"Electricity 101" is a structured series of "lessons" designed to help a student (of any age) gain a better understanding of that mysterious form of energy that we call "electricity". Following the historical evolution of the understanding of electicity starting with analogies to plumbing & flowing water, thru batteries, and continuing into PV cells & modules & systems, these lessons include lots of detailed illustrations and photos.
Here are several excerpts from some of the Electricity 101 lessons.
Teachers & students (of all ages) are welcome to download the entire 28 page PDF.
Created by Gary Vaughn for the NMSEA
Electricity & Plumbing - An Analogy That Works 2
What's an "Analogy" and Why? 2
An Introduction to Force and Pressure 3
Pressure, Flow, Work and Power 5
Siphons and Circuits 8
Pumps and Batteries 11
Battery Cells & Modules 14
Battery Facts 14
Common Battery Cells 15
Battery Modules 16
Building Battery Modules 16
Photovoltaic Cells & Modules 19
Photovoltaic Facts 19
Photovoltaic Cells 20
Photovoltaic Modules 21
Building PV Modules 21
Reading the Performance Graph 23
Tracking the Sun – Or Not 24
Changing DC Electricity into AC Electricity 25
Grid-Tied and Net-Metered 25
The BIG PICTURE of a PV System 26
Questions & Answers 28
ELECTRICITY AND PLUMBING : AN ANALOGY THAT WORKS
Lesson 1: What's an "Analogy" and Why?
a-nal-o-gy n., < Latin analogia < Greek analogia; analogos
1. similarity in some respects between things that are otherwise unlike; partial resemblance
2. explaining something by comparing it with something similar
Electricity isn't easy to understand.
We use electricity every day at home and at school, but we almost never think about it.
We tend to take electricity "for granted", that is, we just assume that our lights and appliances and electrical gadgets will work when we need them to work, and we don't "worry about the details".
But just remember : the "modern" lifestyle that you enjoy would be completely impossible without electricity.
Q1: List everything that you used at home today, and that you used on the way to school, that requires electricity to operate.
Q2: List everything in your classroom, and in your school, that requires electricity to operate.
Q3: Do you remember a time when the electricity at your home or school was "off" - maybe due to a storm or a broken power line? What happened? How did you have to change the way you did things?
Electricity wasn't studied at all before about 1600 AD. And progress after that was very slow. You may have learned that Benjamin Franklin flew a kite in a thunderstorm. That's true – it happened in 1752. Mr. Franklin was very lucky that he wasn't electrocuted! But his clever (and dangerous!) experiment proved that lightning is a form of electricity. No one had any idea of that before then. (Can you be electrocuted if you have no idea what electricity is??)
In 1798 an Italian, Alessandro Volta, invented the first battery.
In the early 1800's a Frenchman, Andre Ampere, made measurements of electrical quantities like voltage and current.
In 1827 a German, Georg Ohm, formulated "Ohm's Law" to explain the relationship between electrical resistance, voltage and current.
In 1831 a Frenchman, Michael Faraday, discovered that an electrical current would be generated if a magnet was moved across a metal wire.
Additional discoveries followed, but electricity was still something that only a few professors and famous scientists thought about.
President Abraham Lincoln never saw a light bulb.
In 1880 (less than 130 years ago) there was no electricity available in the White House or in any other house or business or school anywhere in the world.
Since we can't really "see" or "hear" or "touch" electricity directly, and because electricity is difficult to understand or "visualize" in an intuitive way, it's very common to teach people about electricity by using "analogies" to everyday objects and experiences. Electricity (as well as heat) was originally understood to be a kind of fluid. In fact, many of the names of electrical quantities, like "current", were taken directly from terms that were already being used to describe the movement of fluids such as water.
The most widely used analogy for helping to understand the "flow" of electrons in a metal conductor is to think of the "flow" of water molecules in a pipe.
This water flow analogy can be very useful in describing many aspects of electricity, but it falls apart for others. (A famous English scientist, Oliver Heaviside, didn't think much of this water analogy approach. He called it "Drain-Pipe Theory".)
Lesson 2: An Introduction to Force & Pressure
1) Students will do "thought experiments" using very familiar objects.
2) Introduction to force, pressure and pressure difference.
Objectives: (these will differ depending on the grade level standards)
Time: 20 minutes
• two or more 16 fluid oz clear plastic soda bottles
• a shallow catch pan or bucket.
1) Pour water into both bottles until they're about full.
2) Hold up the bottles as needed to illustrate:
Q1: What would happen if there were a hole in this bottle right here near the bottom?
(the water would flow out)
If you can, show this by using one bottle with a hole near the bottom.
Q2: Why does the water run out of the bottle and fall into the pan?
(the weight of the water due to the force or "pull" of gravity )
Q3: Why does the water go down? Why doesn't it go up?
(gravity always "pulls" things toward the ground - toward the center of the earth)
Q4: If we were in a space ship where there wasn't any gravity, what would happen?
(the water would just sit there, because nothing would be "forcing" it to move or flow in any direction; the water would be "weight-less")
Q5: How much does the water in this bottle weigh?
