Electrostatics

St. John/Hall

Chapter 32

Charge

• there are two kinds of charge, positive and negative • like charges repel, unlike charges attract • positive charge comes from losing electrons; negative charge

comes from gaining electrons • charge is quantized, meaning that charge comes in integer multiples

of the elementary charge e

q = n e • q is the symbol used to represent charge, while n is a positive or

negative integer, and e is the electronic charge, 1.60 x 10-19 Coulombs.

• charge is conserved in isolated systems

Electric Charge – The specifics

Some important constants:

•The symbol for CHARGE is “q”•The unit is the COULOMB(C), named after Charles Coulomb•If we are talking about a SINGLE charged particle such as 1 electron or 1 proton we are referring to an ELEMENTARY charge and often use, e , to symbolize this.

Particle Charge Mass

Proton 1.6x10-19 C 1.67 x10-27 kg

Electron 1.6x10-19 C 9.11 x10-31 kg

Neutron 0 1.67 x10-27 kg

Conduction• An electroscope is a device used to detect the presence of charge

and it's relative amount.  One method of charging the electroscope is by conduction or contact.  This means that a charged object must actually touch the electroscope and transfer charge too. 

When charging something by contact it is important to note the following properties:

• The objects must actually touch and transfer some electrons.• The objects become charged alike.

Inverse Square Laws

Coulomb’s Law Newton’s Law

Similarities:Both the forces are directly proportional to the product of the

interacting commodities (mass and charge) Both the forces are inversely proportional to the square of the

distance of separation Differences:

The Coulombian force can be attractive or repulsive while the gravitational force is attractive only

 

Robert Millikan

• Milikan created an experiment using oil drops placed within a capacitor field to show that all charges are a multiple of 1.6 x 10-19

Practice Problems

• 1. Suppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them.

• 2. Two balloons are charged with an identical quantity and type of charge: -6.25 C. They are held apart at a separation distance of 61.7 m. Determine magnitude of the electrical force of repulsion between them.

• 3. Two balloons with charges of +3.37 C and -8.21 C attract each other with a force of 0.0626 Newtons. Determine the separation distance between the two balloons.

•(Felect = 9.0 x 109 N)

•(Felect = 9.23 x 107 N)

•(d = 1.99 x 10-1)

4. The Q in Coulomb's law equation stands for the _____.• a. mass of a charged object• b. # of excess electrons on the object• c. the current of a charged object• d. the distance between charged objects• e. charge of a charged object

5. The symbol d in Coulomb's law equation represents the distance from ___.

• a. A to B b. A to D c. B to C d. B to D e. C to D f. A to G g. B to F h. C to E

Problemse.

g.

Problems

• 6. Determine the electrical force of attraction between two balloons with separate charges of +3.5 x 10-8 C and -2.9 x 10-8 C when separated a distance of 0.65 m.

 • 7. Determine the electrical force of attraction between two balloons

with opposite charges but the same quantity of charge of 6.0 x 10-7 C when separated a distance of 0.50 m.

  • 8. A balloon has been rubbed with wool to give it a charge of -1.0 x

10-6 C. A plastic rod with a charge of +4.0 x 10-6 C localized at a given position is held a distance of 50.0 cm above the balloon. Determine the electrical force of attraction between the rod and the balloon.