m jar c jar mwater cwater T Eq. 2
Here T is the final minus the initial temperature. In this particular case, we can simplify this equation.
The heat capacity of the jar is equivalent to 23 grams of water. Thus, we can use Equation 3.
Q m jar mwater cwater T Eq. 3
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By equating the electrical energy provided to the heat added to the water, Q, we can solve for the
specific heat of water, cwater.
Electrical Equivalent of Heat Jar
Power supply capable of delivering up to 3 A at 12 V
Stopwatch or clock
A balance for accurately determining the mass of the water heated by the bulb
IMPORTANT: When using the Electrical Equivalent of Heat Apparatus, always observe the following
Do not fill the water beyond the line indicated on the EEH Jar. Filling beyond this level can
significantly reduce the life of the lamp.
Illuminate the lamp only when it is immersed in water.
Never power the incandescent lamp at a voltage in excess of 13 V.
1. Measure and record the room temperature (Tr)
2. Weigh the Electrical Equivalent of Heat (EEH) Jar, with the lid on, and record its mass (Mj)
3. Remove the lid of the EEH Jar and fill the jar to the indicated water line with water. DO NOT
4. Add approximately 10 drops of India ink to the water, so that the lamp filament is just barely visible
when the lamp is illuminated.
5. Weigh the EEH Jar with the water, and record the value (Mjw)
6. Attach patch cords from the output terminal of the power supply to the lid of the EEH Jar, as seen in
7. Turn on the power supply and adjust the power supply voltage to about 11.5 volts, then shut the
power off. DO NOT LET THE VOLTAGE EXCEED 13 VOLTS.
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Figure 1: Connect power supply to Electrical Equivalent of Heat Jar
8. Insert the EEH Jar into the foam calorameter.
9. Insert a stirring rod through the hole in the top of the EEH Jar. Stir the water gently, and then insert
the thermometer through the hole in the top of the EEH Jar.
10. Turn on the power supply.
11. Record the starting time (ti) and the starting temperature (Ti).
12. Record the current (I) and voltage (V). If the current and voltage values shift significantly, use an
average value for V and I in your calculations.
13. When the temperature has increased by approximately 20°C, shut off the power and record the
14. Very gently stir the water with the thermometer. Watch the thermometer until the temperature
peaks and starts to drop. Record the peak temperature (Tf).
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Tr = _________________________________________
Mj = _________________________________________
Mjw = ________________________________________
V = __________________________________________
I = __________________________________________
ti = __________________________________________
tf = __________________________________________
Ti = __________________________________________
Tf = ________________________________________
1. Calculate the electrical power delivered to the lamp using Eq 1. Now calculate the total energy
supplied to the lamp.
2. Calculate the specific heat of water.
3. Look up the accepted value of the specific heat of water on the internet and compare to the
What effect are the following factors likely to have on the accuracy of your determination of cp?
1. The inked water is not completely opaque to visible light.
2. There is some transfer of thermal energy between the jar and the room. (What is the advantage
of beginning the experiment below room temperature and ending it an equal amount above
3. Your temperature difference is off by 2°C. What percentage will this change your value of cp?
Part B – Designing an Experiment to Determine Light Bulb Efficiency
A general definition of efficiency is “what we want” divided by “what we have to put in to get what we
want.” With a light bulb, we want light energy. Our input is electrical energy. In this section of the lab,
you will design your own experiment to determine the efficiency of the light bulb. You can use the same
apparatus that you used for Part A. Here are some questions and comments to help you get started. If
you need additional help, please discuss your experimental setup with your lab instructor.
1. Remember that the light bulb produces both light and heat. In Part A, the light energy was
absorbed by the water, producing heat. What did you do in Part A to cause this absorption? If
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you don’t want the light energy to be absorbed in this section, how would you change the
2. We cannot measure the light energy directly. If we know the electrical energy input and the
amount of heat produced, can you determine the amount of light energy that must be
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