(measure or estimate the weight :
one US gallon of water weighs 8.3 lbs
a ½ liter (16.9 fluid oz) soda bottle full of water weighs about 1 lb)
Q6: How much weight or pressure is the bottom of the bottle supporting?
(all the weight of the water)
Q7: How big is the bottom of the bottle? How could you calculate its area?
( measure the diameter of the bottle at the bottom; calculate the area using the formula :
Area = 3.14 X (diameter/2) 2 ; a typical 16 fluid oz soda bottle has a bottom area of about 5 square inches – but measure yours)
Q8: So if the water weighs (1 lb) and the bottom of the bottle is ( 5) square inches, what is the weight of the water on each square inch of the bottom of the bottle?
( weight of water / square inches = weight per square inch)
( 1 lb / 5 square inches = 0.2 (or 1/5) lb per square inch)
Q9: Pressure is often measured in pounds per square inch or lb/in2 .
If this bottle was 5 times as tall, but it had the same diameter, and you filled it with water, what would it weigh?
( 5 times as much)
What would be the pressure of the water on the bottom of the bottle?
( 5 times as much – or about 1 lb/in2 in our example)
Q10: What if you had a much wider bottle, say 4 times as wide as this one we have now. If you filled that bottle up to the same height as this one:
Would you have to use more water? (of course)
How much more? (estimate how many of the small bottles it would take to "fill" the same amount of space as this large bottle. It would take 16)
How much would all that water weigh? (16 times as much)
What would be the pressure per square inch of the water on the bottom of this large bottle?
(it would be exactly the same as the pressure/square inch on the bottom of the small bottle, because the pressure per square inch on the bottom depends only on the height of the water above it. 16 times more weight & 16 times more bottom area = the same weight per unit area.
Q11: A cubic foot of water weights 62.4 lbs. If you had a container that was 12 inches long and 12 inches wide and 120 inches (10 ft) tall, and you filled it with water, how much would the water weigh? (624 lbs)
Q12: What would the area of the bottom of this container be in square inches?
(12" X 12" = 144 in2 )
Q13: What would the water pressure be at the bottom of this container?
(624 lbs / 144 in2 = 4.3 lb/in2 )
What would the water pressure be at the top of this container? (zero)
Q14: In many parts of the ocean the water is over 1 mile deep. What is the water pressure at a depth of one mile in the ocean? (2270 lb/in2 )
What is the water pressure on the surface of the ocean? (zero)
What would happen to you one mile down if you weren't protected in a submarine?
(you'd be squished like a bug!)
Q16: Air is much lighter than water, but the force of gravity is "pulling" on it too. The earth's atmosphere is several miles thick. The weight of that tall column of air is pressing on the ground all the time. What is the average pressure of all that air?
( the average air pressure at sea level is 14.7 lb/in2 )
Q17: Air pressure of 14.7 lb/in2 is pressing on every square inch of the top of your desk. Why doesn't your desk collapse from all that weight?
(The air pressure is the same everywhere in your room. The air pressure is the same under your desk as on top of it. There isn't any difference in the air pressure above and below your desktop.
Q18: So what would happen if there was a vacuum (zero air pressure) under your desk? ( Now the force or weight on the top of your desk would be very high, because there would be a difference in the weight or pressure of the air above and below the desktop. Calculate what it would be. (desktop area in square inches X 14.7 lb/in2 )
Q19: The air pressure on the outside of your body is 14.7 lb/in2 . You have lots of square inches of skin. Why doesn't your body get squished?
(the average pressure inside your body is also 14.7 lb/in2 , so you're fine.)
Q20: What would happen if you took a space walk (in a vacuum) without a space suit?
(your body would expand like a balloon – not a pretty sight! – because the pressure inside would be much greater than the pressure outside.)
BATTERY CELLS AND MODULES
- Batteries store energy in a chemical form.
- When batteries are activated, they convert their chemical energy into electrical energy.
- Since electricity is essential for our modern lifestyle, batteries are extremely useful.
- The first battery was developed by Professor Alessandro Volta in Italy in 1798.
- The unit of electrical pressure or electro-motive force, the Volt, was chosen to honor Professor Volta. The word Volt is always capitalized because it comes from his last name.
- There are many different types of batteries, and each type uses a different combination of chemicals to store energy.
- A battery "cell" is the simplest form of a battery. The typical or "nominal" voltage of a battery cell doesn't depend on its size, but on what chemicals are used to make the cell.
- The size of a battery cell determines how much chemical material it can hold, and therefore how much energy it can store. But the nominal voltage of a cell doesn't depend on its size.
- Common battery cell types and their typical cell voltage are:
- Alkaline 1.5 V
- Lead-Acid 2.0 V
- Nickel Cadmium (NiCad) 1.2 V
- Lithium Ion 3.0 V
- Battery cells can be connected together to form a battery module that has a higher voltage and a higher amount of stored energy than a single cell.
- Batteries are "polarized", which means that they have a positive terminal and a negative terminal. When you use a battery, you have to pay attention to which terminal is which !
