ELECTRICAL ENERGY
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Part A
Re-familiarize yourself with how an electric circuit works by using a battery to light a light bulb.
Draw your circuit below.
1. Where does the energy come from to light the bulb?
The amount of energy which a battery (or any electrical power supply) can give to the charges
moving through a circuit is determined by the voltage of the battery. Voltage is closely related to
electrical potential energy, and in some elementary school materials voltage is treated
synonymously with energy. However, they are not exactly the same thing. In order to understand
how voltage differs from energy, first consider gravitational potential energy.
If you want to know the gravitational potential energy of an object in this room, one thing you
need to know is how far above the floor the object is. The object will obviously have a different
potential energy if it is near the ceiling than if it is at the height of a table. The object’s height
above the floor is thus closely related to its energy, but the height is not equal to its energy. To
actually calculate the gravitational potential energy of an object, you also need to know the weight of
the object. (There is a big difference in energy between hanging an elephant from the ceiling as
opposed to hanging a mouse from the ceiling!)
For electrical potential energy, voltage plays the role that height plays for gravitational potential
energy. Knowing the voltage of a battery is like knowing the height of an object. The voltage is
proportional to how much energy a charge will get as it moves around a circuit. Since all of the
charges moving in a circuit have the same magnitude of charge, each charge will receive the same
amount of energy from a given battery. In comparison, when you hang various objects from the
ceiling, the energy of the objects will not all be the same if their weights are different.
Think about many of the different sized batteries one can buy - from AAA, like you might use
in a TV remote control, to size D (flashlight batteries). All these batteries are 1.5 Volt batteries.
What is different about them? They all supply charges with the same amount of energy, but the
larger batteries have more total energy stored inside them (in the form of chemical potential
energy). Thus, all other things being equal, the larger sized batteries last longer than the smaller
batteries because they have more total energy stored inside them.
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A voltmeter measures the voltage difference between any two places. If you have a fresh Dcell battery, then a voltmeter will measure about a 1.5 volt difference between the two ends of the
battery. Familiarize yourself with how a voltmeter works, and select three batteries which can each
supply at least 1.5 Volts.
2. Use your voltmeter to determine the voltage two batteries in series can supply, and record
your result. (see diagram below)
+
EN
-
+
EN
-
V = ______________
3.
Check this result by lighting a light bulb by first using one battery, and then two batteries in
series. Describe what you observe when you do this, and sketch the circuits you used below.
Can you explain what you observe from the standpoint of energy?
4.
Now determine the voltage of three batteries in series. Compare this to with what you found
using just one or two batteries, and report on this below. You should also set up a circuit
with a single bulb and three batteries in series and observe the results. What are your
conclusions?
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5.
Now connect two batteries in parallel, and determine the voltage they supply. (see diagram
below) Show how you made this measurement by drawing in your voltmeter on the
diagram.
+
+
EN
EN
-
-
V = ______________
6.
Check this result by lighting a light bulb by first using one battery, and then two batteries in
parallel. Describe what you observe when you do this, and sketch the circuits you used
below. Can you explain what you observe from the standpoint of energy?
7.
Why would anyone want to make a circuit with two batteries in parallel? (Hint: Think about
how fast electrical energy is used up.)
8.
Now try three batteries in parallel and see if you get the results you expect. Report on this
on the next page.
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9.
It should be clear (with a little thought) that the light bulbs in a circuit are able to indicate
how much electrical energy is being converted into light and heat energy. Explain how they
indicate this.
Part B Lights in Series and Parallel
1.
Using two batteries in series, begin by connecting one light bulb to the batteries. Indicate the
brightness of the bulb. (bright, medium, or dim)
2.
Now put two light bulbs in series with the batteries and again indicate the brightness
observed.
3.
Continue putting light bulbs in series (one at a time) until the bulbs will no longer light. As
each bulb is added, keep a record of how many bulbs are in series and how bright they are.
4.
What is your conclusion regarding the ability of the batteries to provide enough energy to
light light bulbs and the number of light bulbs you have in a series circuit?
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5.
What is your conclusion regarding the ability of the batteries to provide enough current to
light light bulbs and the number of light bulbs you have in a series circuit?
6.
Repeat the procedure you just used for questions 1 - 5, but this time put your bulbs in
parallel instead of series. Continue adding bulbs until you have used five of them. As each
bulb is added, keep a record of how many bulbs are in parallel and how bright they are.
7.
What is your conclusion regarding the ability of the batteries to provide enough energy to
light light bulbs and the number of light bulbs you have in a parallel circuit?
8.
What is your conclusion regarding the ability of the batteries to provide enough current to
light light bulbs and the number of light bulbs you have in a parallel circuit?
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PART 3
CONDUCTORS AND INSULATORS
Electrical conductors are objects that allow electrical current to pass through them easily.
