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PPT
Current
Developer Notes
 How to get across the idea of a circuit?
 Show circuit diagrams with source, load, switch? Have students draw them?
 Draw circuit for two-way switch as exercise? Or as demo? extension?
 Should show that static moving is the same stuff as comes from a battery. It is possible to
show that a battery can deposit a charge on an electrophorus. The electrophorus plate can
then induce a charge in an electroscope. I used a 9 V battery to charge an aluminum foil disk
(with PS handles) sitting on a plastic shopping bag on a metal desk. Or a pie tin on a bag on a
sheet of aluminum. Or a sheet of aluminum on a bag on a sheet of aluminum. This would
show that the stuff coming out of a battery is the same stuff that we get by rubbing things
together. This might be best as a demo or a station. Be sure to show that rubbing the disk on
the bag can produce a charge, but that you’re not rubbing it.
 Discuss conventional current (Franklin), vs. electron current. 50-50, he guessed and got it
wrong. Needed later for relation to magnetic field.
 Show salt water conducts? Resistance.
Version
01
02
03
Date
2004/07/06
2004/07/07
2005/02/18
Who
Sc
dk
dk
Revisions
Initial version
 added schematic and activity with circuit board
 simplified schematic and activity
Goals
 Students will know that materials vary in their conductivity.
 Students will know that current does work.
 Students will know that a circuit is required for a continuous flow of current.
 Students will know the difference between series and parallel circuits.
 Students will understand Ohm’s Law, V = IR.
Concepts & Skills Introduced
Area
Physics
Physics
Physics
Physics
(Extension)
Concept
Conductors & Non-conductors
Circuit
Series and Parallel circuits
Ohm’s Law (I = V/R)
(P=I2R=IV)
Standards Addressed
Time Required
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Warm-up Question
 How is an electric current like a water current?
 What does circuit mean?
Presentation
There may be some students who don’t know about conductors and non-conductors. Show a
variety by putting them in series with a bulb and battery. For the most part, metals are the
conductors we use, but even air can be a conductor if the voltage is high enough. Compare cloth,
wood, water, metal, pencil lead (can show resistance using mechanical pencil lead in series –
slide one contact on the lead).
Students should also know that a complete circuit is required in order for charge to flow. This
can be easily shown with a battery and lamp. The bulb won’t light until a circuit is connected
from one end of the battery to the other.
We’ve used an electroscope to demonstrate charge. Charge that doesn’t move is called static
electricity. A moving charge is called a current – moving charge is current electricity. To show
current (moving charge), do a POE.
 Setup - Use two electroscopes. Charge one up. Then show that you will connect the two
electroscopes by making the paper clips touch.
 P – Will charge move from the first electroscope to the second? If so, how far apart will the
vanes be – more, less, or the same as the first one was, and more, less, or the same as the first
one when they’re connected?
 O – Connect the two electroscopes. [The vanes on both electroscopes should be separated,
but less than the first one was originally.]
 E – [Some of the excess charge from the first electroscope moved to the second one because
likes repel. The charges had potential energy, and work was done on them.]
Note that the moving charge did some work in lifting the vanes. The charge wouldn’t move
without a force (electrical in this case). The force operated through a distance, and that’s work.
Make sure the students understand that a lamp works by conducting current from the tip of the
base, through the filament, and to the body of the base. They also need to understand that one
end of the battery has excess electrons compared to the other side. The electrons come from the –
end, the flat end on a D-cell. The voltage is not high because the chemicals in the battery only
react at a certain rate.
Have students build circuit boards or show a demo of one.
 Electric circuit = when a current makes a complete loop
 What are necessary components of an electrical circuit? Source of voltage and charge
(battery), conductor (path for charge to flow) which has some resistance
 How are these components related? How can we tell how strong the current will be? Ohm’s
Law! I=V/R
 Types of circuits-series, parallel
 A circuit board shows that electric current can do work (like light a light bulb).
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In the activity, students should be able to see that current increases with increased voltage, and
that current decreases with increased resistance. They’ll know that voltage is directly related to
current, and that resistance is inversely related to current. Then they should be able to create I ~
V/R. They won’t know if it’s a linear or exponential relationship, however.
Students should also be able to create P ~ IV.
Assessment
Do a prediction where the students need to determine if the circuit is in series or in parallel.
Writing Prompts
Relevance
Answers to Exercises
1.
Answers to Challenge/ extension
1.
Equipment
The goal here is to have a simple and straightforward piece of equipment to show V =IR and P
= IV. We can show that current decreases with increased resistance, and that current increases
with increased voltage. The batteries supply the voltage (and the current), the bulbs are the
resistance, and the brightness of the bulbs indicates the amount of current.
You can make a permanent assembly that will work well. It’s more work and expense to make,
and it will take more space in storage. It will be more reliable, but the connections are less
obvious to the students – it’s more of a black box.
