Charges and fields
Static electricity is a well-known phenomenon. It affects many industries in diverse
environments. Static charge buildup can result in potentially dangerous electrical shocks,
which can cause fires, explosions and severe damage to sensitive electronic components.
Static charge buildup can be caused by friction between two surfaces. This is called
triboelectrification. Electrons migrate from the surface of one material to the surface of
the other. Upon separation of the two surfaces, one surface loses electrons and becomes
positively charged. The other surface gains electrons and becomes negatively charged.
As the pressure and the speed of contact and separation increase, the amount of the
static charge buildup increases. Rapidly moving materials can quickly develop charges,
which produce a potential of more than 25,000 volts.
During this session you will separate electric charges. You will consult the triboelectric sequence to find out which object acquires a positive, and which
object acquires a negative charge. You will then investigate the interaction between
the charged objects.
- Sticky tape
- Rods and rags
- Styrofoam pieces
Open a Microsoft Word document to keep a log of your experimental
procedures, results and discussions. This log will become your lab report.
After each step, write down what you have observed. Address the
points highlighted in blue. Answer all questions in blue in your log.
Obtain a piece of sticky tape, about 15 - 20 cm in length. For ease in handling, make
"handles" by folding each end of tape to form portions that are not sticky.
Press the tape firmly onto a smooth, unpainted surface, for example, onto a textbook
or onto the table.
Then quickly peel the tape off the surface and hang it from a support.
Describe the behavior of the tape as you bring objects, such
as a finger or a pen, towards it.
Make another piece of tape as described above. Bring the second tape toward the
first tape with the non-sticky sides facing each other.
Describe your observations.
It is important,
that during this experiment you keep your hands and other objects away from the tapes.
Explain why this precaution is necessary.
Describe how the distance between the tapes affects the
interaction between them?
Press two pieces of tape onto the surface and write a B (for bottom) on them.
Then press another tape on top of each B tape and label it T (for top).
pair of tapes off the surface as a unit. After they are off the surface, separate the T
and B tapes. Hang one of the T tapes and one of the B tapes from a support.
Describe the interaction between the following pairs of tape
when they are brought near one another.
- Two T tapes
- Two B tapes
- One T and one B tape
Our electrostatic materials kits contains rods and rags made from different
materials. Consult the table of triboelectric materials below.
The items on top are less attractive to electrons and become positively charged
when rubbed against items below, while
the items on the bottom are more attractive to electrons and become negatively charged
when rubbed against items above.
- Human Skin (usually too moist though) (very positive)
- Rabbit Fur
- Human Hair
- Steel (neutral)
- Hard Rubber
- Nickel, Copper
- Brass, Silver
- Gold, Platinum
- Styrene (Styrofoam)
- Saran Wrap
- Polyethylene (like scotch tape)
- Vinyl (PVC)
- Teflon (very negative)
Choose a rod and a rag, for example a PVC rod and wool rag or a Lucite rod
and a vinyl rag. Rub the rod vigorously with the rag
and then hold the rod near newly-made T and B tapes hanging from a
support. Compare the interactions of the rod with the tapes to the previously observed
interactions between the tapes.
similarities or differences.
The rod and the tapes interact, because they are electrically charged.
Answer the following questions based on the
observations you have made thus far.
- How many different types of charge do there appear to be? Explain.
- Which tape, T or B, has a positive charge? Explain.
- How do two objects that are positively charged interact? Explain
how you can tell.
Now remove all tape from the support. Attach a small piece of Styrofoam to
approximately 30 cm of insulating string and hang it from a support.
Touch the Styrofoam piece to a charged rod and observe the behavior of the piece after it touches
Is the piece charged? If so, does the piece have net charge with the same sign
the rod? Explain how you can tell.
The figure below shows two tapes.
What kind of charge could be on tape 1 and tape 2?
Imagine the following situation.
Two metal balls are touching each other. A charged rod is brought near the left
one. While the rod is near, the right ball is taken away. Finally the rod is
taken away. At the end of this procedure, the left-hand metal ball has a
Describe what you think is happening.
