Electric charges and fields

Charge model

• Frictional forces add or remove charge from an object. This is called charging.

• Two kinds of charges exist – positive and negative.

• Neutral objects have equal numbers of each kind of charge – friction separates them.

• Like charges repel; opposite charges attract.• The force between two charged objects is a

non-contact (or field) force.

Charge model

• The magnitude of the force decreases as the distance between charged objects increases.

• There are two types of materials:– Conductors: allow charges to move through– Insulators: charges remain immobile.

• Charge can be transferred from one object to another by contact.

• Charge is conserved (cannot be created or destroyed).

A charged rod is brought near the spheres. The spheres are separated

and the rod is taken away.

A B

C

D

++++++

++++++

After the charged rod is removed, which of the spheres is:

• Positive

• Negative

• Neutral

A charged plastic rod is brought near an uncharged metal rod on

an insulating stand. An uncharged metal ball is hanging near the other end of the metal rod. What happens to the ball?

An electroscope is charged by touching it with a positive glass rod.

The electroscope leaves spread apart, and the glass rod is removed. Then a negatively charged plastic

rod is brought close to the top of the electroscope but doesn’t touch it.

What happens to the leaves?

Charge

• Represented by q

• SI unit of charge is the coulomb (C)

• Protons and electrons have the same charge:

e = 1.60 x 10-19 C

This is called the fundamental charge.

Rank in order from most positive to most negative, the charges qA

to qE of these five systems.A. ProtonB. ElectronC. 17 protons, 19 electronsD. 1,000,000 protons

1,000,000 electronsE. Glass ball missing 3 electrons

How big is a Coulomb?Very, very large...

+1 C 1 m +1 C

F = 9 x 109 N (1,000,000 tons)

Often use: μC = 1 x 10-6 C

pC = 1 x 10-9 C

Coulomb’s Law

• The force law that quantitatively describes the behavior of charged objects.

• F = k q1 q2

r2

• Electrostatic constant k = 9.0 x 10 9 N • m2/C2

+ +

rF1 on 2F2 on 1

q1 q2

Coulomb’s Law

• The net force on a charge is the vector sum of the forces from all other charges.

Adding electric forces in one dimension

Two +10 nC charged particles are 2.0 cm apart on the x-axis. What is the next force on a +1.0 nC charge midway between them? What is the net force if the charged particle on the right is replaced by a -10 nC charge?

+ + +

0 1 2 x (cm)

Adding electric forces in two dimensions

Three charged particles - q1, q2 and q3 - are shown below. What is the net force on q3 due to the other two charges?

- +

+

q1= - 50 nC

2

10.0 cm

5.0 cm

q3= +30 nC

q2= + 50 nC

-

+

1.0 cm

2qg= +10 nC

qp= + 50 nC

A small plastic sphere is charged to -10 nC. It is held 1.0 cm above a small glass bead at rest on a table. The beak has a mass of 15 mg and a charge of+10 nC. Will the glass bead “leap up” to the plastic sphere?

Electrical v. gravitational force

Electric Field from a Point Charge

Electric Field Lines for a Point Charge

Electric Field from Two Point Charges

Electric Field between Two Plates

Movement of a Charged Particle in an Electric Field