Use two 1.5 V batteries. Any size will work, but D-cells last longer.
The bulbs must handle 3 V without burning out, and must still glow visibly with two bulbs in
series at 1.5 V. Radio Shack 272-1132 work well. They’re rated at 2.47 V, 300 mA and they
have E-10 threaded bases.
Simple Assembly
The simple assembly is just three tracks of flat conductive material and paper clips (besides the
batteries and bulbs). The tracks are attached to a flat surface. They can be screwed or nailed to a
piece of wood, or just taped in place on a (non-conductive) desk when needed. The spacing has
to allow the bulbs and batteries with clips to fit (see below).
D cells stand on their ends easily and the + end works with jumbo paper clips. (Duracells worked
in my testing.)
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The bulbs should mount well in paper clips. Threaded bases clip into paper clips better than
bayonet bases.
Track material should be highly conductive, non-corroding, flat, straight, cheap, and not too hard
to make in lengths.
 Galvanized pipe strapping – This worked really well. I don’t know why the difference from
other galvanized metal. Comes in 10’ rolls. Has to be cut to length and straightened.
 Galvanized tie strap (Simpson Strong-Tie with obvious crystal patterns in the galvanization)
– Worked really well some times, but not at all others, even after cleaning with alcohol and
sandpaper. Flat and straight and comes in different lengths. Sheared sheet metal, so some
sharp edges.
 Zinc (I think) plated bracing straps – These didn’t work well, even after cleaning with
alcohol and sandpaper. Flat and straight, and comes in different lengths.
 Aluminum foil – didn’t work well. I suspect aluminum does not work well as a conductor
because it oxidizes on the surface, creating too much resistance.
 Aluminum strap – Has the usual aluminum problems, even after cleaning with alcohol and
sandpaper.
 Aluminum screen – Worse than aluminum foil.
 Gum wrapper – Haven’t tried it.
 Tin cans – These might work, but they’re hard to cut. I haven’t tried them.
 Bare copper wire – This didn’t work as well as I expected. It’s also not flat to set batteries
and bulb/clip assemblies on.
Here’s a standard jumbo paper clip.
To hold a bulb, bend a clip open to 90. The bulb goes in the wide end. You may need to squeeze
the wide end together a little bit to keep the bulb from sliding out.
To make a clip for a battery, bend a clip open straight. The wide end will clip on the + end of a D
battery. Again, you may need to bend it together a little bit. Bend the clip down just past the edge
of the battery so that the small end reaches the table at the – end of the battery.
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Permanent Assembly
It’s convenient to have a small board to attach everything to. Wood or particle board works well,
about ?” x ?”.
The easiest way to hold batteries is with battery holders. If the holders have pre-attached wires,
that’s a bonus.
A socket that is easy to connect to and shows the connection pattern is good. Screw terminals are
easier for most people to work with (as opposed to solder).
Use single stranded wire, 22 AWG or so. Old phone wire works well.
Lay out the board approximately as shown in the schematic of the activity section.
Parts List
Qty
Description
1
Board, ?” x ?”
2
Battery holder, D-cell
2
Battery, D-cell
3
Lamp, 2.5 V, E-10
3
Lamp holder, E-10 threaded
2’
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Wire, single conductor, 22 ga
Screw, wood, flat head, #6,
1/2”
Vendor
na
Radio Shack
Radio Shack
Radio Shack
Radio shack
Radio Shack
Vendor PN
na
270-403
23-870
272-1132
272-357 or
272-360
278-1215
Notes
has wires attached
To mount lamp and
battery holders
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Background
You are familiar with charge. You know that charge moves due to electric force. When charge
moves, it’s called current. Voltage is a measure of the potential energy of the electric field
creating the force. Charge can jump a gap if the voltage is strong enough. Charge can also flow
through and over some materials. Here you will play with voltage and current. Current is the
movement of charge. Current can move at different rates.
Charge moves due to electric force. Batteries provide the force. Potential energy. charge. voltage.
field. There are many ways to look at it!
In this activity, think of
 circuit – the flow of electrons
 voltage – the electric force that makes the electrons flow (batteries)
 resistance – the resistance to electrons flowing (bulbs)
 current – the number of electrons flowing (brightness of the bulbs)
 power – the total amount of energy coming from the bulbs (how many bulbs and how bright)
Problem
Learn about circuits, series and parallel circuits, and find the relationship between voltage,
current, resistance, and power.
When two things are in series, it is like a single lane road, like
this:
When two things are in parallel, it is like a multi-lane road, like
this:
Materials
1
circuit setup
Procedure
For each setup below, note how brightly each bulb is lit.
Copy the table and fill it in. Indicate the brightness for each setup using 1 for dim, 2 for medium,
and 3 for bright. To find the total brightness, multiply the brightness by the number of bulbs. As
an example, the table is filled in for one bulb and one battery.