- The positive terminal is usually marked with a plus sign (+) or a RED mark.
- The negative terminal is usually marked with a minus sign (-) or a BLACK mark.
- Since batteries are polarized, they can only be used in "direct current" or DC circuits.
- Batteries are rated by their terminal voltage and by their total "capacity" to store energy.
- The capacity of a battery is usually listed in Amp-hours (Ah) or in milli-Amp-hours (mAh), which tells you how much electrical current a new battery can supply for how long.
- If you multiply the nominal battery voltage by its rated capacity, you get the battery's nominal Watt-hour rating. A Watt-hour is a unit of electrical energy (1 Watt of power for one hour).
- The capacity rating of a battery will only be accurate if the battery is used EXACTLY the same way that the battery manufacturer tested it.
- If you use a battery in a circuit that needs a higher current flow (and therefore a higher power output) than in the test case, the battery's terminal voltage and capacity will be reduced.
- Some types of batteries are designed to be recharged many times. Other types of batteries are designed to be used only once.
- Alkaline batteries are not designed to be recharged.
- Lead-acid and NiCad batteries are designed to be recharged.
- Your family's car has a large 12 Volt lead-acid battery with a capacity of hundreds of Ah.
- Solar photo-voltaic systems use lead-acid batteries to store electrical energy for use at night.
- Lead-acid batteries are very heavy for their size. Why?
- The sulfuric acid used in lead-acid batteries can eat holes in your clothes and burn your skin.
- Lead is a metal, and it's very poisonous. Always recycle used lead-acid batteries.
- When a lead-acid battery is recharged, both hydrogen and oxygen gases are generated. What happens if an undiluted mixture of hydrogen and oxygen gas is exposed to a spark or a flame?
PHOTOVOLTAIC CELLS & MODULES
- Photovoltaic panels don't store energy like batteries.
- When photovoltaic panels are active (when they're exposed to sunlight) they "transform" or "convert" light energy into electrical energy.
- PV panels can be described as electrical generators or "batteries" that run on light.
- Since electricity is essential for our modern lifestyle, photovoltaic panels are extremely useful.
- The first practical PV cell was built by scientists at Bell Telephone Labs in 1954. It is now in a museum, and it still works just fine. Bell Labs called those first PV cells "Solar Batteries".
- The word "photovoltaic" was built from the Greek word "photo", meaning light, and the word "voltaic" which comes from Volt. A Volt is the unit of electrical pressure or electro-motive force.
- The Volt was named in honor of the Italian, Alessandro Volta, who built the first battery in 1798. That's why Volt is always capitalized.
- There are several different types of PV cells. Each type uses a different material to perform the light-to-electricity conversion. Some materials are better at this conversion than others.
- A PV "cell" is the most basic PV building block. The typical or "nominal" voltage of a PV cell doesn't depend on its size, but on what type of material is used to make the cell.
- The most common PV cells are made out of silicon, which is the main ingredient in ordinary beach sand ("Silicon" isn't the same thing as "Silicone"). Computer chips are also made out of silicon. A silicon PV cell has a nominal cell voltage of about ½ of a Volt.
- The two most critical things required to make a PV cell are sand and brains ! Why?
- The area of a PV cell determines how much light energy it can receive, and therefore how much electrical energy it can produce.
- PV cells can be connected together to form a PV module that has a higher nominal voltage and current rating (and therefore a higher power rating) than a single PV cell.
- PV cells and modules are "polarized", which means that they have a positive terminal and a negative terminal. When you use PV, you have to pay attention to which terminal is which !
- The positive terminal is usually marked with a plus sign (+) or a RED mark.
- The negative terminal is usually marked with a minus sign (-) or a BLACK mark.
- Since PV modules are polarized just like batteries, they can only be used in "direct current" or DC circuits.
- PV cells are rated by their terminal voltage, and by their power output in Watts.
- The nominal current generated by a PV cell is usually listed in Amperes (Amps or A), which tells you how much electrical current the cell can supply when it is exposed to light.
- If you multiply the nominal PV cell voltage by its rated current, you get its Watt rating. A Watt is a unit of power (that is, the amount of electrical energy generated per second).
- A PV panel rated at 100 Watts and exposed to full sunlight for one full hour, can generate 100 Watt-hours of electrical energy. At night, the PV panel won't generate any energy.
- The power rating of a PV cell will only be accurate if the cell is used EXACTLY the way that the cell manufacturer tested it (that is, with the same electrical load and the same amount of light).
- If less light falls on a PV cell than in the test case, the cell will generate about the same voltage, but less current, so it will generate less electric power. (W = V X A)
- PV cells don't "use up" the material that they are made of like batteries do. PV cells don't "wear out". Most PV modules are guaranteed to work for at least 25 years.
- PV cells don't need to be recharged the way some batteries do.
- Photovoltaic modules can be used to recharge lead-acid batteries. The electrical energy that the PV modules generate during the day can be stored in the batteries for use at night.
(28 pages in PDF format)