Name (printed) _______________________________
LABETTE
A SURVEY OF STATIC ELECTRICITY
ultimately ionizes the air molecules in between and
allows excess charge to flow from the charged object
to the conductor (this is the essence of “grounding”).
Static electricity is everywhere – especially on dry
days. Rub one insulator (a non-conducting substance
like fur, fabric, or plastic) against another insulator
and you’ll probably get a charge transfer. It happens
all the time with clothes in the dryer. They’re
insulators and the heat makes the environment very
dry.
Now most people really don’t understand static
electricity, and fewer have toyed with it in any
organized way. That’s the whole reason for doing this
labette. It’s kind of fun to play with and if you do it
systematically you’ll begin to be able to visualize the
movement of the charge. Visualizing the movement
of the charge is very important, because you’ll never
be able to actually see any charges and if you can’t
picture what’s happening, then the more important
topic of current electricity is sure to frustrate you.
INTRODUCTION
When it’s very dry out, I can get a static electric
shock just about every time I get out of my truck. All
I have to do is open the door, step to the ground, and
then reach to close the door. ZAP! Every time. It’s
not really painful, just ... well … a shock. The same
type of shock often occurs when you walk across a
carpeted floor and then touch a doorknob or, heh heh,
an unsuspecting little sister (a favorite former activity
of mine). When you move around on a fabric
automobile seat or walk across carpeting with the
right type of shoes, you often transfer electric charge.
This is called charging by contact. (There is another
type of charging mechanism called charging by
induction.) When I reach for the metal door of the
truck, or the doorknob, or the little sister (all
conductors – that’s important) there is spark
discharge that occurs right before the touch. The
close proximity of the charged object (me) to the
conductor causes a growing electric field that
PURPOSE
•
To become familiar with charging by conduction and induction.
•
To build and understand the operation of a capacitor.
PART 1: THE ELECTROPHORUS
PROCEDURE
1.
If there are not enough already available, hot-glue a Styrofoam cup to the top of a pie plate.
2.
Obtain a Styrofoam plate and a piece of fabric or fur.
3.
Rub the Styrofoam plate with the fabric. This charges both the Styrofoam and the fabric by contact.
4.
Let the pie plate rest on the Styrofoam. The pie plate and Styrofoam together are referred to as an electrophorus.
5.
Touch the pie plate with your finger. This charges the pie plate by induction.
6.
Lift the pie plate by its Styrofoam cup handle. The cup is an insulator, so no charge can flow to or from your
hand to the plate.
7.
Touch the pie plate to an unsuspecting ear or other extremity. The small spark you hear discharges the pie plate.
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QUESTIONS
1.
Assume the Styrofoam is negatively charged. Show carefully using the series of drawings below, how the
electrophorus is charged by induction. Use caption boxes to carefully explain your drawings.
Before
grounding
While
grounding
After ground
is removed
2.
Now use the electrophorus to practice charging an electroscope by conduction and induction. When you feel
confident, call me over to test your ability to use both charging methods.
Passed charging by conduction
GET CHECKED
BEFORE MOVING ON
Passed charging by induction
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PART 2: THE CAPACITOR
PROCEDURE
1.
Fill a film canister 3/4 to the top with water. Snap the top onto the canister. Dry the outside thoroughly.
2.
Push a nail into the top of the canister and down through until it touches the bottom.
3.
Wrap aluminum foil around the canister and under its bottom. This is your capacitor.
4.
Charge the electrophorus.
5.
Now hold the capacitor in one hand, making contact with the aluminum foil. Touch the electrophorus to the
nail. You should hear a small spark. This charges the capacitor by conduction.
6.
Repeat the previous two steps several more times.
7.
Now, while holding the aluminum foil with one hand, touch the nail with the other hand. You should notice
a larger spark. This grounds the capacitor.
EVALUATION
Use the space below to illustrate:
1. The multiple steps of the capacitor being charged by conduction.
2. What happens when the capacitor gives a shock.
GET CHECKED
BEFORE MOVING ON
3
QUESTIONS AND PROBLEMS
STATIC ELECTRIC CHARGES AND FORCES
1.
Use the series of diagrams to the right to explain the
charging by conduction by an object that is charged
positively.
The top diagram to the right shows an uncharged
conducting sphere. Note the number of each charge
present.
++-
++-
a. Show what happens to the charges in the sphere
when the charged rod is brought close. Explain
below.
+ + + + + + + + + + + + +
b. Show what happens to the charges in the rod and
sphere when the rod touches the sphere (both the
rod and sphere are conductors). Explain below.
c.
Show the charges in the sphere after the charging
rod is removed. Explain below.
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+-
2.
Use the series of diagrams to the right to explain the
charging by induction by an object that is charged
positively.
The top diagram to the right shows an uncharged
conducting sphere. Note the number of each charge
present.
++-
+-
+-
+-
a. Show what happens to the charges in the sphere
when the charged rod is brought close and the sphere
is grounded (the sphere is a conductor). Explain
below.
+ + + + + + + + + +
b. Show what happens to the charges in the sphere
when the rod stays close to the sphere, but the ground
is removed. Explain below.
+ + + + + + + + + +
c. Show the charges in the sphere after the rod is
removed. Explain below.
GET CHECKED
BEFORE MOVING ON
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1.
a. What is the force between two charged spheres, one with an excess of -1.6 µC and one with an excess of
3.2 µC. that are separated by 50 cm? What does the sign on your answer represent?
b. Now imagine the original charges are present and the spheres are moved to 1.5 m apart. How much force
would be exerted between the two charged spheres now? Try to do this conceptually.
