Energy Conversions I
Energy can take many forms, but any one form can
usually be converted into another. And no matter what
form we talk about, we can use conversion factors to
calculate equivalent amounts of energy that could
(theoretically) be derived from changing one form into
another. For example, we show in this lesson that
electrical energy can be converted into heat energy
using an immersion coil. In a similar fashion we can
convert heat, light, mechanical energy, chemical energy,
electrical energy or nuclear energy from one form into
another. Can you supply examples of energy
conversions and units of measure that would complete
the matrix below?
Form
Definition
Figure 1:
Equipment for Energy Conversions I
balance
graduated cylinder (100 mL or 250 mL)
immersion heater (with split cord)
multimeter
power supply
ringstand
stopwatch
Styrofoam® cup
thermometer
Example
Unit of
measure
(most common one)
HEAT
energy from a hot object
unit on a gas kitchen range
calorie or Btu
LIGHT
MECHANICAL
CHEMICAL
ELECTRICAL
NUCLEAR
One of the most important considerations in our search for energy is that it be easily
converted from one form to another. As we have noted, the sun is a primary source of energy on
our planet. Captured solar energy provides us with derived energy from green plants. After
millions of years of decay, plant and animal matter convert into fossil fuels like coal and
petroleum. These can be burned and converted into electricity — a form of energy that our
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Energy Conversions I -- Page 1 of 6
society converts daily into many forms to serve our needs. For example, the electrical outlet in
our kitchen can be tapped if we want to heat water for making hot chocolate.
In this lesson we will look at the conversion of electrical energy to heat water. We do this
every day, but rarely think about it. This is what happens on a large scale in electrical hot water
heaters. Have you ever wondered how much energy is lost in using electricity to heat water?
Let's explore this on a reduced scale by using a small immersion coil that heats rapidly when we
plug it into an electrical outlet. If the coil is immersed in a cup of water, the heat is transferred to
the water and the water temperature goes up. For each gram of water that is heated by one degree
Celcius a single calorie is absorbed. {One thousand of these is known as a kilocalorie — the
food Calorie we encountered earlier.} We can measure the power of the immersion coil in watts,
defined as the product of voltage and current. Then we can compare our results to the rating
shown on the label for that electrical appliance. We will use this power input in calculating the
efficiency of heating water this way. Electrical energy is measured in watt-hours or kilowatthours, which is what the power company charges us for in a monthly bill. Some basic energy
definitions are found in Appendix A and energy conversion factors are found in Appendix B.
But enough of this! Let's see what happens when we heat a cup of water using electricity.
Objectives [At the end of this lesson students will be able to...]
•
•
•
•
•
describe the steps needed to collect data on electrical voltage and current, and use these to
calculate the power in watts that is the rate of energy consumption of a small electrical
appliance.
determine the number of calories absorbed by a given quantity of water heated through a
measured temperature change.
describe the steps needed to collect data and calculate the efficiency of heating water
using a small immersion coil.
compare the energy consumed by the coil with the known relationship between watthours and calories, and describe the efficiency of heating water this way.
work problems requiring energy conversion from watt-hours to calories and vice-versa.
Start-up questions
1. Suppose we want to heat a cup of water using a small electrical immersion coil. What
does the power rating of the coil (in watts) tell us about the time required to heat the
water? Predict the temperature change expected from 2.0 minutes of heating using the
coil provided.
2. How much energy is required to heat 250. milliliters of water from 15° Celcius to 95°
Celcius? Does this sound like a lot or a little to you?
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Energy Conversions I -- Page 2 of 6
3. Do you think this will prove to be an efficient way of heating water? In other words, does
most of the electrical energy used by the immersion coil go directly into raising the
water's temperature? How could we find out?
Using Electricity to Heat Water
Now we will heat water with the small immersion
coil. We need to know how much water, since the
quantity of heat needed depends on how much water
we are heating. Also, we should know the beginning
and ending temperature of the water, since each
Celcius degree we raise the temperature of a gram of
water requires another calorie of heat. And we will
determine the wattage rating of the immersion coil so
we can discuss the energy it provides. The energy
input is the power applied (in watts) from the coil
multiplied by the time it was applied (in hours). WattFigure 2: Immersion Heater Set-up
hours can be converted to calories, since 860. calories
equals one watt-hour. Efficiency is the ratio of energy
output divided by energy input, converted to per cent. Let's see how efficient our little coil is in
heating water...