Electrical insulators are objects that do not allow electrical current to pass through them easily.
1.
Find at least five conductors and five insulators in this room and list them below. Be sure
you test them!
CONDUCTORS
INSULATORS
2.
If you have not already done so, test to see if water is a good conductor. (Make your test
using a D-cell battery and a light bulb.)
3.
It is well known that adding salt to water increases its ability to conduct electricity. Try
adding salt to the water, and note below if you detect any changes in the water’s ability to
conduct electricity.
4.
Examine the wires that you submerged in the saltwater. Do they look clean, or do they seem
a little “dirty”?
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5.
Now test the conductivity of your saltwater in a little bit different way. Construct the circuit
shown below in which the bare (plastic insulation removed) portion of the wires are pressed
tightly on the tops of the nails.
What do you observe when you immerse the two
nails into the saltwater? Do you observe anything
interesting about the nails?
nails
When a current flows through water, chemical reactions can occur near the electrodes (the
wire tips or nails immersed in the water). As a result of these reactions, water molecules can
be broken apart into hydrogen and oxygen gas molecules. Bubbles of hydrogen gas form at
the negative electrode, and oxygen gas forms at the positive electrode. If you did not notice
bubbles before, be sure you check for them now. Do you have the same amount of bubbles
forming at each electrode, or does one electrode produce more than the other? [Is this
consistent with the idea that water (H 2O) contains two hydrogen atoms for every atom of
oxygen?]
Now remove the nails from the solution and examine the nails. Do you observe anything
about them?
Note: As a result of chemical reactions at the electrodes, various residues can form on the metal.
These residues can serve as insulating layers that stop the current flow. Since nails have
more surface area than wires, the nails worked better as electrodes in this activity.
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Most people know that electricity should not be used around water. Consequently, many people
expect water to be an excellent conductor. In fact, pure water is a very poor conductor of
electricity. However, water that has something like salt dissolved in it can be a good conductor.
This is especially true when your body is immersed in the water. Please be careful!
Electricity and Magnetism:
Wrap a coil of wire around a compass. Observe that running a current through the wire can cause
the compass to deflect. (Do this for only a short time as the battery and wires may get very hot!)
The above activity illustrates an important link between electricity and magnetism. Since
compasses deflect due to magnetic fields, we can infer that a current-carrying wire generates a
magnetic field. This phenomenon can be very useful. Recall that when you studied magnetism you
saw that some materials, such as paper clips and nails, behaved like magnets when in contact with a
true magnet, but they lost their magnetic properties when removed from the true magnet. Such
temporary magnets can also be produced by current-carrying wires, in which case one has an
electromagnet. Your assignment is to construct an electromagnet by wrapping a very long coil of
wire (e.g., 100 cm) around the entire length of a large iron nail. Be sure to leave enough room at
each end of the wire for connecting to a battery. The nail should behave as a magnet when a current
is run through the wire. When no current flows, the nail should return to its non-magnetic state.
Sketch your electromagnet below:
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Electricity Homework
1. a. If a positive charge is brought near to a negative charge, does the potential energy of the
charge system increase or decrease? Why?
b. If a negative charge is brought near to another negative charge, does the potential energy of
the charge system increase or decrease? Why?
2. The relative sizes of a 9 volt transistor radio battery and a 12 volt car battery are shown below.
In terms of energy, what is the likely consequence of the car battery being so much bigger than
the 9 volt battery?
12 volt
9 volt
3. Wires in electrical circuits can become noticeably hot if the current through the circuit becomes
too large. Where does this heat come from?
4. If the electric company charges 20 cents per kilowatt hour, how much would it cost to keep a
100 watt bulb lit for 24 hours?
5. a. Is air a good conductor or a good insulator? How do you know?
b. Lightning occurs when the air conducts electricity between a cloud and the ground. What
does this tell you about the voltages involved in lightning strikes?
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QUESTIONS, PROBLEMS, AND EXTENSIONS
Electricity
6. Consider the three circuits below:
A
B
C
In circuit A the bulbs would be dim. In circuit B the bulbs would be of medium brightness.
In circuit C the bulb would be bright. Using the four designations dim, medium, bright, or off,
label each of the bulbs in every circuit below. Check any circuits you are not sure of
experimentally!
i.
iv.
vii.
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ii.
iii.
v.
vi.
ix.
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QUESTIONS, PROBLEMS, AND EXTENSIONS
7.
Consider the following combinations of batteries. For each combination, indicate what the
difference in voltage would be if measured at either end of the combination. (All of the
batteries are 1.5 volts.)
a.
b.
c.
d.
8.
e.
Which light bulb below will remain lit the longest? Explain your answer.
a.
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b.
c.
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QUESTIONS, PROBLEMS, AND EXTENSIONS
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