$\cal{SCIENCE}$1 $\cal{FUN}$# 3:

CHARGE!

2 Feb. 2000

As a team, choose one or the other of the following problems to explore and explain.

Physics Homework Question 2:

Earth's Extra Electrons

The Earth's atmosphere is a thin electrical insulator which has an excess of positive charge at higher altitude and an excess of negative charge near the Earth's surface, which acts like a spherical conductor. This charge separation gives rise to an approximately constant electric field pointing down through the atmosphere. However, above the atmosphere there is no electric field due to the Earth unless it has a net charge. Assuming that only electrostatic and gravitational forces are involved, calculate how many excess electrons the Earth could hold.

(Hint: what will happen to a free electron just outside the atmosphere?)

-- OR --

Extension of Electrostatics ``Kit'' Experiment:

Scotch Tape Van de Graaf

Take two strips of 3M brand Scotch $^{\hbox{\tiny\sf TM}}$ tape and fold over one end on each so that you have a short non-sticky end for handling the tape. Attach some very small weights to the other end of each strip. (Be able to determine the magnitude of the weights.) Take two more tape strips of about the same length and stick them down flat on a smooth surface like a table top. Now stick both the original strips down on top of those strips. Make sure the tape is ``stuck tight'' and then pull it loose in one motion (one for each strip, that is). If you let the weights dangle and then carefully bring the tops of the tape strips closer together, what happens? Why? Can you estimate how much charge each strip has ``pulled loose'' from the tabletop?

This is the basic principle of the Van de Graaf generator, a device for creating very large electric fields.

Suppose you took a whole roll of Scotch $^{\hbox{\tiny\sf TM}}$ tape and mounted it on a spindle connected to an insulated metal sphere by ``brushes'' touching the smooth surface of the tape on the roll just underneath where the loose tape has been pulled off the roll. Now you grab the end of the tape and run down the hall until you have pulled all the tape off the roll. (Don't try this with our tape!)

Calculate the net charge on the hypothetical metal sphere.

Procedure for   $\cal{SCIENCE}$1 $\cal{FUN}$:

1.

Find two other people to form a team of three. Please avoid classmates from the same high school if possible.

2.

In your team, everyone should participate on all tasks; but the primary responsibilities should be assigned as three roles: select one person as Leader, another as Computer and a third as Recorder.

• The Leader's role is to analyze the problem, propose methods for solution (including measurements), compose the conclusions and (if time permits) make a presentation; much depends on the Leader's creative ingenuity, but (s)he is not a dictator - important decisions should be reached by consensus.
• The Computer solves mathematical problems - both abstract calculus and/or algebra when needed for the problem's ``formal solution'' and numerical calculations once the problem reaches that stage; but (s)he participates fully in the planning and data-gathering stages and may dragoon the others into making calculations too, if there is nothing else left to do.
• The Recorder writes down the team's decisions regarding strategy, the principles utilized, any data obtained by measurement, the abstract and concrete calculations of the Computer, and the final results and conclusions (in consultation with the others).