Heating water: data and calculations
Value
1. Volume of water in cup
2. Mass of water that was heated
3. Initial temperature of water
4. Final temperature of water
5. Wattage of immersion coil
6. Time during which coil was energized
7. Temperature change of water
8. Energy input = watts × time of heating (hours)
9. Energy input in calories (above × 860. cal/watt·hr.)
10. Energy output (mass × sp. heat × temp. change)
11. Efficiency = (energy output / energy input) × 100%
Unit(s)
ml
g
°C
°C
W
s
C°
watt·hr
cal
cal
%
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Energy Conversions I -- Page 3 of 6
Clarifying points:
•
The equation relating energy E, mass m, specific heat of a substance c, and change in
temperature ∆T, is:
E = m × c × ∆T
•
•
•
•
All energy are convertible from one form to another and the second law of
thermodynamics says that you cannot break even. You are going to lose useful energy
when converting from one form of energy to another. The second law is frequently
referred to as increase in entropy (or disorder or chaos).
In general, efficiency is defined as: Efficiency = (Output/Input)×100%
Efficiency of energy conversion in a heat engine where there is a heat source and a cold
sink can be expressed as: Efficiency = (Thigh - Tlow)/Thigh < 1 or < 100%
The best efficiency one can expect from most engines is about 30% (usually 10%).
What are some of the consequences of the lack of efficiency?
How can one develop higher efficiency systems?
The conversion process is usually between electrical energy, thermal energy, or light
energy (photo or electromagnetic) and mechanical work. These basic conversion
concepts and measurement of their efficiency are demonstrated throughout these energy
conversion lessons.
Suppose we are interested in how much electricity is required to heat up a cup of water to
compare this with the energy accounted for in calories when the water temperature rises.
We've all heard that energy cannot be created or destroyed. So what if some energy gets
"lost" in conversion from electrical energy to thermal energy (heat). Like, a lot of energy
gets lost! We can examine the process of energy conversion by comparing the energy
output in a process with the energy input to that process. The ratio (converted to percent)
is called efficiency.
Assessment questions
1. Do you recommend an immersion coil as a means of heating water? If so, what
advantages does this method have over other ways of heating water for hot chocolate
[microwave oven, stove top, lower cup into active volcano, etc.]? Can you think of other
ways to heat water in the event of electrical power failures?
2. What are some factors that might have influenced the data you collected in this
experiment? Does the type of cup holding the water have any effect on the results? For
example, would you get the same results from styrofoam cups, heavy metal mugs, glass,
or large pots?
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Energy Conversions I -- Page 4 of 6
Example problem
You heated 250. ml of water from 15.0°C to 85.0°C with an immersion coil heater rated at 150.
watts. The coil was plugged into an electrical circuit for 600. seconds.
a. How many calories of heat were absorbed by the water?
b. How many watt-hours of energy were produced by the coil?
c. What is the efficiency of the process?
Solution:
Energy
a. Output =
temp. change
1 cal
× mass × g·C°
1g
1 cal
= (85.0°C - 15.0°C) × 250. mL × mL × g·C° = 1.75×104 cal
Energy
b. Input
1 hr
= 150. W × 600. s × 3600 s = 25.0 W·hr
Energy Output
1.75×10 cal
W·hr
c. Efficiency = Energy Input × 100% = 25.0 W·hr × 860. cal × 100% = 81.4%
4
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Energy Conversions I -- Page 5 of 6
Homework
1. Suppose you are given 1.00×10² grams of an unknown liquid and told that you must heat
it from 15.0°C to 65.0°C.
• Show the steps that you would use to determine its specific heat using an
immersion coil. All critical data measurements must be shown in your method.
• At what point would you stop adding heat (before, just as, or after the
thermometer reached 65.0°C)?
2. If a 100.-watt immersion coil left on for 185 seconds heated the liquid through the
temperature change above, what is the specific heat of the liquid (assume 100%
efficiency)?
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