Why is the left ball negatively charged at
the end of the procedure?
Assume two identical metal balls are suspended by insulating
strings. Both balls have the same net charge. Do not assume the
balls are point charge.
Draw a separate free-body diagram for each ball.
Label the forces to indicate
the object exerting the force,
the type of force (gravitational, tension, etc.),
whether the force is a contact or a non-contact
Predict what will happen if the charge on the ball 2 is reduced, so it is less than that
on the ball 1. Draw a sketch to illustrate your answer.
Is the angle that ball 1 makes with the vertical,
greater than, less than, or equal to the angle that ball 2 makes with the
vertical? Explain your reasoning.
How does the free-body diagram for each ball in this
case compare to the corresponding free-body diagram that you drew
before the charge on ball 2 was reduced? If the magnitudes or
directions of any of the forces change, explain
how they change.
Use an on-line simulation from the University of
Colorado PhET group to explore the electric field produced by different
distribution of charges.
Link to the simulation:
- Explore the interface!
- Create one positive charge in the middle. Take
out one E-field sensor and explore its behavior as you put it at various
points around the positive charge. The E-field sensor is a small positive
test charge. Does the E-field sensor
behave as you would expect? Explain.
- Remove the positive charge and place a negative charge
in the middle. What changes? Does the E-field sensor behave as you would expect?
- Set up an electric dipole: a positive and negative charge
separated by a small distance.
Sketch electric field lines associated with
this charge distribution. Where do the field
lines begin and where do they end?
- Create a plane of positive charge and a plane of
negative charge separated by a about 4 major grid divisions.
Sketch electric field lines associated
with this charge distribution. Where do the
field lines begin and where do they end?
The flux of a vector field through a surface area is the amount of whatever the
field represents passing through the area. The total flux depends on
strength of the field, the size of the surface area it passes through, and on
how the area is oriented with respect to the field. You can think of
flux as the amount of something crossing a surface. The surface is a two
dimensional (real or imagined) boundary. It can be open or closed.
An open surface could be a the area of a door, the area of a sheet of paper, the
area of a bowl, etc. A closed surface could be the surface area of a sphere
or a cube, etc. Flux is measure at a single point in time. Flux is
the total amount of something crossing the surface, it is not something per unit area,
Field lines help us to visualize the field. The density of the
field lines is proportional to the strength of the field. The number of
field lines passing through a geometrical surface of given area depends on three
- the strength of the field
- the surface area
- the orientation of the surface
The number of field lines passing through an area A is proportional
to the flux through that area.
To visualize electric flux
in 3D, explore the demonstration below.
Click Download Demonstration as CDF. You can then open it in
your Browser. Use the sliders and drag the image with the mouse. Try some of
the "Things to Try".
The net flux through a surface enclosing a volume can be
positive or negative. It is proportional to the number of field lines
leaving a volume minus the number of field entering the volume.
net flux = constant*(# of field lines leaving - # of field
If there is no source or sink of field lines inside the volume,
than what flows in must flow out, the net flux through its surface is zero.
The sources and sinks of the electric field lines are charges. If there
are no charges inside a volume, then the net flux of the electric field through
its surface is zero.
If there is a net charge Q inside a volume , then field lines
start or end there. The number of field lines starting or ending is
proportional to the charge Q. We have a net outward or inward flux
proportional to Q. Gauss' law tells us that the proportional
constant is ε0.
To visualize electric flux through a closed surface in 3D,
explore the demonstration below.
Click Download Demonstration as CDF. You can open it in your
Browser. Use the slider and drag the image with the mouse. Try some of the
"Things to Try".
What is the net electric flux through the
surfaces S1 - S5 shown below?
For each surface calculate the flux in units
The drawing is 2D, but imagine 3D boxes with flat tops
and straight-up walls.
Convert your log into a session report, certify with you signature that
you have actively participated, and hand it to your instructor.