2.
a. How far apart are two charged spheres, each with 4.0 µC of excess positive charge, if the force they exert
on each other is 0.0030 N?
b. If distance is changed so that the force is 0.012 N, what is that new distance?
3.
The diagrams to the right show three charges. Calculate the net force on each of the charges due to the
other two.
(Regular physics do the top diagram;
1.0 µC
-1.20 µC
0.50 µC
Honors physics do the bottom
diagram.)
1.0 µC
0.50 µC
-1.20 µC
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For the following multiple choice questions, give clear and complete evidence for your choice in the space provided.
4. _____
The electric force between two charged spheres is 18 units. If the distance between them is tripled, the
force between them will be
a. 2 units
b. 3 units
c. 6 units
d. 54 units
5. _____
If the charge on each of a pair of charged objects is doubled and the distance between them is also
doubled, the electrostatic force between them is
a. quartered
b. halved
c. unchanged
d. doubled
6. _____
When two small charged spheres are separated by 2.0 m, the electric force of attraction between them is
6.0 N. If the charge on each sphere is doubled and the separation is reduced to 1.0m, the force of
attraction will now be:
a. 6 N
b. 16 N
c. 24 N
d. 48 N
e. 96 N
7.
In each of the cases below “P” is positive and has the same size charge as the + and - charges. Rank the force
felt by “P” from greatest to least. Circle any that have the same force.
In the space below, provide rationale for your choices above.
GET CHECKED
BEFORE MOVING ON
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QUESTIONS AND PROBLEMS
ELECTRIC FIELDS
For the two multiple choice questions, give clear and complete evidence for your choice in the space provided.
1. _____
2. _____
3.
+ + + + + +
As an electron between a pair of parallel oppositely charged plates approaches
the positive plate, the force on the electron
a. increases.
b. decreases.
c. remains the same
_ _ _
_ _ _
A block of aluminum rests on a wooden table in a region where a uniform electric field points straight
up. What can be said concerning the electric charge on the block’s top surface?
a. The top surface is charged positively.
b. The top surface is charged negatively.
c. The top surface is neutral.
d. There is not enough information to determine the charge on the top surface.
Draw appropriate field lines in the drawings below to show the electric field around the charged objects.
+
+
-
+
-
-
+
+
+
+
+
+
+
+
For this drawing, consider the circle to be a
neutral conductor and also remember the effects
of induced polarity.
GET CHECKED
BEFORE MOVING ON
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QUESTIONS AND PROBLEMS
VOLTAGE
For the first two questions, consider two slides at a park. One is twice the height as the other. Children (all the same
size) can slide down either slide. Use this scenario as an analogy to electric energy and voltage. Give clear and
complete evidence for your choice in the space provided.
1. _____
The amount of “voltage” in the shorter slide is higher than the “voltage” of the taller slide.
a. True always.
c. It depends on the number of children sliding down each slide.
b. False always.
2. _____
The amount of energy from the shorter slide is higher than the energy from the taller slide.
a. True always.
c. It depends on the number of children sliding down each slide.
b. False always.
3.
A flashlight uses two 1.5 V batteries (giving its charge a potential of 3.0 V). How much charge must flow
through the light bulb filament for 100 J of energy to be delivered to the light bulb?
4.
10 Coulombs flow through a circuit, delivering 12 J of energy. What is the voltage of the source?
5.
A 12 V car battery delivers 15 Coulombs per second for 3.0 s as it provides energy to start the car. How much
energy was used to start the car?
GET CHECKED
BEFORE MOVING ON
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QUESTIONS AND PROBLEMS
OHM’S LAW, ELECTRICAL ENERGY AND POWER
For all problems assume that the cost of electricity = $0.12 per kilowatt-hour
1.
What is the resistance of a resistor through which 8.0 x 104 C flow in one hour if the potential difference
across it is 12 V?
2.
How much current flows through the 240-Ω, tungsten filament of a light bulb when it is connected to a
120-volt outlet?
3.
Let’s say that you dry your hair for five minutes a day with a 1400 W hair dryer. How much energy do you
use per year drying your hair?
4.
The element in an electric frying pan has a resistance of 86 ohms. If the frying pan is hooked up to a
standard 120-volt source, how long will it take to generate 50,000 J of heat?
5.
In 3.0 minutes, an immersion heater delivers 36,000 J of energy to a coffee cup full of water. The
immersion heater is connected to a standard 120-volt source. What is the resistance of the immersion
heater?
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6.
Ever heard of the cake circuit? That’s right, an electrical circuit with the resistance load being some cake
batter. It may sound odd, but the cake circuit is every bit as genuine as an electric lamp, a refrigerator, or a
toaster. The electric lamp circuit produces light (actually it’s mostly heat), the refrigerator circuit produces
motion (the motor), and the toaster produces heat. The cake circuit is most like the toaster because it
produces heat that results in the cake being cooked. When the cake batter is connected to a standard 120-V
potential it draws 1.5-A and the cake takes 12 minutes to cook.
a. Calculate the resistance of the cake.
b. Calculate the power being used to cook the cake.
c. Calculate the energy used to cook the cake.
d. Calculate the cost of cooking the cake.
7.
When you buy a box of 60-watt light bulbs, the assumption is that you will use the standard 120-volt
potential provided by electrical energy providers throughout the country. What is the resistance of the
filament in these light bulbs?
8.
How much current does a 1500-watt hair dryer draw when it is connected to a 120-volt source?
9.
How much does it cost to run a 5.0-watt alarm clock for an entire year?
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