Wkb Chapter 1: Static ElectricityActivities, vocabulary & Facts:

Guidelines for the activities in Sherwood & Chabay's "Electric and Magnetic Interactions" Chapter 1

Charge: two kinds, positive and negative.

There are two tapes located at each table, a U tape and beneath it a L tape.

Grasp the L tape and pull it off the table, rub both sides with your fingers until it isn’t attracted to you, then pull the U & L tapes apart quickly.

Hang them from the rod stand about a foot apart.

Bring things, your hand, a pen, something metal, etc, close to each of them to see which attracts what.

1. Record your observations here.

Establish that attraction only indicates that one of the two objects are charged, not which.

Next take a U tape and bring it close to both a U and L tape and record the reaction of each tape to the U tape.

2. Record your observations here.

The relationship between this force and the distance between objects:

Look at S&F Active Figure 15.6. (23.7 in 214) Start with the charges 48x10^-12 m apart.

Discharging objects. Make a note of how close you must bring your hand to the bottom of the U tape to make it move toward you approximately 2 cm.

3. Record your observations here.

Now partially discharge the U tape by holding onto the bottom with one hand while rubbing your finger down one half of the width of the slick side of the tape.

Again make a note of how close you must bring your hand to the bottom of the U tape to make it move toward you approximately 2 cm.

4. Record your observations here.

Next discharge the entire tape by rubbing the entire width of the tape with your finger, and check to see if it will now attract anything. (How about your L tape?)

Mysteries we have manufactured.

#1. Why do both kinds of charge attract everything?

#2. How, exactly, do things become charged? (See C&S pg 11, 1.3.2)

#3. How does rubbing the slick side of the tape remove charge from it? (See C&S pg 11, 1.3.3)

Basic Atomic Physics Facts:

Materials are made up of 3 particles, electrons, protons, and neutrons. (Yeh, Yeh, we all know this is false.)

The basic element of charge is the charge on an electron .

Charge on an electron q_e, or simply -e = -1.6 x 10^-19 Coulombs (C)

Mass of an electron m_e = 9.11 x 10^-31 kg.

Charge on an proton q_p, or simply e = 1.6 x 10^-19 Coulombs (C)

Mass of an proton, m_p = 1.67 x 10^-27 kg.

Charge on an neutron = 0

Mass of a neutron m_n = 1.67 x 10^-27 kg

Protons and neutrons are embedded in the nucleus of atoms, while electrons go whizzing around the nucleus in orbits, or "shells".

Like charges repel each other and unlike charges attract each other.

Charge can neither be created nor destroyed.

Compare Forces: Given a proton and electron 10^-10 m apart, find:"

F_E =

Conclusion: It doesn't take much charge to cause these forces we're seeing!

Definition: (Ben Franklin's) Rubber (polystyrene) rubbed by fur causes excess negative charge on the rubber.

Glass (Acrylic) rubbed by silk causes a deficit of negative charge on the glass rod, and hence it is said to be positively charged.

Read C&S page 16.

Determine the sign of the charge on the U and L tapes (you will have to "recharge them first.)

5. Record your observations here.

U tape has ___________ charge.

L tape has ___________ charge.

1.7.3 Find out if electric forces can act thru paper shield. This may be rather weak, hold the paper in front of one of your L tapes and wave the charged rubber rod back and forth behind it. Next read the discussion on page 24 of C&S.

6. Record your observations here.

Does the force act thru the paper? __________

The Super-Position Principle: 1.7 Page 20

Do on your own: 1.7.1a,b,c, 1.7.2 c

Steps to finding total F .

1. Draw frame of reference (coordinate axes).\

2. Draw a free body diagram for the point or point charge.

3. Find the magnitude of each F , do NOT use the signs of charges .

4. Find the x & y components of each (Pay attention to the signs of these).

5. Add the x components and then the y components to find the x and y components of the total F or E.

6. If necessary, use Pythagoras’ Thm to find the magnitude of the vector.

7. Use the inverse tangent function to find u = tan^-1 (F_y/F_x) .

Show’em the chart method for 2 dimensional problems.