Electric Circuits
The word “circuit” comes from the same root as the words “circle” and “cycle.” An electric
circuit is a device which moves electric charge around in a circle or at least a loop. It is related
to other cycles such as the water cycle.
Consider a simplified “water cycle” which might be a water feature in a lawn or garden:

Water cascades down a short waterfall. In the process it turns a small water wheel. At the
bottom of the waterfall the water flows through a drain and into a pipe. The water flows
through the pipe to a water pump. The pump pushes the water up through another pipe to
the top of the waterfall. The process repeats itself constantly.
Now consider a simple electric circuit:

One end of a battery (or cell) is connected to a wire. The wire is connected to a glowing
light bulb. The other “end” of the light bulb is connected to another wire. This second
wire leads back to the other end of the battery.
This water feature and this electric circuit are closely related! In the space below, draw a sketch
of the water feature on the left and a sketch (or diagram) of the electric circuit on the right.
Below your diagrams try to identify the corresponding pieces of the two systems. (For example,
what piece of the electric circuit corresponds to the water pump of the water feature? What part
of the water feature corresponds to the wire from the battery to the light bulb?)
1
Charge: the stuff that moves.
The stuff that moves through the water pipes is, um, water. The stuff that moves through the
wires is called electric charge. A few experiments will illustrate the nature of electric charge.
1. The sticky tape experiment(s).
The equipment for this experiment is extremely simple and cheap. Even so, correctly
setting it up and doing the experiment can be tricky. It is best to get “instructions” from
somebody else, usually called an “instructor.” If at all possible, watch your instructor do
the experiment once before you try to do it yourself.
 Take a piece of sticky tape (such as “magic tape”) about two or three inches long
and fold one end over on itself so that you have a non-sticky tab (a quarter to a
half an inch long) on one end.
 Smash the tape down on a hard smooth surface (such as a clean tabletop).
 Grasp the non-sticky tab of your tape and quickly peel it off the table.
 Let your tape hang down (you might need to shake it off of your hand) and try
holding it near the tape of one of your classmates. What happens?
Sticky Tape II (the sequel)






That was so much fun we’ll do it again with TWO sticky tapes placed on top of
each other. In order to tell them apart we’ll need to label them, so you will need a
pencil or pen that can write on your sticky tape.
Make one sticky tape and smash it down on the table as you did above. This will
be the “lower” tape so label the non-sticky tab with a lower case “l” for lower.
Now make another sticky tape (the same size) and smash it down directly on top
of the lower one. The two non-sticky tabs should be right on top of each other.
This is the “top” tape so label the non-sticky tab with a lower case “t” for top.
Grasp BOTH STICKY TABS between the thumb and finger of one hand and
quickly lift them off of the table together. Once they are hanging (together),
grasp the two sticky tabs, one with each hand, and quickly pull them apart.
Slowly bring your “t” tape near your “l” tape. What happens?
Bring your “t” tape near the “t” tape of a classmate. Bring your “l” tape near the
“l” tape of a classmate. What happens?
2
(Sticky tape, continued)
Talk to your classmates and try to come up with a general rule for the behavior of “t” and
“l” sticky tapes. Your rule might sound like another rule you have heard before.
As a clue to what’s going on here, try looking at your sticky tapes sideways. One of them
will have a sideways “t” and one will have a sideways “l”. What do they look like?
(This is where you say something like, “Oh, now I get it!”)
Benjamin Franklin figured out that electric charges come in two types. If we have equal
amounts of the two types then they cancel each other out. He recognized that these are
the properties of positive and negative numbers so he called one kind of charge positive
and the other kind negative.

Based on what you have done in class (or elsewhere) have you seen anything other than a
label (such as “l”, “t”, “red” or “black”) that clearly identifies which kind of charge is “+”
and which kind of charge is “-” ? (Do light bulbs care? Would the sticky tape
experiment work differently if we reversed the labels?)
Interesting historical fact: Mathematicians of the 1700s were undecided about whether or
not negative numbers were “real” or whether they should be taught in school. Franklin’s
discovery ended that debate.

The “t” sticky tapes have more positive charge than negative charge. The “l” tapes have
more negative than positive. Do you think your hand has more positive or negative
charge? Why do you think so? What evidence do you see?

