1. AP Physics
Section 16-1 to 16-6
Electric charge
and the
electric force

2. History
The ancients Greeks noted that amber objects, such as buttons, attracted hair and fur. Thales of Miletus recorded experiments of rubbing fur on amber (Anc.Gk: ēlektron) and producing sparks.
In 1600, the English scientist William Gilbert coined the neo-Latin term electricus, providing English with electric and electricity.

3. In 1660, Otto von Guericke invented the first electrostatic generating machine, and in 1675 Robert Boyle did experiments showing that electrostatic forces worked across a vacuum.
von Guericke
Stephen Gray developed the concept of conductors and insulators in His work was expanded in 1739 by Charles François de Cisternay du Fay who developed the two–fluid theory of electricity. He believed electricity to be created by two fluids, vitreous (glassy) and resinous.
du Fay

4. Benjamin Franklin did experiments in which he proved that lightning was an electrostatic discharge. Franklin developed the one–fluid theory. He believed that some materials had an excess of electric fluid (positive) or lacked some of the fluid (negative).
Franklin also developed the theory of the conservation of electric charge.

5. Electrostatics
Prior to the 19th century, most of the electrical work involved static charges, not electric current.
Charges were created mostly by rubbing one material against another. Creation of charge through friction is called the triboelectric effect.
A list of materials with their relative static charge is known as the triboelectric series.
Materials such as glass, fur, silk, and skin were found to become positively charged. Other materials such as sulfur, rubber, and ebonite became negatively charged.

7. Electrons and charge
The electron was discovered by J. J. Thomson in 1897, and the its unit of elementary charge was determined by Robert Millikan in the famous oil drop experiment of After that time, theories of electrostatics involved electron charges.
On the atomic level, friction removes electrons from the surface of some substances and deposits them on others.
Electrostatic generators use a rotating disk or cylinder of one material and friction to create a net charge to be stored.

8. von Guericke’s machine
Franklin’s generator
Carré machine
Holtz machine
Toepler machine
Van de Graaff

9. Martin van Marum generator, Teylers Museum, Netherlands

10. Thomas Kim on YouTube has made many historical electrostatic machines, and performed electrostatic experiments such as induction, in his videos. His channel address is:
Watch these: (linked in videos on Chapter 16 page)
von Guericke type friction generators
Toepler-Holtz electrostatic generator
Lord Kelvin's thunderstorm
Pfaff and Svanberg electrostatic multiplier
Wimshurst generators made from CDs
Van de Graaff generators

11. Electric force History
David Bernoulli made rough measurements of the force created by electric charges.
Henry Cavendish was the first scientist to realize that the electric force follows an inverse square law just like gravity. Recall that Cavendish was the scientist that determined the value for the gravitational constant, G.
Cavendish
Cavendish suffered from extreme shyness, and did not publish his results on electric force.

12. Properties of electric force
By the late 18th century, scientists had noted several properties of electrically charged objects.
• Two kinds of charge existed, called positive and negative by Franklin.
• Charges could exert a force on other charges, even through a vacuum, as noted by Boyle.
• The force became stronger when the charges were closer together.
• Like charges repelled and opposite charges attracted each other.

13. Storing charge
Devices were invented for storing and detecting charge in the mid 18th century. The primary device used to store charge was a type of capacitor called a Leyden Jar.
The jar was invented by Pieter van Musschenbroek and Andreas Cunaeus of the University of Leyden in 1744, and independently discovered by German researcher Ewald Georg von Kleist.

14. Measuring charge
The presence, and to some extent the amount of charge, was measured with an electroscope. The first were pith ball electroscopes invented in 1754 by John Canton. – +
In 1787, Charles Bennet invented a much more sensitive electroscope, the gold leaf electroscope. –

15. Charging by conduction
Charge can be transferred from one object to another by direct contact. This is charging by conduction. When a metallic conductor, such as an electroscope or an isolated metal sphere, is charged, the charge spreads through the object.
Nonconductors must be rubbed over their entire surface.
Charging an electroscope by conduction

16. Charging by induction
Charging two
objects by
induction
Charging one
object by
induction
An opposite charge can be induced by a few methods.

17. Coulomb’s experiment
In 1785 Charles Coulomb used a torsion balance to measure the force created by electric charges.

18. Coulomb’s Law F ∝ qAqB F ∝ F ∝ F = ke
Coulomb found that the electric force is directly proportional to the product of the charges.
F ∝ qAqB
Unit of charge (q):
coulomb (C)
Coulomb found that the electric force was inversely proportional to the square of the distance between the charges, just as Cavendish had.
F ∝ 1 r2
Therefore:
F ∝ r2
qAqB
F = r2
qAqB ke
ke = 9 × 109 Nm2/C2
Coulomb’s constant

19. Point charge problems F = ke One dimensional problems – + r2 qAqB
The force on a point charge exerted by surrounding charges can involve one, two or three dimensions. The problems are therefore vector force problems.
One dimensional problems
1. A positive and negative charge, each of magnitude 25 μC, are separated by a distance of 15 cm. Find the force on each of the charged particles. + –
+25 μC
–25 μC
F = r2
qAqB ke
0.15 m
(25 × 10–6 C)
(25 × 10–6 C)
F =
(9 × 109 Nm2/C2)
(0.15 m)2
F =
2.5 × 102 N toward each other

20. F = ke r2 = ke + + ? r2 qAqB F qAqB (80 × 10–6 C) (30 × 10–6 C) r2 =
2. A force of 2.4 × 102 N exists between a charge of +80 μC and a charge of +30 μC. What distance separates the charges? +
+80 μC
+30 μC + ?
F = r2
qAqB ke
r2 = F
qAqB ke
(80 × 10–6 C)
(30 × 10–6 C)
r2 =
(9 × 109 Nm2/C2)
(2.4 × 102 N)
r2 =
0.09 m2
r =
0.3 m

21. F = ke – + r2 qAqB (2 × 10–6 C) (3 × 10–6 C) FAC = (9 × 109 Nm2/C2)
3. What total force is exerted on the positive charge? –
–2.0 μC
–4.0 μC
0.050m
0.030m +
+3.0 μC
F = r2
qAqB ke A C B
(2 × 10–6 C)
(3 × 10–6 C)
FAC =
(9 × 109 Nm2/C2)
(0.050 m)2
FAC =
21.6 N to the left
FAC =
–22 N
(4 × 10–6 C)
(3 × 10–6 C)
FBC =
(9 × 109 Nm2/C2)
(0.030 m)2
FBC =
120 N to the right
FBC =
+120 N
Net force:
F =
FAC + FBC
= –
= +98 N
F =
98 N to the right