Physics 1bl Electric Potentials And Fields (Page 2 of 6) in pdf

Physics 1BL

Electric Potentials & Fields

Summer Session II 2010

Group Activity:

Copy the three charge configurations shown below to your whiteboard.

For each

configuration, first draw what you would expect the electric field lines to look like. Then,

draw the corresponding equipotential

surfaces from the field lines you just

drew. Once you finish, ask your TA

to look over your work, as it is

essential for the rest of the lab that

you understand the relation between

electric field lines and equipotential

surfaces. You will be performing the

experimental

part

this

lab

(activities A, B & C) with a different

configuration from the one you used

in your whiteboard activity. Preserve

your whiteboards as a class resource

for reference to the experimental data

acquired in activity B1 and B2.

In this experiment you will use a black paper with special properties called teledeltos paper.

This was originally invented for use in telegraphy, transmitting information, in this case

images, by phone lines. It was possible to use an applied voltage at a point on the paper to

burn away a tiny part of the paper’s surface. By varying the voltage applied at different

points on the paper an “image” could be made and transmitted. This was a powerful

technical advance and enabled black and white photographs to be sent electronically via

undersea cable from London to New York in 1939. The paper allows very small amounts of

charge to flow along the direction of the local electric field. This makes it neither an

insulator nor a conductor, but a member of a very important set of materials called

“semiconductors”. In this lab it allows us to measure and visualize the potentials around

conducting shapes (electrodes) we draw with a pen using silver conducting ink. In this lab

we will not be using large enough potentials to burn the paper.

Activity A: Set-up the Equipment

A1. Each group should have a piece of black teledeltos paper with a diagram of a system

of charged conductors in a configuration different from the configuration the group

used for the whiteboard activity. Each person must trace or copy their group’s

configuration of conductors onto the photocopy of the Teledeltos paper. Similar

gridlines appear on both the teledeltos paper and the photocopy. The photocopy goes

into your lab book for your record. Do not make any marks on the black teledeltos

paper apart from the silver electrodes. The same sheet needs to be used for the entire

lab period.

A2. Make sure that the plastic pushpins attach the black teledeltos paper to the corkboard.

A3. Put a metal pushpin through each of the silver ink conductors. Choose a point where

the silver line is solid and wide, and seat the pushpin securely. Select two wires from

the stock in the lab, one red and one black. Choose wires that have banana plugs on

Physics 1bl Electric Potentials And Fields Page 2

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