Everything we encounter in our daily lives contains both positive and negative charges.
What must be true about the positive and negative charges in most of these things? Why?
3
2. Other “charged” stuff.
Franklin’s definitions: We can identify positive and negative charges using sticky tape
but Franklin’s definitions were based on what was available at the time.
o If you rub a piece of amber with rabbit fur, the amber will become negatively
charged. If you don’t have amber and rabbit fur, you can use a balloon and your
own hair. Rub a balloon against your hair and the balloon becomes “negative.”
o If you rub a piece of glass with silk, the glass will become positively charged.
You can substitute acrylic for glass if you want and you can substitute plastic food
wrap (such as Saran Wrap) for the silk.

What does it mean to say that a balloon is negatively “charged”? Does that mean it has
no positive charges in it?
WARNING: This use of the word “charged” is different from the use of the word
“charged” or “charging” when referring to a battery! Referring to a battery as “charged”
or “uncharged” is actually sloppy use of language but it is very common.

Before it has been rubbed with fur or hair, we say it is “uncharged”. What does this
mean?

An uncharged balloon is rubbed against some uncharged fur. The balloon becomes
negatively charged. What do you suppose happens to the fur?
You should be able to find a collection of things you can use as substitutes for Franklin’s
amber, fur, glass, and silk. Predict what will happen if we “charge” these objects in
various ways and bring them close to each other. Once you have made some predictions,
test your predictions with some experiments. Record your observations below.
4
Wires: “pipes” for electric charge
When looking at an electric circuit, the wires don’t move or light up. When an electrical device
quits working we usually don’t change the wires. The wires might not seem to “do” anything.
1) Set up an electric circuit with a couple of cells and one round bulb. Find a compass (the
kind that points north, not the kind that draws circles) and place it on the table far from
metal objects. When the needle comes to rest, lay one of the wires of your circuit directly
over the needle (it should run right over the needle in the same direction as the needle,
not perpendicular to it). Without moving this wire or compass, connect and disconnect
the circuit a couple of times. Do you see evidence of something happening at the wire?
2) Prepare to set up an electric circuit with a couple of light bulbs (both long or both round)
in series but BEFORE YOU CONNECT THE CIRCUIT, predict which light bulb will
light first when you do connect the circuit.
a. Discuss this with your classmates. You will probably hear different hypotheses
about which bulb will light first and why. Write down some of these hypotheses
so you can test as many as possible.
b. Now connect the circuit. Which bulb lights first? Do you all agree about which
bulb lights first? Does this change if you switch the order of the bulbs? Does it
change if you reverse the batteries? What do you think: which bulb lights first?
3. You can think of your wire as solid piece of copper. In copper wire the positive charges
(nuclei of atoms) stay put along with most of the electrons, while only a small fraction of
the negatively charged electrons move. Does this sound more like a hollow water pipe or
a pipe full of sand? Would this kind of water pipe lend itself toward charge that moves
fast or slow? How does this analogy compare with your observations in part 2b, above?
5
4. Thinking about water… When you turn on a water tap, how long does it take before
water starts to flow?
5. Jane is shopping for an apartment. She sees one that she really likes but she notices that
it is a couple of miles away from the nearest water reservoir. She reasons that water will
come out of the tap at a rate of a couple of feet per second. If the reservoir is 10,000 feet
away, then she figures she would have to wait 5,000 seconds for water to reach her from
the reservoir. She doesn’t want to wait 5,000 seconds (that’s well over an hour) every
time she needs a glass of water so she decides to look for another apartment. What do
you think of Jane’s reasoning? What’s wrong with it?
6. When we use a water tap at GRCC, the water starts to flow right away. Immediately
before we started to use the water tap where was that water?
7. So here’s a little paradox to figure out.
 The electric charge that moves through a wire is like a fluid moving through sand the
size of atoms. It flows very slowly. One centimeter per minute is not unusual and
when speeds get up to one millimeter per second you can expect the wire to get hot!
 When you flip a light switch, the lights seem to go on or off instantaneously. Even if
lights are across the room they seem to light immediately when the switch is flipped.
How can both of these statements be true? (Which they are.)
Check your ideas with an instructor.
6
Batteries: the “pumps” of electric charge.
First a question about our water feature (remember the waterfall, water wheel, and the pump).
