18.3 Conductors and Insulators

• Electric charge can exist on an object and can move through an object.

• Different materials have different abilities to allow electric charge to move or be conducted through them.

Electrical conductors

• Electrons are conducted from negatively charged object toward the positively charged object.

• Electrical conductors: substances that readily conduct electric charge.

• Ex. metals

Electrical Insulators

• Materials that conduct electric charge poorly.• Ex. Rubber, plastics, wood

What makes a material a conductor or insulator?

• The atomic structure• Electrons in the outer orbits experience a

weaker force of attraction to the nucleus than do those in the inner orbits.

• Valence electrons (outermost electrons) can be dislodged more easily than the inner ones.

• Electrons are able to move readily away from the negative end and toward the positive end.

• Ready movement of electrons is a characteristic of a good conductor.

• In an insulator: very few electrons are free to move throughout the material. Little flow of charge.

18.4 Charging By Contact and By Induction

• When objects are touch excess electrons are transferred.

• Charging by contact: process of giving on object a net electric charge by placing it in contact with another object that is already charged.

• It is also possible to charge a conductor that does not involve contact.

• Pg. 541• Negatively charged rod is brought close to a

sphere.• Electrons move to end of rod closest to sphere.• Sphere becomes positively charged near rod

and negatively charged on the opposite end.

• Induced or persuaded to form because of repulsive force between the negative rod and the free electrons in the sphere.

• If the rod was removed the electrons would spread out evenly again.

Charging by Induction

• Process of giving one object a net electric charge without touching the object to a second charged object.

18.5 Coulomb’s Law

THE FORCE THAT POINT CHARGES EXERT ON EACH OTHER

• The electrostatic force that stationary charged objects exert on each other depends on the amount of charge on the objects and the distance between them.

• Greater the charge, closer together they are, the greater the force.

Coulomb’s Law

Example 2: A Large Attractive Force

Two objects, whose charges are +1.0 and -1.0 C, are separated by 1.0km. Compared to 1.0 km, the sizes of the objects are small. Find the magnitude of the attractive force that either charge exerts on the other.

• In a lab charges are small so often they are measured in microcoulombs.

• 1uC = 10^-6C• Coulomb’s Law is very similar to Newton’s Law

of Gravitation

Both depend on the distance.Difference is the force between the object can

either repel or attract. Gravitational force always attract.

Example 3: A Model of the Hydrogen Atom

In the Bohr model of the hydrogen atom, the electron is in a circular orbit about the nuclear proton at a radius of 5.29 x 10^-11 m. The mass of the electron is 9.11 x 10^-31 kg. Determine the speed of the electron.

Physics of Adhesion

• Distance between tape and surface is small.• Electrons shift over the small distances

between the tape and the surface.• Materials become oppositely charged.• Pg. 545

THE FORCE ON A POINT CHARGE DUE TO TWO OR MORE OTHER POINT CHARGES

• Q1 and q2 are point charges.• Now a third point charge is introduced q3.• What would be the net force on q1 due to both

q2 and q3?• First find the magnitude and direction of the force

exerted on q1 by q2 (ignoring q3).• Then determine the force exerted on q1 by q3

(ignoring q2).• The net force on q1 is the vector sum of these

forces.

EXAMPLE 4: THREE CHARGES ON A LINEThree point charges that lie along the x axis in a

vacuum. Determine the magnitude and direction of the net electrostatic force on q1.

Example 5: THREE CHARGES IN A PLANEThree point charges that lie in the x, y plane in a

vacuum. Find the magnitude and direction of the net electrostatic force on q1.

The forces F12 and F13 are resolved into x and y components.

Then the x components are combined to give Fx and the y components are combined to give Fy.

The magnitude and direction of F can be determined using trigonometry.

Practice Problems