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1
Bulbs
1
2 in parallel
2 in series
Bright
2
Total
2
Batteries
2 in parallel
Bright
Total
2 in series
Bright
Total
One battery
1. One bulb
2. Two bulbs in parallel
3. Two bulbs in series
Two batteries in parallel
4. One bulb
5. Two bulbs in parallel
6. Two bulbs in series
Two batteries in series
7. One bulb
8. Two bulbs in parallel
9. Two bulbs in series
Summary
Remember, batteries supply voltage, bulbs are resistance, brightness indicates current, and total
amount of light is power.
1. What is required for any bulb to light?
2. What kinds of energy are being created from the electrical energy in the battery? Name at
least two.
3. What does increasing the resistance in the circuit (putting bulbs in series) do to the current?
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4.
5.
6.
7.
8.
Current
a. Are resistance and current directly or inversely related?
b. Write an expression relating current (I) and resistance (R).
What does increasing the voltage (putting batteries in series) do to the current?
a. Are voltage and current directly or inversely related?
b. Write an expression relating current and voltage (V).
Write an equation relating current to voltage and resistance.
What does putting bulbs in parallel do to the current?
a. Based on your equation, what does that indicate about the resistance?
What does putting batteries in parallel do to the current?
a. Would a single bulb stay on longer this way than with one battery? Why?
Power is the total brightness of the bulbs.
a. Which setup uses the most power?
b. Are power and voltage directly or inversely related?
c. Are power and current directly or inversely related?
d. Write an equation relating power to current and voltage.
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Reading
In the Charge activity, when you created sparks, you generated thousands of volts but not much
charge, so when a spark happened, there wasn’t much current. A battery is different. It doesn’t
generate a lot of voltage, but it has a lot of charge and can supply a current for a long time.
Batteries supply the field and the charge (electrons) to make a current. Charge only flows if there
is a path for it, a circuit. Most of the time, charge flows through wires and other good conductors,
but if the voltage is high enough, electrons can flow through the air – examples are sparks and
lightning.
Current is defined as the amount of charge/ time, q/t. In units, it is Coulombs/ second, and is
given the name amperes (A), after
In quantities
I = q/t
In units
1 A = 1 C/s
If there is a field, and if the resistance is low enough, charge will flow, making a current. The
higher the voltage, the greater the current. The higher the resistance, the lower the current.
I = V/R
The units for resistance are ohms ().
A = V/
This is Ohm’s Law, named after
A good analogy for electricity is water. The amount of water is the charge. If the water moves, it
makes a current. Think of water flowing through a hose like electric current flows through a
wire. If there’s greater voltage (water pressure), more water will flow. If there’s resistance
(someone standing on the hose), less water will flow. The same amount of water (current) can
flow with a small amount at a high speed (high voltage and resistance) or a large amount at a
lower speed (low voltage and resistance).
Electricity is a form of energy. It can do work. If it can do work, we can measure its power. As
indicated in the activity, electric power is directly related to both current and voltage. Remember,
volts is energy/charge. Multiply by the current, charge/time, and you end up with energy/time, or
power.
Current is charge/ time
I = q/t
Volts is potential energy/ charge
V = PE/q
Current times volts power
q/tPE/q = PE/t = P
In quantities
P = IV
In units
J = AV
The power of water coming from a hose is a combination of the amount of water and the speed at
which it comes out. Firemen use hoses with a large current and high pressure – the flow is strong
enough to knock people over. A garden hose doesn’t generate as much current or pressure.
Power is energy/time. The amount (mass) and velocity of water coming from the hose every
second determine its power.
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Electrical quantities
Quantity
Symbol
Charge
q
Current
I
Volt
V
Resistance
R
Power
P
Current
Equation
I = q/s
V = PE/q, V = IR
R = V/I
P = IV
Unit
Coulomb
Ampere
Volt
Ohm
Joule
Symbol
C
A
V

J
Unit Measure
6.25e18 electrons
1 A = 1 C/s
1 V = 1 J/C
1  = 1 V/A
1 J = 1 AV
A safety tip - When you deal with electricity, remember – current kills. A higher voltage makes
it more likely that a current will flow, but it’s the number of electrons that flow through you that
do the damage.
Exercises
1. Check your family’s electric bill for a recent month.
a) What are the units on the bill? [kWh, kilowatt-hours]
b) Think about the formula for power. Why do the units on your electric bill make sense?
[The formula for power is W/t, work/time. Watts are a form of work (energy), so
multiplying by time leaves you with the work done, or energy consumed. Power doesn’t
make sense for an electric bill because it depends on how much time it took to use the
energy.]
2. 3 variable Ohm’s Law
3. 3 variable power equation
4. current and time?
5.
Challenge/ extension
1.
Glossary

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