Does the pump create the water? Does the pump store the water? Does the water “come from”
the pump? What does the pump do?
A water pump can do a combination of a couple of things. It can raise water up, or it can
increase the pressure in the water, or both. This is similar to what is done at hydroelectric dams.
There is a name for this. A water pump increases what property of the water? (Ask your
instructor if you just don’t know.)
Electric potential
Scientists use the word “Potential” to mean “potential energy per unit of stuff.” Exactly what is
the most useful “unit of stuff” in any given field of science is something which is defined by
experience and tradition. Here are some examples:
𝐹𝑙𝑢𝑖𝑑 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 =
𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦
𝑈𝑛𝑖𝑡 𝑜𝑓 𝑣𝑜𝑙𝑢𝑚𝑒
𝐺𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 =
𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 =
𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦
𝑈𝑛𝑖𝑡 𝑜𝑓 𝑚𝑎𝑠𝑠
𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦
𝑈𝑛𝑖𝑡 𝑜𝑓 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙𝑠
(the “unit of chemicals” is usually the “mole”)
For electricity, the “stuff” is electric charge, so the unit of stuff is the unit of electric charge. The
standard unit of electric charge is the “coulomb” but don’t worry about that yet. Just take it for
granted that we have a unit for a certain amount of electric charge.
𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 =
𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦
𝑈𝑛𝑖𝑡 𝑜𝑓 𝑐ℎ𝑎𝑟𝑔𝑒
Electricity is probably the area of science where the concept of potential is used the most.
7
So based on our analogy with the water feature, what do you think batteries do?
Let’s test your ideas. Set up a circuit where you light two light bulbs in series (and you might as
well use long ones) with one cell. That’s not very impressive.
If one cell raised the potential by one unit (we’ll get to the units of electric potential in a minute)
what do you predict two cells in series would do to the potential? What do you predict this
would do to your circuit? Test your prediction.
Go ahead and test what happens with three cells in series and four cells in series. You might
burn out a bulb but you can try it this once. Record your observations.
Now what do you expect would happen if you set up the circuit below? This circuit should
behave the same way as what circuit you’ve seen before? Test your prediction. (Pay careful
attention to the arrangement of the cells and you might need extra hands to hold all of the cells.)
8
Now what do you expect would happen if you set up the circuit below? Notice that it is not the
same as the last circuit. Even so, this circuit should behave the same way as what circuit you’ve
seen before? Test your prediction. (Pay careful attention to the arrangement of the cells and you
might need extra hands to hold all of the cells.)
Electric potential is “potential energy per unit of charge”. If you know the electric potential
difference between two points (which might be the ends or “terminals” of a battery), how could
you figure out how much energy is required to move a certain amount of charge through the
circuit?
One more step. Electric potential is “potential energy per unit of charge”. Power is “energy per
unit of time”. If you know the electric potential difference between two points (which might be
the ends or “terminals” of a battery), how could you figure out how much power is required to
operate an electric circuit?
9
Light bulbs and other workers:
Incandescent light bulbs are essentially made out of wire with some fancy packaging (glass and a
metal case). The filament of a light bulb is just wire, and yet there are other wires in the circuit.
Why do you suppose the filament glows? Aside from being bright, what other word might
describe the filament while it is glowing?
The filament is a lot thinner than the other wires in the circuit. What difference would that make
to the electric charge flowing through it? (You can think of water flowing through a water pipe.)
The “flow rate” of electric charge is called “current”. In the circuit below, how would the flow
rate (or current) through bulb A compare to the flow rate (or current) through bulb B?
A
B
In the circuit above, how would the brightness of bulb A compare to the brightness of bulb B?
(You can set up the circuit if you like.)
What have you noticed about the relationship between the brightness of bulbs and potential
difference? If two (identical) bulbs have the same brightness, what would you say about the
potential difference of the two bulbs?
10
A
B
x
For historical reasons, we draw “arrows” for electric current as though it comes out of the
positive end of a battery and flows into the negative end (and all the way through the battery as
well). To remind yourself of this, draw a clockwise arrow around the circuit above to indicate
the direction we say the current is flowing.
Now imagine we follow the current around this loop, starting at the “x” in the corner. The first
