1. Quick Question
If you wipe a CD with a clean cloth,
why does the CD then attract dust?
How does the distance between 2 charges
impact the force between them? If the
distance is decreased while the charges
remain the same, what happens to the
force?
Two electrons in an atom are separated by
1.5 x meters, the typical size of an
atom. What is the electrical force between
them?

2. Quick Question
If you wipe a CD with a clean cloth,
why does the CD then attract dust?
Rubbing the CD charges it. Electrons are either transferred to it making it negative, or removed from it, making it positive. When neutral particles such as dust
approach it, the particles in the
dust that are the opposite charge
are attracted to it and move to
that side of the dust particle. The attractive force between the particles hold them together.

3. How does the distance between 2 charges
impact the force between them? If the
distance is decreased while the charges
remain the same, what happens to the
force?
Electric force is inversely proportional to the distance squared. As the distance decreases and charges remain the same, the force increases as the square of the distance.

4. Two electrons in an atom are separated by
1.5 x meters, the typical size of an
atom. What is the electrical force between
them?
= 9x109 Nm/C2 (1.6 x 10-19C) (1.6 x 10-19C)
(1.5 x m)2
= X Nm
2.25 x m2
= x 10-8 Nm

5. Your assignment due today: Read Pages 541- 553.
Answer questions:
Page 545 Answer Problems 1-8
Page 552 Complete problems
Page 553 Answer problems 14-21

6. Kinetic and Potential Energy
Quiz Tomorrow on
Kinetic and Potential Energy
and Chapter 20.

7. Today’s Objective: I can calculate the potential difference from the work required to move a charge.

8. Inflate and tie off two balloons
Inflate and tie off two balloons. Attach a ½ meter length of string to each balloon.

9. Rub one balloon back and forth on your shirt eight times, causing it to become charged. Hang it from the lab table so it does not hit the floor.

10. Rub the second balloon the same way and then suspend it from its string.
Slowly bring the 2nd balloon toward the suspended one How to do the balloons behave?

11. Tape the second balloon so it hangs by its string next to the first one.
Bring your hand toward the charged balloons. Observe what happens.
What did you observe as ```` the 2 balloons were brought near each other? What happened when you hand was brought near the balloons?

12. Where else have you observed two objects that react without touching like this?

13. An electric field is the open space where a charged object can have an influence on another object…much like demonstrated with the two balloons.

14. object with a charge of 5 coulombs be at the same point?
The strength or intensity of an electric field is the force exerted on a small test charge placed at some point in the field. It is measured in newtons per coulomb.
If a test charge of 1 coulomb
is subjected to a force of 10
N, then the field is 10 N/C at
that point.
What would the force on an
object with a charge of 5 coulombs be at the same point?
50 N

15. .

16. Two basic formulas for determining the strength of an electric field (E):
where F is the force and
q is the charge of the particle; and
Where k is Coulomb’s law constant,
q is the charge of the particle,
and r is the radius or distance
between the particles’ centers.

17. An electric field is measured using a positive test charge of 3
An electric field is measured using a positive test charge of 3.0 x 10-6 C. This test charge experiences a force of 0.12 N at an angle of 15° north of east. What are the magnitude and the direction of the electric field strength at the location of the test charge?
Q = x 10-6 C
F = 0.12 N
0.12 N
3.0 x 10-6 C
E = ?
4.0 x 104 N/C

18. An electric field is measured using a positive test charge of 3
An electric field is measured using a positive test charge of 3.0 x 10-6 C. It experiences a force of 0.24 N at an angle of 15 North of East. What are the magnitude and direction of the electric field strength at the location of the test charge?
.24 N
Q = x 10-6 C
3.0 x 10-6 C
F = .24 N
E = 8.0 x 104 N/C
E = ?

19. E = -4 X 10 5 N?C toward the sphere (or to the left)
What is the electric field strength at a point that is 0.30 m to the right of a small sphere with a charge of -4.0 x C?
q = x 10-6 C
d = m
E = ?
Kq1q2 r2
E = k q r2
(9.0 x 10 9 )(-4.0 x 10-6 C)
(0.30 m)2
E = -4 X N?C toward the sphere (or to the left)

20. Creating and Measuring Electric Fields
Read Section 21.1 about
Creating and Measuring Electric Fields
Page 565 problems 1-5
Page 566 problems 6-10 &
Answer questions 11 – 15 on page 568

21. Most of the time when we speak of electricity, we are meaning electric current.