thing that happens to the charge is that it hits that battery. When that happens, does the electric
potential go up or down? What do you think? (You might want to check your answer with an
instructor.)
Then the charge travels across the wire at the top of the diagram. It turns out that very little
happens to the potential in a large wire. No measureable work is done there.
Work is done when the charge gets to the bulbs. If the charge has to do work as it moves
through bulb A, what happens to its potential as it moves through A? Does the potential increase
or decrease? (Think about what happens if you have to do work or a spring has to do work.)
What happens to the potential as the charge moves through bulb B?
If A and B are identical bulbs, how would the change in potential in A compare to the change in
potential in B?
The charge then moves back to the battery along a thick wire and not much happens to the
potential along that wire. Put it all together. What happens to the potential as the charge moves
around the loop?
11
C
D
x
That was fun, let’s do it again. The difference in this circuit is that there are two loops that pass
through the battery which the charge can follow. The first loop goes from the x through bulb C
and back to the x. The second goes from the x, through bulb D and back to the x. You may want
to use colored pencils, but draw both loops in the diagram above.
To start out, let’s assume that bulb C and bulb D might be very different. One might be a huge
spotlight and the other one is a small flashlight bulb. Is it necessarily true that the current (or
flow rate) through bulb C will be the same as the current (or flow rate) through bulb D?
Now the honors question: there is also current flowing through the battery. You can think of
this as the current at the x if you like. Looking at your arrows for your two loops, you should be
able to come up with a relationship between the current through the battery, the current through
bulb C, and the current through bulb D. What is that relationship?
Now think about potential differences. Look at the loop through bulb C. The electric potential
rises as the charge moves through the battery and it drops as it moves through bulb C. If it all
comes back to the same spot at the x, what must be true about the jump in potential at the battery
and the drop in potential at the bulb? (Think about the water feature if you like.)
Look at the other loop. What can you say about the potential difference across bulb D?
What can you say about the potential difference across the battery the potential difference across
bulb C, and the potential difference across bulb D?
12
A
C
D
B
x
x
Putting it all together:
We would say that light bulbs A and B are connected in ___________________
We would say light bulbs C and D are connected in ___________________
When two light bulbs are connected in series, what must be the same in those two bulbs?
When two light bulbs are connected in parallel, what must be the same across those two bulbs?
(Note on language: Notice the use of the words “in” and “across” in the previous two questions.
That isn’t an accident. Current runs through things. Potential difference is measured between
the two ends of things. Charge moves. Current flows. Potential difference doesn’t move.)
While we are on the subject of language:
Electric current is measured in units called amperes or amps (abbreviated A).
Electric potential is measured in units called volts (abbreviated V).
Another name for electric current is amperage.
The difference in electric potential between two points goes by the special name voltage.
Voltage is also measured in volts.
13
Resistance: What makes a filament light up.
When we rub our hands together, we say our hands get warm because of friction.
When we run current through a wire, we say the wire gets warm because of resistance.
Electric current through an object (a light bulb) increases when the potential difference (or
voltage) across the light bulb increases. If the potential difference (or voltage) is zero, then the
flow rate is zero. What sort of a relationship does that sound like?
Electric current through an object (a light bulb) decreases when the resistance of the light bulb
increases. If the resistance becomes infinitely big, then the flow rate is zero. What sort of a
relationship does that sound like?
The previous statements can be summarized in one equation. The equation is called Ohm’s law
and it can be written in a couple of ways.
𝑐𝑢𝑟𝑟𝑒𝑛𝑡 =
𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑜𝑟
𝑣𝑜𝑙𝑡𝑎𝑔𝑒 = (𝑐𝑢𝑟𝑟𝑒𝑛𝑡)(𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒)
Think about water pipes full of sand or gravel. If you want a water pipe with very low resistance,
what properties would you give it?
Think about what happens when you add more light bulbs in series (to a constant number of
cells). What appears to be happening to the current? What must be happening to the resistance?
Think about what happens when you add more light bulbs in parallel (to a constant number of
cells). What appears to be happening to the current through the battery? What must be
happening to the resistance?
14